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+---
+title: "Bringing RISC-V to Guix's bootstrap"
+
+subtitle: What's done and what we need to do
+
+license: CC-BY-SA
+author: Ekaitz Zárraga
+links-as-notes: true
+lang: spanish
+polyglossia-lang:
+    name: english
+how-to: pandoc -f markdown+smart -t beamer contents.md -o beamer.pdf --pdf-engine=xelatex --template=template.tex
+header-includes:
+    - \usepackage{multicol}
+    - \newcommand{\hideFromPandoc}[1]{#1}
+    - \hideFromPandoc{
+        \let\Begin\begin
+        \let\End\end
+      }
+...
+
+## Who I am
+- Telecommunication engineer (EEE equivalent)
+- Freelance engineer/programmer at [ElenQ.Tech](https://elenq.tech)
+- Guix user and contributor
+- You might remember me from my talk last year:
+  *"A year of RISC-V adventures: embracing chaos in your software journey"*
+
+
+## Intro
+
+- Last year I asked [NlNet for a grant][nlnet-grant].
+- I wanted to push the bootstrapping effort for RISC-V, and they funded me to
+  do so.
+- In this talk I'm going to introduce what I did[^1], what I think it's more or
+  less done and what's missing.
+
+[nlnet-grant]: https://nlnet.nl/project/GNUMes-RISCV/index.html
+
+[^1]: Read the longer version here: <https://ekaitz.elenq.tech/tag/bootstrapping-gcc-in-risc-v.html>
+
+# Intro to bootstrapping
+
+## Free software is not enough
+
+We love Free Software because it helps us audit our programs.
+
+![&nbsp;](img/Source-Binary.svg){width=200px}
+
+But do we know if the source code we read actually maps to the binary we are
+executing? **Not really**
+
+## Reproducibility
+
+![The relation is one-way: the compiler is in the middle](img/Source-Binary-Compiler.svg){width=200px}
+
+In Guix we have **reproducibility**, so we can make sure some inputs (the
+source, the compiler and the environment where it runs) always produce the same
+outputs.
+
+**We can challenge the binaries**, so nobody will give us a malicious binary.
+
+## Trusting trust
+
+![But what if the bad actor is not a person but a program?](img/Sneaky-Compiler.svg){width=200px}
+
+## Trusting trust
+
+![But what if the bad actor is not a person but a program?](img/Sneaky-Compiler.svg){width=160px}
+
+Reproducibility here will only make sure we generate the same **corrupt**
+binary.
+
+> This kind of attack [can be done in real life][ken-trust].
+
+[ken-trust]: https://niconiconi.neocities.org/posts/ken-thompson-really-did-launch-his-trusting-trust-trojan-attack-in-real-life/
+
+## Recursive problem, recursive solution
+
+The compiler is a program too. **This issue is recursive**: a corrupt compiler
+could corrupt it's output compiler!
+
+![What's the exit point?](img/Bootstrapping.svg){width=300px}
+
+## Recursive problem, recursive solution
+
+![What's the exit point?](img/Bootstrapping.svg){width=200px}
+
+If we could have a compiler that we can make sure its output is not corrupt
+(**but how?**), we could make sure all the chain is correct.
+
+## In practice
+
+GNU+Linux distributions often rely in many prebuilt binaries: Bash, GCC,
+Coreutils, Python...
+
+Some distributions like Guix are interested on reducing the amount of binaries
+they have to trust.
+
+We can compile most of **The World** from source using a powerful compiler (GCC
+FTW). But we can't use a pre-built compiler (remember the previous slides?)
+
+**The key**: Who is compiling the compiler?
+
+## In practice - II
+
+Let's try with GCC:
+
+0. The World (requires a modern GCC)
+1. Modern GCC (requires ISO C++11 compiler)
+
+## In practice - II
+
+Let's try with GCC:
+
+0. The World (requires a modern GCC)
+1. Modern GCC (requires ISO C++11 compiler)
+2. GCC 11 (requires ISO C++98 compiler)
+
+## In practice - II
+
+Let's try with GCC:
+
+0. The World (requires a modern GCC)
+1. Modern GCC (requires ISO C++11 compiler)
+2. GCC 11 (requires ISO C++98 compiler)
+3. GCC 4.8 (requires ISO C89 compiler)
+
+## In practice - II
+
+Let's try with GCC:
+
+0. The World (requires a modern GCC)
+1. Modern GCC (requires ISO C++11 compiler)
+2. GCC 11 (requires ISO C++98 compiler)
+3. GCC 4.8 (requires ISO C89 compiler)
+4. GCC 3.4 (requires K&R compiler)
+
+## In practice - II
+
+Let's try with GCC:
+
+0. The World (requires a modern GCC)
+1. Modern GCC (requires ISO C++11 compiler)
+2. GCC 11 (requires ISO C++98 compiler)
+3. GCC 4.8 (requires ISO C89 compiler)
+4. GCC 3.4 (requires K&R compiler)
+5. ...
+
+## In practice - II
+
+Let's try with GCC:
+
+0. The World (requires a modern GCC)
+1. Modern GCC (requires ISO C++11 compiler)
+2. GCC 11 (requires ISO C++98 compiler)
+3. GCC 4.8 (requires ISO C89 compiler)
+4. GCC 3.4 (requires K&R compiler)
+5. ...
+
+*I didn't mention libraries here, that's also a lot of fun*
+
+## Guix's bootstrapping
+
+0. The World
+1. Modern GCC
+2. GCC 7.5
+3. GCC 4.6.4
+4. GCC 2.95
+5. TinyCC
+    - Bootstrappable TinyCC
+6. GNU Mes
+7. Stage0-POSIX => **SOURCE CODE**
+
+## GNU Mes
+
+> GNU Mes is a Scheme interpreter and C compiler for bootstrapping the GNU
+> System. Since version 0.22 it has again helped to halve the size of opaque,
+> uninspectable binary seeds that are currently being used in the Further
+> Reduced Binary Seed bootstrap of GNU Guix. **The final goal is to help create
+> a full-source bootstrap as part of the bootstrappable builds effort for
+> UNIX-like operating systems**.
+
+> The Scheme interpreter is written in ~5,000 LOC of simple C, and the C
+> compiler written in Scheme and these are mutual self-hosting. Mes can now be
+> bootstrapped from M2-Planet and Mescc-Tools.
+
+<https://www.gnu.org/software/mes/>
+
+
+## Stage0-POSIX
+
+> It bootstraps all these from a single 256 byte seed (which you will find in
+> the folder bootstrap-seeds). **The ultimate goal is for this to bootstrap all
+> the way up to GCC**.
+
+> There is only one "missing" part that is not bootstrappable from the hex0
+> seed: a kernel. This issue is not yet solved and at the moment the kernel is
+> trusted.
+
+<https://github.com/oriansj/stage0-posix>
+
+
+## Boostrapping - wrapping up
+
+![&nbsp;](img/meme.jpg)
+
+
+# RISC-V support
+
+## Guix's bootstrapping - RISC-V support
+
+\Begin{multicols}{2}
+
+0. The World
+1. Modern GCC
+2. GCC 7.5
+3. GCC 4.6.4
+4. GCC 2.95
+5. TinyCC
+    - Bootstrappable TinyCC
+6. GNU Mes
+7. Stage0-POSIX
+
+\columnbreak
+
+- N/A
+- YES
+- YES
+- NO
+- NO
+- YES
+    - NO
+- PARTIAL
+- YES
+
+\End{multicols}
+
+
+## Guix's bootstrapping - RISC-V support *SPOILER*
+
+\Begin{multicols}{2}
+
+0. The World
+1. Modern GCC
+2. GCC 7.5
+3. GCC 4.6.4
+4. GCC 2.95
+5. TinyCC
+    - Bootstrappable TinyCC
+6. GNU Mes
+7. Stage0-POSIX
+
+\columnbreak
+
+- N/A
+- YES
+- YES
+- ~~NO~~ *I backported this*
+- ~~NO~~ *We will remove it*
+- YES
+    - ~~NO~~ *I backported this*
+- PARTIAL
+- YES *I made some of this*
+
+\End{multicols}
+
+# What I did
+
+## GCC
+
+GCC uses a Davidson-Fraser model. Meaning that it uses an intermediate language
+that is machine dependant: RTL (Register Transfer Language).
+
+```
+HLL -> GIMPLE -> RTL -> OPTIMIZATIONS -> RTL -> ASSEMBLY
+```
+
+GCC is only a coordinator: it calls `as` and `ld` from binutils as the
+assembler and linker.
+
+- `GIMPLE -> RTL`: is done using identifiers. The GIMPLE nodes match insn
+  identifiers.
+
+- `RTL -> OPTIMIZATIONS`: RTL matches the RTL templates we write in the
+  backend part of GCC. Those can be expanded to other RTL expressions.
+
+- `RTL -> ASSEMBLY`: The expanded RTL expressions are matched against RTL
+  templates that also describe their equivalent in assembly and assembly is
+  generated from them.
+
+## GCC
+
+RTL templates are written in LISP in machine descriptor files (`*.md`), they
+look like this:
+
+\small
+``` lisp
+(define_insn
+  "adddi3"          ;; Identifier
+
+  ;; The behavior of the instruction
+  [(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"    ;; Predicate to test
+  "add\t%0,%1,%2"   ;; Assembly output template
+
+  ;; Attributes
+  [(set_attr "type" "arith")
+   (set_attr "mode" "DI")])
+```
+\normalsize
+
+## GCC
+
+Apart from that GCC needs tons of other definitions in order to get another
+target:
+
+- Target description macros and functions
+- Libraries like libgcc and many others
+
+## GCC - What I did
+
+Cherry picked the RISC-V support from GCC 7.5 to GCC 4.6.4
+
+1. There were missing insns => Used older ones that were equivalent.
+2. Some RTL constructs (`int_iterator`) didn't exist in 4.6.4 => Expanded the
+   iterator by hand.
+3. There were missing predicates => Copied them.
+4. The internal GCC API moved from C to C++ in the meantime => I had to convert
+   the code from using a class to the older interface.
+5. Memory barriers didn't exist back then => Always introduce a `fence` to make
+   sure code is correct.
+6. `libgcc` is a mess => Play around until it works
+
+**TL;DR**: Touch everything until it works.
+
+## GCC - What I did
+
+Finally I managed to make a GCC 4.6.4 that is able to generate RISC-V binary.
+
+See the blog for a more detailed description of the changes:
+
+- <https://ekaitz.elenq.tech/bootstrapGcc3.html>
+- <https://ekaitz.elenq.tech/bootstrapGcc4.html>
+
+
+## Bootstrappable TinyCC
+
+TinyCC has RISC-V support but it's not boostrappable using GNU Mes.
+
+The bootstrappable fork of TinyCC GNU Mes uses is old => Backport again.
+
+## Bootstrappable TinyCC - What I did
+
+Copy the relevant files from the upstream TinyCC and:
+
+0. Prepare a reproducible way to build the Bootstrappable TinyCC.
+1. Just read the code and make it match.  
+   **SURPRISE**: The code is really hard to read... But I eventually managed to
+   make it work.
+2. Some core code was needed for the backend to work => Remove it! It was only
+   some optimization code!
+
+More detailed description of the changes:
+
+- <https://ekaitz.elenq.tech/bootstrapGcc6.html>
+
+## Bootstrappable TinyCC - What I did
+
+\tiny
+\Begin{multicols}{2}
+
+**OPTIMIZED VERSION**
+```
+0000000000000000 <main>:
+   0:   fd010113    addi    sp,sp,-48
+   4:   02113423    sd  ra,40(sp)
+   8:   02813023    sd  s0,32(sp)
+   c:   03010413    addi    s0,sp,48
+  10:   00000013    nop
+  14:   fea43423    sd  a0,-24(s0)
+  18:   feb43023    sd  a1,-32(s0)
+  1c:   0130051b    addiw   a0,zero,19
+  20:   fca42e23    sw  a0,-36(s0)
+  24:   05a0051b    addiw   a0,zero,90
+  28:   fca42c23    sw  a0,-40(s0)
+  2c:   fdc42503    lw  a0,-36(s0)
+  30:   00051463    bnez    a0,38 <main+0x38>
+  34:   0180006f    j   4c <main+0x4c>
+  38:   fd842503    lw  a0,-40(s0)
+  3c:   00051463    bnez    a0,44 <main+0x44>
+  40:   00c0006f    j   4c <main+0x4c>
+  44:   0010051b    addiw   a0,zero,1
+  48:   0100006f    j   58 <main+0x58>
+  4c:   00008537    lui a0,0x8
+  50:   7005051b    addiw   a0,a0,1792
+  54:   00000033    add zero,zero,zero
+  58:   02813083    ld  ra,40(sp)
+  5c:   02013403    ld  s0,32(sp)
+  60:   03010113    addi    sp,sp,48
+  64:   00008067    ret
+```
+\columnbreak
+**UNOPTIMIZED VERSION**
+```
+0000000000000000 <main>:
+   0: fd010113    addi    sp,sp,-48
+   4: 02113423    sd  ra,40(sp)
+   8: 02813023    sd  s0,32(sp)
+   c: 03010413    addi    s0,sp,48
+  10: 00000013    nop
+  14: fea43423    sd  a0,-24(s0)
+  18: feb43023    sd  a1,-32(s0)
+  1c: 0130051b    addiw   a0,zero,19
+  20: fca42e23    sw  a0,-36(s0)
+  24: 05a0051b    addiw   a0,zero,90
+  28: fca42c23    sw  a0,-40(s0)
+  2c: fdc42503    lw  a0,-36(s0)
+  30: 00051463    bnez    a0,38 <main+0x38>
+  34: 01c0006f    j   50 <main+0x50>
+  38: fd842503    lw  a0,-40(s0)
+  3c: 00051463    bnez    a0,44 <main+0x44>
+  40: 0100006f    j   50 <main+0x50>
+  44: 0010051b    addiw   a0,zero,1
+  48: 0140006f    j   5c <main+0x5c>
+  4c: 0100006f    j   5c <main+0x5c>
+  50: 00008537    lui a0,0x8
+  54: 7005051b    addiw   a0,a0,1792
+  58: 00000033    add zero,zero,zero
+  5c: 02813083    ld  ra,40(sp)
+  60: 02013403    ld  s0,32(sp)
+  64: 03010113    addi    sp,sp,48
+  68: 00008067    ret
+```
+\End{multicols}
+
+\normalsize
+
+# What needs to be done
+
+## GCC - What needs to be done
+
+- Properly package the GCC-4.6.4 to include C++ support, and fix all the
+  libraries that might be missing.
+- Build the backported GCC-4.6.4 using TinyCC
+- Build GCC-7.5 using the backported GCC-4.6.4
+
+## TinyCC - What needs to be done
+
+- Build the bootstrappable TinyCC using GNU Mes
+- Decide if we need the upstream TinyCC to build GCC or not
+    - If we do: build the upstream TinyCC with the bootstrappable TinyCC
+
+## GNU Mes - What needs to be done
+
+- Review the current RISC-V support and prepare it to be merged
+
+## Guix - What needs to be done
+
+- Describe the whole compiler compiler chain in the bootstrapping packages so
+  everyone can benefit from it
+
+## Extra
+
+Do it all in real hardware
+
+# Last words
+
+## Last words
+
+There's still a lot of work to be done, most of it being the integration of the
+work I already did in the past thanks to NlNet.
+
+The future is bright though. We are probably going to get more funds from NlNet
+to involve more people on this.
+
+Wanna join?
+
+## Contact and take part
+
+- Email me: <ekaitz@elenq.tech>
+- Relevant IRC channels: `#bootstrappable`, `#guix`, `#guix-risc-v`
+
+# Thank you