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authorEkaitz Zarraga <ekaitz@elenq.tech>2022-03-14 23:52:34 +0100
committerEkaitz Zarraga <ekaitz@elenq.tech>2022-03-14 23:52:34 +0100
commit57ebc576bf74ca887f9c77cb6b7771b9ff2b1843 (patch)
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parent0c2fa8a18d664ddb470cdae489dd08bbcc12d590 (diff)
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diff --git a/content/bootstrapGcc/00_intro.md b/content/bootstrapGcc/00_intro.md
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+Title: Intro to GCC bootstrap in RISC-V
+Date: 2022-02-14
+Category:
+Tags: Bootstrapping GCC in RISC-V
+Slug: bootstrapGcc0
+Lang: en
+Summary:
+ Introduction to my new adventure bootstrapping GCC for RISC-V. Why, how,
+ and who is going to pay for it.
+
+You probably already know about how I spent more than a year having fun with
+RISC-V and software bootstrapping from source.
+
+As some may know from my [FOSDEM talk][fosdem22], [NLNet / NGI-Assure put the
+funds][nlnet] to make me spend more time on this for this year and I decided
+to work on GCC's bootstrapping process for RISC-V.
+
+[nlnet]: https://nlnet.nl/project/GNUMes-RISCV/
+[fosdem22]: https://fosdem.org/2022/schedule/event/riscvadventures/
+
+### Why GCC
+
+GCC is probably the most used compiler collection, period. With GCC we can
+compile the world and have a proper distribution directly from source, but who
+compiles the compiler?[^1]
+
+[^1]: *wHo wATcHes tHE wAtchMEN?*
+
+Well, someone has to.
+
+### The bootstrap
+
+Bootstrapping a compiler with a long history like GCC for a new architecture
+like RISC-V involves some complications, starting on the fact that the first
+version of GCC that supports RISC-V needs a C++98 capable compiler in order to
+build. C++98 is a really complex standard, so there's no way we can bootstrap a
+C++98 compiler at the moment for RISC-V. The easiest way we can think of at
+this point is to use an older version of GCC for that, one of those that are
+able to build C++98 programs but they only require a C compiler to build. Older
+versions of GCC, of course, don't have RISC-V support so... We need a
+*backport*[^2].
+
+[^2]: Insert "Back to the Future" music here.
+
+So that's what I'm doing right now. I'm taking an old version of GCC that only
+depends on C89 and is able to compile C++98 code and I'm porting it to RISC-V
+so we can build newer GCCs with it.
+
+Only needing C to compile it's a huge improvement because there are *Tiny C
+Compilers* out there that can compile C to RISC-V, and those are written using
+simple C that we can bootstrap with simpler tools of a more civilized world.
+
+In summary:
+
+- C++98 is too complex, but C89 is fine.
+- GCC is the problem and also the solution.
+
+### What about GNU Mes?
+
+When *we*[^3] started with this effort we wanted to prepare GNU Mes, a small C
+compiler that is able to compile a *Tiny C Compiler*, to work with RISC-V so we
+could start to work in this bootstrap process from the bottom.
+
+[^3]: "*We*" means I shared my thoughts and plans with other people who have a
+ much better understanding of this than myself.
+
+Some random events, like someone else working on that part, made us rethink our
+strategy so we decided to start from the top and try to combine both efforts at
+the end. We share the same goal: full source bootstrap for RISC-V.
+
+### Tiny C Compilers?
+
+There are many small C compilers out there that are written in simple C and are
+able to compile an old GCC that is written in C. Our favorite is TinyCC (Tiny C
+Compiler).
+
+[^4]: But there are some others that are really interesting (see
+[cproc](https://sr.ht/~mcf/cproc/), for example)
+
+GNU Mes is able to build a patched version of TinyCC, which already supports
+RISC-V (RV64 only), and we can use that TinyCC to compile the GCC version I'm
+backporting.
+
+We'd probably need to patch some things in both projects to make everything
+work smoothly but that's also included in the project plan.
+
+### Binutils
+
+Binutils is also a problem mostly because GCC, as we will talk about in the
+future, does not compile to binary directly. GCC generates assembly code and
+coordinates calls to `as` and `ld` (the GNU Assembler and Linker) to generate
+the final binaries. Thankfully, TinyCC can act as an assembler and a linker,
+and there's also the chance to compile a modern binutils version because it is
+written in C.
+
+In any case, the binary file generation and support must be taken in account,
+because GCC is not the only actor in this film and RISC-V has some weird things
+on the assembly and the binaries that have to be supported correctly.
+
+### Conclusion
+
+This is a very interesting project, where I need to dig in **BIG** stuff, which
+is cool, but also has a huge level of uncertainty, which scares the hell out of
+me. I hope everything goes well...
+
+In any case, I'll share all I learn here in the blog and I keep you all posted
+with the news we have.
+
+That's all for this time. If you have any question or comment or want to share
+your thoughts and feelings with me[^5] you can find my
+[contact information here](https://ekaitz.elenq.tech/pages/about.html).
+
+[^5]: Or even hire me for some freelance IT stuff 🤓
+
+---
+
+> PS: Big up to NlNet / NGI-Assure for the money.
+
+<style>
+.container{
+ display: flex;
+ flex-flow: row wrap;
+ justify-content: center;
+ gap: 40px;
+}
+.no-side-margin{
+ margin: 0px;
+}
+</style>
+<div class="container">
+<img class="no-side-margin" src="{attach}/bootstrapGcc/nlnet.svg" width=200px>
+<img class="no-side-margin" src="{attach}/bootstrapGcc/NGIAssure.svg" width=200px>
+</div>
diff --git a/content/bootstrapGcc/01_internals.md b/content/bootstrapGcc/01_internals.md
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+Title: GCC internals — From a porting perspective
+Date: 2022-03-08
+Category:
+Tags: Bootstrapping GCC in RISC-V
+Slug: bootstrapGcc1
+Lang: en
+Summary:
+ Deep diving into GCC's internals from the perspective of someone who
+ wants to port GCC for a new architecture.
+
+
+In the [previous post]({filename}00_intro.md) of the
+[series]({tag}Bootstrapping GCC in RISC-V) the problem of the GCC bootstrapping
+was introduced. In this post we'll describe how GCC works, from the
+perspective of someone who wants to port it so we understand what's the job we
+have to do.
+
+1. [Disclaimer](#disclaimer)
+2. [Overview](#intro)
+ 1. [The compiler generation framework](#cgf)
+ 2. [GCC as a coordinator](#gcc-coordinator)
+3. [Source code parsing](#parsing)
+ 1. [GENERIC](#generic)
+4. [GIMPLE](#gimple)
+5. [Register Transfer Language](#rtl)
+ 1. [Target-dependent code](#target-dependent)
+ 2. [Machine description files](#md)
+ 1. [Machine modes](#mm)
+ 2. [RTL Templates](#rtl-templates)
+ 3. [Target description macros and functions](#target-desc)
+6. [Assembly code generation](#assembly)
+7. [Summary](#summary)
+8. [My job in the backport](#job)
+9. [Last words](#last)
+10. [Learn more](#more)
+
+### Disclaimer {#disclaimer}
+
+- This post may be only valid for old GCC versions, like 4.something, because
+ that's the one I'm interested in. More recent versions may have different
+ details, but I don't expect them to be very different to what is described
+ here. More specifically: I'm working on GCC 4.6.4, and the first GCC with
+ RISC-V support is GCC 7.0.0.
+- This post will focus on how GCC compiles C programs because that's the part
+ we care about. Some other languages have differences on how they are treated
+ but that's not very relevant for us, as it has no implications on the
+ *back-end*.
+
+Both of these points will get clearer later.
+
+### Overview {#intro}
+
+GCC is structured as a pipeline of several steps that run one after the other.
+
+1. Source code parsing
+2. GIMPLE IR generation (target-independent)
+3. Some GIMPLE tree optimizations
+4. RTL IR generation (target-dependent)
+5. RTL optimizer
+6. Assembly code generator
+
+Before starting to analyze each of the steps independently there are a couple
+of things to clarify.
+
+#### The Compiler Generation Framework {#cfg}
+
+An important point to note is GCC is a **compiler collection** meaning that it
+is able to compile code from many high level languages (HLL) and for many
+different targets. This has implications on how some steps are mapped to GCC's
+source code.
+
+The most important thing of all this is to differentiate between GCC's code and
+an actual `gcc` executable. The key point here is that GCC's codebase includes
+what is called CGF (Compiler Generator Framework) that can generate `gcc`
+executables from GCC's code. The CGF generates `gcc` executables according to
+the input (target machine, host machine...) we give it, but the generated `gcc`
+executables may differ one from another even if they were generated from the
+same codebase.
+
+Any `gcc` executable is able to compile any input HLL[^language] (C, C++,
+Objective-C, Ada, Fortran and Go[^java]), so GCC's code must include parsers
+for each of these languages.
+
+[^language]: When calling `gcc` you can choose which language you are compiling
+ using `-x language` option or you can let `gcc` guess from the extension.
+
+[^java]: And also Java in the past!
+
+On the other hand, `gcc` executables are only able to generate code for one
+target (x86, MIPS, ARM, *RISC-V*...), that must be chosen when GCC is compiled.
+In order to make the porting efforts easier, GCC has a set of tools that
+generate the target-dependent code from some configuration files called Machine
+Descriptions (MD).
+
+Putting all this together, source code parsing and AST generation depend on the
+input HLL, and the code that runs for each HLL is **selected** when `gcc` runs
+(*steps 1*). The intermediate representation, GIMPLE, is target-independent so
+everything related with that is **copied** inside the final `gcc` executable
+(*steps 2 and 3*). The RTL (Register Transfer Language) representation and
+assembly code generation are target-dependent and the code related to that is
+**generated** from MD files when GCC is compiled (*steps 4, 5 and
+6*)[^rtl-opt].
+
+[^rtl-opt]: The RTL optimizer contains many steps, most of them being target
+ independent. That doesn't really matter here, but those are not generated but
+ copied from GCC's source, as GIMPLE is.
+
+This all means if we want to be able to read the source code of GCC we have to
+have clear in mind how the source code maps to the actual executable, i.e. if
+we generate a `gcc` executable for **x86** it won't contain the code for other
+architectures and **it won't even check if it was correctly programmed**,
+because it's not going to compile it.
+
+
+#### GCC as a coordinator {#gcc-coordinator}
+
+Many GCC users or C programmers (or me, not that long ago) might think there is
+something missing on the list of steps we recently reviewed. The normal usecase
+for calling `gcc` like
+
+``` bash
+$ gcc -o helloworld helloworld.c
+```
+
+does several steps internally that we need to separate:
+
+1. Preprocessing: the resolution of preprocessor macros like `#define` and
+ stuff like that.
+2. Compiling to assembly: the generation of assembly code files per compilation
+ unit (a file that is the output of the preprocessor).
+3. Assembly: the conversion from an assembly file to ELF object file.
+4. Linking: the executable or library generation from the ELF object files
+ created in the previous step.
+
+The reality is there's more than one program involved here and `gcc` is just a
+coordinator that makes other programs run if needed.
+
+The preprocessor is called `cpp` and it is generated from the GCC codebase. The
+compiler is `gcc` itself but the assembler and the linker are generally
+obtained from GNU Binutils' `as` and `ld` respectively.
+
+So, one of the most important things to understand is GCC only generates
+assembly, but it looks like it doesn't[^tinycc-asm].
+
+This means we need a proper support for our architecture on the assembler and
+the linker too. But we'll keep that story for another day[^post-long].
+
+[^tinycc-asm]: Other compilers have different approaches for this. For example,
+ TinyCC generates machine code directly, without the intermediate assembly
+ file generation step, and is also able to link the files by itself.
+
+[^post-long]: This post is already long enough and we only made the
+ introduction.
+
+---
+
+### Souce code parsing {#parsing}
+
+> HLL dependent. Generates appropriate IR
+
+So the first step of the compiler is to process the input text and convert it
+to the appropriate Intermediate Representation. The most used intermediate
+representation is GENERIC, that was designed for C but fits other procedural
+languages pretty well[^fortran].
+
+[^fortran]: The case of FORTRAN is a little bit weird, as it generates its own
+ representation that is later converted to GENERIC, we don't really care about
+ this at this point.
+
+This parsing process not really relevant for us, as we want to add a new
+target, but it's interesting to note because it gives shape to the codebase.
+GCC splits the code for the different input languages in folders named like:
+`gcc/$LANGUAGE`.
+
+#### GENERIC {#generic}
+
+GENERIC is just a representation, we don't need to care that much about it but
+a word is not going to hurt anyone. GENERIC is a tree representation: a set of
+nodes with some extra common information. Those nodes can be read at
+`gcc/tree.def`.
+
+A simple example of this could be a function declaration, that would take the
+a node of type `FUNCTION_DECL` that has some sub-nodes: one for the return
+type, another for the body of the function and another for the arguments of the
+function.
+
+It's a simple AST you could come up with yourselves, except the fact that it is
+pretty complex. 😅
+
+### GIMPLE {#gimple}
+
+> HLL- and Target- independent representation
+
+The next step is called *Gimplification* (see `gimplify.c`), the process of
+converting to GIMPLE. Normally, representing the AST as GIMPLE is too complex
+to be done in one step, so the GENERIC (+ some extensions) is used as a
+previous step that is easier to create.
+
+GIMPLE is the central internal representation of GCC. It's target-independent
+and High-Level-Language-independent. At this point some optimizations can be
+applied, those related with the structure of the source code, like loop
+unfolding or dead code elimination.
+
+From the porting perspective, this representation is important, as it's the
+border line between the front-end and the back-end, and we are interested in
+the latter. A really interesting part is to understand is how is this converted
+to the next representation, RTL.
+
+### Register Transfer Language (RTL) {#rtl}
+
+> Target-dependent low level representation
+
+The next part of the compiler work is done using the RTL intermediate
+representation. The RTL representation is based on LISP, so we have a reason to
+love it, and it serves two purposes:
+
+1. Specify target properties via the Machine Descriptor files. These Machine
+ Descriptor files are text files that look like LISP and are processed at
+ compilation time.
+2. Represent a compilation. Meaning that the RTL is also an intermediate
+ representation, a low-level one, that represents sets of instructions.
+
+GCC does not make any distinction between the first and the second purpose,
+calling both RTL, but there some difference on the purpose and the shape of the
+RTL. RTL has both, an internal form represented by structures (case 2) and an
+external form represented as a text file (case 1).
+
+The RTL is formed by a set of objects: expression, integers, wide integers,
+strings or vectors. In the textual form they are represented like in LISP,
+using double quotes for strings, brackets for vectors... and a lot of
+parenthesis. The internal representation you can imagine, structures for
+expressions, integer types for integers, `char*` for strings, etc.
+
+The most interesting RTL objects are expressions, aka RTX, that are just a name
+(an expression code) plus the amount of possible arguments.
+
+This is how a piece of RTL may look, it represents an instruction that sets the
+register 0 to the result of the addition of the register 1 and the constant
+integer 10 (see `rtl.def` for more information):
+
+``` lisp
+(set (reg 0)
+ (plus (reg 1)
+ (const_int 10)))
+```
+
+In the example the only things that are not expressions are the numbers (0, 1
+and 10), all the rest you can find in `rtl.def` and see what they mean.
+
+From GIMPLE, there are two steps left to reach our target, assembly code, and
+both involve RTL. The first maps the GIMPLE nodes to pattern names in a
+target-independent way, generating a list of RTL `insn`s. The second matches
+those `insn` lists to RTL templates described in Machine Description files and
+uses those matches to generate the final assembly code.
+
+Those `insn`s are objects that represent code in RTL. Each function is
+described with a doubly-linked list of `insn`s. You can think about them as
+*instructions* in the RTL world.
+
+In the first step, the RTL `insn` generation step, only the names matter (and
+they are hardcoded in the compiler), while in the second the structure of the
+`insn` is going to be analyzed as we'll see later.
+
+#### Target-dependent code {#target-dependent}
+
+As we previously said, target-dependent steps are generated at compile time and
+then inserted in the final `gcc` executable. All this code is located in one
+folder per target, under `gcc/config/$TARGET`, so the CFG is able to load the
+target we choose at compile time (using `--target=`) and insert that in the
+final executable.
+
+That is done in different ways depending on the type of file we are working
+with: Machine Description files are processed by the programs (`gencodes`,
+`gentags`...) that generate C code files from them, while target description
+macros and functions, which are C files, are inserted in the building process
+as any other C file.
+
+I'd like to insist here on the fact that the `--target` is the only one to be
+processed and loaded and the other possible targets are going to be ignored.
+The build process is not going to complain if a target is broken or anything
+like that if it isn't the target we chose. It just doesn't care.
+
+#### Machine Description files {#md}
+
+Machine Description files (`.md` extension) let us define `insn` patterns,
+which are incomplete RTL expressions that can be matched against the `insn`
+list generated from the GIMPLE, `attributes` and other interesting things we
+may not try to decipher here.
+
+`define_insn` is a RTX we can use to define new `insn` patterns. It receives
+four or five operands:
+
+1. An optional name. It's going to be used to match against GIMPLE.
+2. An RTL template. A vector of *incomplete* RTL expressions which describe how
+ should the instruction look like. *Incomplete* in this context means it uses
+ expressions like `match_operand` or `match_operator` which are designed to
+ match against the RTL `insn` list and see if they are compatible or not.
+3. A condition. A final condition to say if the `insn` matches this pattern or
+ not.
+4. An output template. A string that contains the output assembly code for this
+ `insn`. The string can contain special characters like `%` to define where
+ should the arguments be inserted. If the output is very complex we can write
+ C code on this field too.
+5. An optional list of attributes.
+
+This is an actual example from the RISC-V code we are backporting:
+
+``` lisp
+(define_insn "adddi3"
+ [(set (match_operand:DI 0 "register_operand" "=r,r")
+ (plus:DI (match_operand:DI 1 "register_operand" "r,r")
+ (match_operand:DI 2 "arith_operand" "r,I")))]
+ "TARGET_64BIT"
+ "add\t%0,%1,%2"
+ [(set_attr "type" "arith")
+ (set_attr "mode" "DI")])
+```
+
+You can see the name `adddi3` is something like: `add` + `di` + `3`. This means
+it's the `add` instruction with the `di` mode and `3` input arguments. That's
+the way things are named.
+
+Next block is a vector with the RTL template. If you try to ignore
+`match_operand` expressions you can see the template is not very different to
+the RTL example we gave before. In this case it's something like:
+
+``` lisp
+(set (reg 0)
+ (plus (reg 1)
+ (reg 2)))
+```
+
+It's basically storing in the first register the result of the addition of the
+other two.
+
+The next field is the condition. In this case it needs to have `TARGET_64BIT`
+defined in order to work because the machine mode is `DI` (we'll explain that
+soon).
+
+The output code is simple, just a RISC-V `add` instruction:
+
+``` asm
+add %0,%1,%2
+```
+
+Where `%N` is going to be replaced by the register numbers used as arguments
+for this instruction.
+
+The last field are the attributes, which can be used to define the instruction
+size and other kind of things. We are not going to focus on them today.
+
+##### Machine modes {#mm}
+
+Machine modes are a way to describe the size of a data object and its
+representation.
+
+- QI: quarter integer
+- HI: half integer
+- SI: single integer
+- DI: double integer
+- SF: single floating
+- DF: double floating
+
+And so on.
+
+The standard `insn` names include machine modes to describe what kind of
+instruction they are. The example above is `addddi3`, meaning it uses `di`
+machine mode: double integer. That's why it needs the target to be a 64 bit
+RISC-V machine.
+
+Machine modes also appear in some RTL expressions like `plus` or
+`match_operand` meaning that they operate in that machine mode, that is, with
+that data size and representation. For example `(plus:SI ...)`.
+
+##### RTL Templates {#rtl-templates}
+
+`match_*` expressions are what make RTL expressions *incomplete*, because they
+are designed to be compared against the `insn` list that comes from the
+previous step.
+
+In the example above we had:
+
+``` lisp
+(set (match_operand:DI 0 "register_operand" "=r,r")
+ (plus:DI (match_operand:DI 1 "register_operand" "r,r")
+ (match_operand:DI 2 "arith_operand" "r,I")))
+```
+
+`(match_operand N predicate constraint)` is a placeholder for an operand number
+`N` of the `insn`. When the `insn` is constructed, the `match_operand` will be
+replaced by the corresponding operand of the `insn`. When the template is
+trying to match an `insn` the `match_operand` forces the operand number `N` to
+match the `predicate` in order to make the `insn` match the template.
+The `match_*` expressions are what defines how `insn`s should be.
+
+The `predicate` is a function name to be called. The function receives two
+input arguments: an expression and a machine mode. If the function returns `0`
+the function does not match.
+
+`predicate`s can also be combined in Machine Description files like this:
+
+``` lisp
+(define_predicate "arith_operand"
+ (ior (match_operand 0 "const_arith_operand")
+ (match_operand 0 "register_operand")))
+```
+
+So the `arith_operand` shown in the example above can be a
+`const_arith_operand` *or* (that's what `ior` means) a `register_operand`.
+They can be more complex but this is more than enough to understand how they
+are built. In the end, they always check against C functions, but you can
+combine them with the convenience of the Machine Description files.
+
+The `constraint` allows to fine-tune the matching. They define if the argument
+is in a register or the memory and stuff like that. `r`, for example, means the
+operand comes from a register.
+
+
+
+There are other matching expressions too, but `match_operand` is the most used
+one and it's the one that explains this concept of *incomplete* expressions the
+best.
+
+
+
+#### Target description macros and functions {#target-desc}
+
+Apart from the machine descriptor files, there are other files involved. For
+example, the constraints defined above need to be defined in code somewhere.
+
+The most important of these are the target description macros and functions,
+normally defined in `gcc/config/$TARGET/$TARGET?(.h|.c)`. The `.c` should
+initialize the `targetm` variable, which contains all the machine information
+relevant to the compiler. It is initialized like this:
+
+``` c
+struct gcc_target targetm = TARGET_INITIALIZER;
+```
+
+That `TARGET_INITIALIZER` is a huge macro, defined in `gcc/target.h`, that
+initializes the `targetm` structure. This macro is split in smaller macros with
+reasonable defaults that may be overwritten by pieces. Each target should have
+a file that includes both `target.h` and `target-def.h` and overwrites any
+inappropriate default by redefining new macros and ends with the initialization
+line we just introduced. This is normally done in
+`gcc/config/$TARGET/$TARGET.c`, while the `.h` is normally used to define some
+macros that are needed in the `.c` file.
+
+As a reference, the RISC-V code we need to backport (see
+`gcc/config/riscv/riscv.c`) uses the file to introduce the amount of registers,
+the type, the size, and that kind of things, and many others.
+
+All this information contained in `targetm` is used by the compiler to decide
+how registers have to be allocated, which ones have preference, the cost of
+them, and many other things.
+
+### Assembly code generation {#assembly}
+
+Having the previous step clear is enough to understand how does the assembly
+generation work. Each of the `insn`s in the list obtained from GIMPLE is going
+to be compared against the RTL templates and the best match is going to be
+chosen. Once the match is chosen, the corresponding assembly is going to be
+generated from the corresponding field of the `define_insn` RTL expression.
+
+As simple as that, but also that complex.
+
+Why do I say it's complex? Because many things have to be considered and GCC
+does consider them. Each instruction has a size, that has to be considered to
+calculate addresses, but also they have some execution time associated and GCC
+calculates the best matches to make the final assembly file as optimum as
+possible.
+
+The RTL step has a lot of optimization passes, too. It's a complex step but
+it's not really important for us because we just need to make a temporary
+compiler that lets us compile a better one. It doesn't really matter if it's
+not perfect, at least at this point.
+
+
+### Summary {#summary}
+
+So, in summary, the process is the following:
+
+1. The HLL language is parsed to a tree, normally GENERIC.
+2. GENERIC is converted to GIMPLE.
+3. GIMPLE optimizations are applied.
+4. GIMPLE is matched to a `insn` list using pattern names.
+6. The `insn` list is matched against the RTL templates defined in the Machine
+ Description files.
+7. RTL optimizations are applied.
+8. The matches convert the RTL to assembly code also taking in account the
+ information obtained from the target definition macros and functions.
+
+From our perspective, the most important things to remember are these:
+
+- The front-end is not very relevant for us, from the parsing to GIMPLE we can
+ ignore for the moment.
+- RTL step is pretty complex, and the GIMPLE->RTL conversion is too.
+- GCC is a compiler collection that has a very powerful compilation process,
+ the Compiler Generator Framework (CFG), in order to modularize the code and
+ make it easier to port.
+- The machine description files and the target definition macros and functions
+ are designed to make the porting process simpler. Those are the only files we
+ need to touch.
+
+---
+
+<div style="
+ text-align: center;
+ font-size: smaller;
+ padding-left: 3em;
+ padding-right: 3em;
+ padding-top: 1em;
+ padding-bottom: 1em;
+ border-top: 1px solid var(--border-color);
+ border-bottom: 1px solid var(--border-color)">
+If you like my job here, consider hiring <a href="https://elenq.tech">ElenQ
+Technology</a>. <br> Even if I'm busy with this, I still have some time slots
+available.
+</div>
+
+---
+
+
+### My job in the backport {#job}
+
+With all the process more or less clear, we can be more specific on the job I
+need to do. I share some specifics in this section, so if you like reading code
+you are going to have some fun[^examples].
+
+[^examples]: I'll link to some examples of the code on RISC-V's GitHub account.
+ This code is already merged in GCC.
+
+First, I need to make sure all the used RTL expressions are compatible with the
+old version of the compiler. If they are not, I have to translate them to the
+old way to make them. Some examples of this are iterators like
+[`(define_int_iterator ...)`][int-iterator] which is not available in old GCC
+versions, so I need to unfold a couple of loops by hand and make them only use
+the old constructs.
+
+[int-iterator]: https://github.com/riscv-collab/riscv-gcc/blob/ca312387ab141060c20c388d83d6fc4b2099af1d/gcc/config/riscv/riscv.md?plain=1#L342
+
+Second, I need to convert the target description macros and functions to the
+old internal C-based API instead of the more [modern C++-based one][c++api], as
+the recent port uses. These changes involve many layers and I didn't yet
+analyze this in detail. They can be simple like converting from `rtx_insn`
+class to `rtx`, the older way to do this. But they can also be complex, like
+removing the 40% of the `#include` directives from `riscv.c`, which has many
+that were not available in the past. It's going to be a lot of fun I predict.
+
+[c++api]: https://github.com/riscv-collab/riscv-gcc/blob/ca312387ab141060c20c388d83d6fc4b2099af1d/gcc/config/riscv/riscv.c
+
+Third, as this whole compilation process is complex, I decided to make it as
+accessible as possible, so other people can audit and replicate my work. For
+that I'm using Guix, my package manager of choice. I added a [`guix.scm`][guix]
+and [`channels.scm`][channels] file to the repository so my work can be
+replicated precisely by myself in the future, or by others[^github].
+
+The Guix package also provides a better interaction with the building process
+of GCC, letting us replace inputs in a very simple way. I'm thinking about the
+next steps of the project here, when we need to compile my backported compiler
+with TinyCC, test if it works and then patch TinyCC until it does. Having the
+`guix.scm` file makes it easy to replace the current compiler with a patched
+TinyCC and ensures nothing is interfering in the compilation process because
+the compilation is done in an isolated container.
+
+[^github]: I'm hosting this on GitHub at the moment because the repository is
+ huge. I'll probably move all this to my server and edit the post after that.
+
+[guix]: https://github.com/ekaitz-zarraga/gcc/blob/guix_package/guix.scm
+[channels]: https://github.com/ekaitz-zarraga/gcc/blob/guix_package/channels.scm
+
+That's mostly the job I need to do in the backport.
+
+Something to keep in mind is that we don't need to make it perfect, we just
+need it to work. The backported GCC is not going to be used as a production
+compiler, but just as a bridge with the next GCC version, so **there's only one
+program it needs to be able to compile correctly: GCC 7**. Once we make the
+bridge with the next version, we can use that to compile anything we want.
+
+
+### Last words
+
+I know this post is long, and the lack proper diagrams make everything a little
+bit hard to understand. That's exactly how I felt reading about GCC, but the
+difference was I had to read some documentation that is... About 100 times
+longer than this post (see [Learn more](#more) below). Not that bad after all.
+
+There are many things I decided to leave out, like peephole optimizations,
+instruction attributes, and some other constructs that are not that important
+from my perspective. You may want to make your research on those on your own.
+
+In any case, if you have any question you can always contact me[^contact] and
+ask me any questions you have or send me some words of support.
+
+In the next post I'll describe a little bit about ELF, the executable and
+linkable format, just the bare minimum to understand the format, as it will be
+relevant for us in the future. And you might be thinking, why is it relevant if
+GCC compiles to assembly? Well, that's one of the questions that we will be
+answering in the next post.
+
+Now I leave you with a couple of interesting links on the next section.
+
+Good luck with your ports!
+
+
+[^contact]: You can find my contact info in the [About
+ page](/pages/about.html).
+
+
+
+### Learn more {#more}
+
+- [The GCC internals documentation](https://gcc.gnu.org/onlinedocs/gccint/): if
+ you are interested on my work you should read an older version of the
+ documentation. See [Disclaimer](#disclaimer).
+- [The GCC source code](https://gcc.gnu.org/git.html): of course, this has
+ everything you need to understand GCC, but the problem is that GCC is a huge
+ codebase, you probably need to spend months reading it in order to understand
+ everything. That's why I think posts like this one are interesting, they help
+ you focus on the parts you are interested in.
+
diff --git a/content/bootstrapGcc/02_elf.md b/content/bootstrapGcc/02_elf.md
new file mode 100644
index 0000000..c684c63
--- /dev/null
+++ b/content/bootstrapGcc/02_elf.md
@@ -0,0 +1,595 @@
+Title: ELF format — why not?
+Date: 2022-03-14
+Category:
+Tags: Bootstrapping GCC in RISC-V
+Slug: bootstrapGcc2
+Lang: en
+Summary:
+ Some introduction to ELF as we'll need to deal with this in the future.
+
+In the [previous post]({filename}01_internals.md) of the
+[series]({tag}Bootstrapping GCC in RISC-V) we introduced GCC and how it
+generates assembly code and we left a question unanswered: *"Why is learning
+about ELF interesting if GCC generates assembly?"*. In this post we are going
+to answer that question (not interesting) and maybe understand the very basics
+of ELF file format (more interesting).
+
+
+### What's ELF
+
+ELF is a file format with two main goals:
+
+- Represent an executable file
+- Represent a linkable file
+
+Apart from that, ELF can also represent core dumps, but if you think about that
+all of the possible options have something in common: they represent contents
+on the memory. We can simply say ELF is a file format that acts as a picture of
+the state of the memory. In the case of the executables, the state will be
+loaded from the file, but in the case of the core dumps the state is obtained
+from the memory and dumped in a file.
+
+Linkable files are those files that can be combined with others to generate
+executables or shared objects, so they can also fit that definition because
+they are going to end up in the memory anyway.
+
+For efficiency reasons, the ELF format has two separate views of the same
+contents:
+
+- The **Linking** view is based on sections and needs a *section header*.
+- The **Executable** view is based on segments and needs a *program header*.
+
+#### ELF header
+
+The ELF header is the only thing that has a fixed position in the file, at the
+beginning. The ELF header has information that defines how to identify the
+file, the machine, the endianness and that sort of things, but it also says
+where are the headers located and identifies the size of their entries and
+their entry count.
+
+It's not that interesting, honestly. The most important thing is it points to
+the descriptions to both of the views (the headers) so we can check them.
+
+#### Linking view
+
+Based on sections, the linking view is the most detailed view of the file and
+it defines how the file should be linked with others in order to create an
+executable file.
+
+Sections, the basic unit of the linking view, are consecutive sequences of
+bytes that do not overlap.
+
+There are [different types of sections according to their possible contents and
+meaning](https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-PDA/LSB-PDA.junk/sections.html),
+the most interesting are:
+
+- `SYMTAB` and `DYNSYM` that hold a symbol table. The `DYNSYM` is for dynamic
+ linking symbols, while `SYMTAB` normally is used for static linking but may
+ contain both.
+- `STRTAB` holds a string table.
+- `RELA` contains relocation entries with addends and `REL` contains
+ relocations without addends.
+- `NOTE` section contains some information of the file.
+- `HASH` contains a symbol hash table, necessary for dynamic linking.
+- `DYNAMIC` for dynamic linking information.
+
+Each section has also a `name`, an `address` if it is supposed to appear in the
+memory of running process, an `offset` that defines where in the file do the
+section's contents appear, a `size`, and some extra data fields that all
+together form a section header entry.
+
+The section header entries are all located where the ELF header says, one after
+the other (like a C array of structures), so the programs just need to access
+that position in the file and read all the headers in a row. The contents of
+the sections are located throughout the file, where the section headers point.
+
+##### String section
+
+The string section (`STRTAB`) is one of the simplest. It contains all the
+strings of the file: the section and symbol names. It's simply a set of null
+terminated strings, written one after the other (it also starts with a null
+character but whatever).
+
+Anywhere in the file where we are supposed to get an string what we get is an
+index that points to the first position in this section to read from. We should
+read from that until we reach a null character. For example in the following
+string section:
+
+```
+ \0 h e l l o \0 n a m e \0
+```
+
+If a name of a section says `1`, the actual name of the section is `hello`
+and if it says `7` it would be `name`. Also, if it says `9` it would be `me`,
+this trick could be used too.
+
+##### Symbol table
+
+The symbol table contains information needed to locate and relocate a program's
+symbolic definitions and references. The symbol table is formed as an array of
+symbol elements that are defined with a `name`, obviously a `value`, their
+`size`, some extra `info`, the index of the section header they relate to
+(`shndx`) and some `other` stuff.
+
+The `info` field manages symbol's type (`OBJECT` for data, `FUNC` for
+function...) and binding attributes, which define the linking visibility and
+behavior of the symbol (local vs global...).
+
+The `value` can be interpreted in several ways too, depending on the type of
+the symbol you are dealing with. But that's not really relevant for us at the
+moment.
+
+##### Relocation
+
+According to the ELF documentation I got from somewhere I don't really
+remember:
+
+> The relocation is the process of connecting symbolic references with symbolic
+> definitions.
+
+I hope it's more explanatory for you than what it is to me, but I don't have a
+clue of what that is supposed to mean. The
+[Wikipedia](https://en.wikipedia.org/wiki/Relocation_(computing)) does a **much
+better** job in the specifics right here:
+
+> Relocation is the process of assigning load addresses for position-dependent
+> code and data of a program and adjusting the code and data to reflect the
+> assigned addresses.
+
+If this doesn't really help, you have a really good example later, but we can
+basically say that it's a way to adjust the code to point to the correct
+addresses, at linking or loading, or even execution, time.
+
+ELF files have, as we said, sections that let us define relocations. These will
+point to some parts of the file and tell the linker or the loader that that
+positions of the file must be reprocessed.
+
+There are two types of relocation sections and in both of them the relocation
+section is an array of entries where each of them represents one relocation.
+In the simple one (`REL`) each relocation only contains an `offset` and an
+`info` word, which also includes the type of relocation to apply. The more
+complex one (`RELA`) is mostly the same but it includes an `addend` which
+includes a constant value to use in calculation of the relocation.
+
+The calculus of the final addresses are specific to the ISA and the relocation
+type, because processors have different instruction formats and different ways
+to pack addresses in instructions. RISC-V has no way to pack a full address
+inside of an instruction, while x86 does, so they have to patch the
+instructions in a different way.
+
+
+##### Special sections
+
+Some sections have a special treatment according to their name, normally the
+ones that start with a dot. These you might have found in the past in assembly
+files, defined like `.data` (for data), `.rodata` (for read only data) or
+`.text` (for code).
+
+These are interesting to have in mind because they appear the same way they do
+in assembly, and we are going to disassemble some of them and play around with
+them.
+
+Other special sections like `.got` or `.dynamic` don't appear in assembly but
+they have a strong meaning in the resulting file, we are not going to deal with
+those today because we want to finish this post someday. If you need to deal
+with those I recommend you to read ELF's documentation on special sections and
+the loading process.
+
+
+#### Executable view
+
+The executable view is another way to access the same contents, but with a
+different perspective. It's based on *segments* rather than *sections*.
+Segments are also pieces of the file, as sections are, but segments can contain
+one or more sections.
+
+Like in the linking view, the base unit, sections for the linking view but for
+segments for the executable view, are described in a header. The header of the
+executable view is called program header and it is, like the section header, a
+bunch of structures piled together, each describing one of the segments.
+
+The program header describes the position and size in the file of each of the
+segments but also some important information about them: how they are supposed
+to be loaded in the memory and where (virtual address and physical address),
+the type of the segment, and some info more.
+
+The most interesting segment types are the following:
+
+- `LOAD` is used for loadable segments, with the other fields of the segment
+ the position and the size this segment will have in memory are described.
+- `DYNAMIC` are segments that have some dynamic linking information. It has to
+ contain the `.dynamic` section.
+- `INTERP` gives the location and size of a null-terminated path name to invoke
+ as an *interpreter*. Interpreter in this context usually means a dynamic
+ linker, which will be called instead of loading this file to memory and the
+ dynamic linker will be the one that will load the parts of the file it
+ considers.
+
+You can see how segments are interesting for loading the file in the memory,
+that is, they are mostly interesting for executable files or shared objects.
+
+#### Segments vs Sections
+
+If you want to have a clear idea about the difference between segments and
+sections, you can consider a file with multiple sections: `.text`, `.rodata`
+and `.data`.
+
+A file that contains those sections can be understood from a linking
+perspective as a file that has some code (`.text`), read-only data (`.rodata`)
+and read-write data (`.data`). Each of those parts must be managed in a
+different way by the linker, but the reality is that the program loader doesn't
+really care about some of the differences of them.
+
+The code and the read-only data are loaded in the memory in the same way, with
+read and execute permission but no write permission, so the executable view can
+put both sections in the same segment, and make the loader's life easier.
+
+Also, the linker doesn't really care about how is the memory loaded so the
+section header does not hold that information. It does care about the section's
+goals though, as it will need to put them together in order during the linking.
+On the other hand, the loader is not really interested on what's the goal of
+the contents of the file but only on what to do with those contents, so it only
+has that information.
+
+
+### So, why do we need to learn it?
+
+We don't really need to learn it very deeply, just learn how it works in a
+high-level way and make sure we are able to read it with the tools we have
+available. The good news for you is if the reasons I give you are not good
+enough it doesn't really matter because you already learned[^gotcha]. Continue
+reading and you'll realize how much you understand now.
+
+[^gotcha]: Ha! Gotcha!
+
+First, let me tell you a personal story. I have previous experience working
+with assembly, but only in small devices that have two memories, one for data
+and other for code (Hardvard Architecture). In those small devices you often
+don't really need to think about how the code and the data is mapped to memory
+because your programs are small and the separation is clear. Computers are a
+different thing, and I have had issues understanding this whole assembly thing.
+
+Computers store both code and data in the same memory, the main memory, (Von
+Neumann Architecture) and they normally have memory segmentation, pagination,
+memory management units and all that kind of stuff, because there are many
+processes running and they want to separate one from the other. That forces us
+to think about how the code and the data are mapped to the memory. Also, modern
+operating systems also use dynamic linkers, which are not available in small
+devices, and we need to be able to deal with that amount of complexity.
+
+ELF allows us to make that all, because it was born for that. ELF is a
+distillation of many of the ideas from System V Unix, that include exactly all
+I mentioned. It's a great way to understand how memory, linking and processes
+work in a *modern* operating system. This is why you need to learn it, at least
+a little. It makes you a cultivated person, which is always good[^system-v].
+
+[^system-v]: It also makes you understand the complexities of the system so you
+ can criticize it. Changing the world requires to learn about it first.
+
+#### The specifics
+
+As I'm sure you are not satisfied totally with the answer of being a cultivated
+person[^some-of-you], let me go for some specifics.
+
+[^some-of-you]: For those that really are. That's the good attitude in life.
+ High five. You can read the whole section still, it has interesting points I
+ think.
+
+So in this project GCC is not the only software we are dealing with, GNU
+Binutils and TinyCC are part of the party too, and I need to make them fit
+together in the best way possible. In those I need to make sure the
+relocations, formats and other things work properly, following the RISC-V ABI
+specification for ELF. That might be a point of failure, so being prepared on a
+high-level at least is interesting.
+
+Of course, GCC's output we need to analyze too, and in order to do that we need
+to make sure we know what it means. We already saw that some ELF sections are
+directly mentioned in the assembly, so in order to know their meanings ELF is a
+good way to understand them. They are really an OS related thing and ELF only
+reflects it, but learning them from the ELF perspective makes the path easier
+probably.
+
+Relocations are a huge point in all this mess, because they are machine
+specific (instructions are too, but those I expect us to know already), and
+they are something I didn't need to research on all the RISC-V adventures I had
+last year. I have to do it sometime.
+
+In general, there are many sharp edges where we can get hurt, so it's better if
+we wear gloves.
+
+### Tools
+
+For all this process there are a couple of tools that were designed to help.
+GNU Binutils has many of them but we are going to focus on two, as they are
+more than enough for many usecases: `objdump` and `readelf`.
+
+The example below uses both of them to analyze a piece of code and its
+compilation result. As you'll see, the main problem they have is their output:
+it's not always clear, the formatting is a little bit chaotic, it's not
+obvious at all to get right and it's really hard to use it procedurally.
+
+There is a really cool tool you should investigate though, called GNU Poke,
+that is designed specifically to fight against those issues. I recommend you to
+[take a look to it](https://www.gnu.org/software/poke/).
+
+### Example
+
+Starting from a very simple C file we can follow a really interesting process
+and understand some of the ELF internals:
+
+``` c
+long global_symbol;
+
+int main() {
+ return global_symbol != 0;
+}
+```
+
+We compile it to assembly with:
+
+``` asdf
+$ riscv64-linux-gnu-gcc -S b.c -O0
+```
+
+This are the contents of the assembly file:
+
+``` asm
+ .file "b.c"
+ .option pic
+ .text
+ .globl global_symbol
+ .bss
+ .align 3
+ .type global_symbol, @object
+ .size global_symbol, 8
+global_symbol:
+ .zero 8
+ .text
+ .align 1
+ .globl main
+ .type main, @function
+main:
+ addi sp,sp,-16
+ sd s0,8(sp)
+ addi s0,sp,16
+ lla a5,global_symbol
+ ld a5,0(a5)
+ snez a5,a5
+ andi a5,a5,0xff
+ sext.w a5,a5
+ mv a0,a5
+ ld s0,8(sp)
+ addi sp,sp,16
+ jr ra
+ .size main, .-main
+ .ident "GCC: (Debian 10.2.1-6) 10.2.1 20210110"
+ .section .note.GNU-stack,"",@progbits
+```
+
+Assemble the file with `as`:
+
+``` asdf
+$ riscv64-linux-gnu-as b.s -o b.o
+```
+
+And this is what we get in `b.o`. The `.text` section contains the following:
+
+``` asm
+$ riscv64-linux-gnu-objdump --disassemble b.o
+
+b.o: file format elf64-littleriscv
+
+
+Disassembly of section .text:
+
+0000000000000000 <main>:
+ 0: ff010113 addi sp,sp,-16
+ 4: 00813423 sd s0,8(sp)
+ 8: 01010413 addi s0,sp,16
+ c: 00000797 auipc a5,0x0
+ 10: 00078793 mv a5,a5
+ 14: 0007b783 ld a5,0(a5) # c <main+0xc>
+ 18: 00f037b3 snez a5,a5
+ 1c: 0ff7f793 andi a5,a5,255
+ 20: 0007879b sext.w a5,a5
+ 24: 00078513 mv a0,a5
+ 28: 00813403 ld s0,8(sp)
+ 2c: 01010113 addi sp,sp,16
+ 30: 00008067 ret
+```
+
+### Relocations
+
+There are some relocations!
+
+``` asdf
+$ riscv64-linux-gnu-objdump b.o -r
+
+b.o: file format elf64-littleriscv
+
+RELOCATION RECORDS FOR [.text]:
+OFFSET TYPE VALUE
+000000000000000c R_RISCV_PCREL_HI20 global_symbol
+000000000000000c R_RISCV_RELAX *ABS*
+0000000000000010 R_RISCV_PCREL_LO12_I .L0
+0000000000000010 R_RISCV_RELAX *ABS*
+```
+
+But in order to understand those relocations properly we need to check the
+value of the symbols too:
+
+``` asdf
+$ riscv64-linux-gnu-objdump -t b.o
+
+b.o: file format elf64-littleriscv
+
+SYMBOL TABLE:
+0000000000000000 l df *ABS* 0000000000000000 b.c
+0000000000000000 l d .text 0000000000000000 .text
+0000000000000000 l d .data 0000000000000000 .data
+0000000000000000 l d .bss 0000000000000000 .bss
+0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack
+000000000000000c l .text 0000000000000000 .L0
+0000000000000000 l d .comment 0000000000000000 .comment
+0000000000000000 g O .bss 0000000000000008 global_symbol
+0000000000000000 g F .text 0000000000000034 main
+```
+
+If you pay attention to the offsets of those relocations (`0x0c` and `0x10`)
+they exactly match the instructions `auipc a5, 0x0` and `mv a5, a5` and those
+are expanded from the `lla a5, global_symbol` (load local address)
+pseudoinstruction from the assembly.
+
+The `mv` is not really a `mv`. `mv` is a pseudoinstruction too, that should be
+expanded to an `addi a5, a5, 0`. The `objdump` is playing with us, making the
+opposite conversion so we can read better but in fact is tricking us.
+
+The `auipc` + `addi` couple in RISC-V appears pretty often, because it's the
+method it has to load addresses in memory. The first instruction, `auipc` adds
+a high part of an immediate to the program counter and stores the result in a
+register, the `addi` adds then another, in this case low, immediate to the
+register i.e. they make a `x[reg] = pc + immediate` operation in two steps:
+`x[reg] = pc + hi20(immediate)` followed by `x[reg] = x[reg] + lo12(immediate)`.
+
+As we have relocations in both `auipc` and `addi` this means their `0` values
+(the immediates) are going to be overwritten with something else at linking
+time, and there's when RISC-V has something to say. All the relocations we can
+see are RISC-V specific, and you can read about them in [RISC-V ABI
+Specification](https://github.com/riscv-non-isa/riscv-elf-psabi-doc).
+
+In our case we have really some simple ones, the easiest to understand (what a
+coincidence, huh?):
+
+> `R_RISCV_PCREL_HI20`: High 20 bits of 32-bit PC-relative reference,
+> `%pcrel_hi(symbol)`. The formula is: `S+A-P` [but only obtains the highest 20
+> bits].
+
+> `R_RISCV_PCREL_LO12_I`: Low 12 bits of a 32-bit PC-relative,
+> `%pcrel_lo(address of %pcrel_hi)`, the addend must be 0. The formula is:
+> `S-P` [but it only obtains the lowest 12 bits].
+
+Both the `HI20` and the `LO12` have a similar formula, this is the meaning of
+the elements on the formula:
+
+- `S`: Address of the symbol
+- `A`: Addend of the relocation
+- `P`: Position of the relocation
+
+If you match their formulas with the description of what we just said about how
+do `auipc` + `addi` couples work, you can easily understand the formulas and
+their meaning. We are not going to do it, do something yourself!
+
+The other relocation:
+
+> `R_RISCV_RELAX`: Instruction can be relaxed, paired with a normal relocation
+> at the same address.
+
+Is an addition our example doesn't use but it could. The `R_RISCV_RELAX`
+basically means that if the relocation it points at is not needed it can be
+discarded. And when does that happen? Easy, when we can get `global_symbol`'s
+address with only one of them, we can remove the other instruction from the
+program.
+
+#### Relocation resolution
+
+If we link the file and generate an executable, we can see the final value
+those zeroes get.
+
+``` asdf
+$ riscv64-linux-gnu-gcc b.o -o b.out
+```
+We link it like this because `ld` needs a lot of input fields and we don't want
+to set them all by hand, but you can do it with `ld` if you feel like it.
+
+``` asdf
+$ riscv64-linux-gnu-objdump --disassemble b.out
+...
+00000000000005e4 <main>:
+ 5e4: ff010113 addi sp,sp,-16
+ 5e8: 00813423 sd s0,8(sp)
+ 5ec: 01010413 addi s0,sp,16
+ 5f0: 00002797 auipc a5,0x2
+ 5f4: a6878793 addi a5,a5,-1432 # 2058 <global_symbol>
+ 5f8: 0007b783 ld a5,0(a5)
+ 5fc: 00f037b3 snez a5,a5
+ 600: 0ff7f793 andi a5,a5,255
+ 604: 0007879b sext.w a5,a5
+ 608: 00078513 mv a0,a5
+ 60c: 00813403 ld s0,8(sp)
+ 610: 01010113 addi sp,sp,16
+ 614: 00008067 ret
+...
+```
+
+There you see the relocation was resolved (`0x5f0` and `0x5f4`) by the linker
+and the final values have been added. `objdump` is intelligent enough to tell
+us where are those instructions pointing (says `2058 <global_symbol>`). Just to
+make sure we can search in the symbol table for the `global_symbol`:
+
+``` asdf
+$ riscv64-linux-gnu-objdump -t b.out | grep global_symbol
+0000000000002058 g O .bss 0000000000000008 global_symbol
+```
+
+> NOTE: We could try to calculate the address of the `global_symbol` as the
+> linker did, but it's a little bit complicated because we also linked the file
+> with the standard library and the startup files, which adds the `crt` files
+> on top of the file. It's really that we get more code than what we had in the
+> assembly file. If you want to see that, you can see the rest of the output of
+> the command, or even try with `--disassemble-all` and calculate the symbol
+> address by hand. Good luck.
+
+#### More sections
+
+If you want the review some simple things, like a string section, you can use
+`readelf` for that. The `-p` flag (equivalent to `--string-dump=`) displays the
+contents of the section as strings. You can read the `.comment` section that
+way:
+
+``` asdf
+$ riscv64-linux-gnu-readelf -p .comment b.o
+
+String dump of section '.comment':
+ [ 1] GCC: (Debian 10.2.1-6) 10.2.1 20210110
+```
+
+This is what we had inserted in `.ident` on the assembly file by the compiler.
+We have it in the binary too.
+
+In other distros the output is a little bit different. Look the output we have
+in Guix:
+
+``` asdf
+String dump of section '.comment':
+ [ 1] GCC: (GNU) 11.2.0
+```
+
+### Conclusion
+
+So this whole this just to explain that ELF files are some kind of dual files
+that have two different goals at the same time. The executable one is kind of a
+picture of the memory state that can be used for loading that state in the
+memory, while the linking one just describes how different parts of the
+contents relate to each other and has tons of funny tricks to make the files
+relocatable, position independent and that kind of things. Cool.
+
+There are still many fields of ELF we didn't talk about but I consider this
+introduction more than enough. Having a simple understanding about how is the
+file organized and what kind of information it has is probably enough for the
+things we are going to need.
+
+The proposed example shows that with the knowledge obtained by this short
+introduction we can dig a little bit on the files that result from a
+compilation and analyze their internals. That's mostly the work I'll need to do
+when I start combining compilers in a pipeline of death and destruction.
+
+If I ever need to dig on something deeper, I'll do.
+
+Anyway, I'm still unsure if I answered the question we left in the previous
+post[^cliff]:
+
+> Why is learning about ELF interesting if GCC generates assembly?
+
+Did I?
+
+[^cliff]: It was a good cliffhanger, though.
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