Re: [PATCH v2 07/50] helper-to-tcg: Introduce get-llvm-ir.py

[Anton Johansson via qemu development <qemu-devel@nongnu.org>]

On 04/08/26, Alessandro Di Federico via qemu development wrote:
> On Thu, 30 Jul 2026 05:09:41 +0200
> Anton Johansson via qemu development <qemu-devel@nongnu.org> wrote:
> 
> > ---
> >  subprojects/helper-to-tcg/get-llvm-ir.py | 145 +++++++++++++++++++++++
> >  subprojects/helper-to-tcg/meson.build    |   8 ++
> >  2 files changed, 153 insertions(+)
> >  create mode 100755 subprojects/helper-to-tcg/get-llvm-ir.py
> > 
> > diff --git a/subprojects/helper-to-tcg/get-llvm-ir.py b/subprojects/helper-to-tcg/get-llvm-ir.py
> > new file mode 100755
> > index 0000000000..982b87f791
> > --- /dev/null
> > +++ b/subprojects/helper-to-tcg/get-llvm-ir.py
> > @@ -0,0 +1,145 @@
> > +#!/usr/bin/env python3
> > +
> > +##
> > +##  Copyright(c) 2026 rev.ng Labs Srl. All Rights Reserved.
> > +##
> > +##  This program is free software; you can redistribute it and/or modify
> > +##  it under the terms of the GNU General Public License as published by
> > +##  the Free Software Foundation; either version 2 of the License, or
> > +##  (at your option) any later version.
> > +##
> > +##  This program is distributed in the hope that it will be useful,
> > +##  but WITHOUT ANY WARRANTY; without even the implied warranty of
> > +##  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > +##  GNU General Public License for more details.
> > +##
> > +##  You should have received a copy of the GNU General Public License
> > +##  along with this program; if not, see <http://www.gnu.org/licenses/>.
> > +##
> > +
> > +import argparse
> > +import json
> > +import os
> > +import shlex
> > +import sys
> > +import subprocess
> > +
> > +
> > +def log(msg):
> > +    print(msg, file=sys.stderr)
> > +
> > +
> > +def run_command(command):
> > +    proc = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
> > +    out = proc.communicate()
> > +    if proc.wait() != 0:
> > +        log(f"Command: {' '.join(command)} exited with {proc.returncode}\n")
> > +        log(f"output:\n{out}\n")
> 
> The whole program should fail if a subcommand fails, printing about it
> is not enough.

makes sense

[...]

> > +def generate_llvm_ir(
> > +    compile_commands_path, clang_path, output_path, input_path, target
> > +):
> > +    command = find_compile_commands(
> > +        compile_commands_path, clang_path, input_path, target
> > +    )
> > +
> > +    flags_to_remove = {
> > +        "-ftrivial-auto-var-init=zero",
> > +        "-fzero-call-used-regs=used-gpr",
> > +        "-Wimplicit-fallthrough=2",
> > +        "-Wold-style-declaration",
> > +        "-Wno-psabi",
> > +        "-Wshadow=local",
> > +        "-c",
> > +    }
> 
> How did you come up with this list?
> We should put some indication to make its maintenance easier.
> For instance, explicitly mention what part of QEMU introduced it, so
> one can easily check if it's still relevant or if it disappeared.

Agree, these are a bit arcane.  I'll double check if they're still
necessary and comment.

[...]

> > +def main():
> > +    parser = argparse.ArgumentParser(
> > +        description="Produce the LLVM IR of a given .c file."
> > +    )
> > +    parser.add_argument(
> > +        "--compile-commands", required=True, help="Path to compile_commands.json"
> > +    )
> 
> I'm not super happy that we use `compile_commands.json`, however:
> 
> 1. `compile_commands.json` is emitted unconditionally by meson at build
>    time.
> 
> 2. There's no good alternative. In rev.ng we configure a dedicated
>    (throwaway) clang QEMU build with `-fembed-bitcode` and then extract
>    the IR downstream.
> 
>    Maybe here we could manage to build a set of sources with
>    `-fembed-bitcode`, but that won't work unless the compiler is clang.
> 
>    We could configure a temporary clang QEMU build in a subdirectory,
>    but that's not very nice.
> 
> So, in the end, this makes sense to me.
> 
> The proper solution would to have `meson` provide the full invocation
> used to produce a certain object file that we can then manipulate, but
> AFAIU there's not such a feature.
> 
> One day we could maybe explore adjusting meson, but I'd say not today.

Yeah compile_commands.json isn't that nice but atleast there's precedent
for it in QEMU (`scripts/modinfo-collect.py`,
`scripts/check_sparse.py`).

> 
> > +    parser.add_argument("--clang", default="clang", help="Path to clang.")
> > +    parser.add_argument("--llvm-link", default="llvm-link", help="Path to llvm-link.")
> > +    parser.add_argument("-o", "--output", required=True, help="Output .ll file path")
> > +    parser.add_argument(
> > +        "--target-path", help="Path to QEMU target dir. (e.q. target/i386)"
> > +    )
> > +    parser.add_argument("inputs", nargs="+", help=".c file inputs")
> > +    args = parser.parse_args()
> > +
> > +    outputs = []
> > +    for input in args.inputs:
> > +        output = os.path.basename(input) + ".ll"
> > +        generate_llvm_ir(
> > +            args.compile_commands, args.clang, output, input, args.target_path
> > +        )
> > +        outputs.append(output)
> > +
> > +    run_command([args.llvm_link] + outputs + ["-S", "-o", args.output])
> 
> Maybe we should emit bitcode (binary form) instead of textual LLVM IR.
> If you go this route, don't forget to rename the output to `.bc`.
> 
> I know this is easy for debugging, but bitcode is what one should use
> "in production" and you're one `opt -S` away from getting the textual
> IR again.

AGH, yes I meant to use bitcode but forgot to change it before sending:)

> 
> > +
> > +
> > +if __name__ == "__main__":
> > +    sys.exit(main())
> > diff --git a/subprojects/helper-to-tcg/meson.build b/subprojects/helper-to-tcg/meson.build
> > index 8ab58adb39..97bce186fe 100644
> > --- a/subprojects/helper-to-tcg/meson.build
> > +++ b/subprojects/helper-to-tcg/meson.build
> > @@ -42,6 +42,14 @@ endif
> >  sources = [
> >  ]
> >  
> > +clang = bindir / 'clang'
> > +llvm_link = bindir / 'llvm-link'
> > +
> > +get_llvm_ir_cmd = [python, meson.current_source_dir() / 'get-llvm-ir.py',
> > +                   '--compile-commands', 'compile_commands.json',
> > +                   '--clang', clang,
> > +                   '--llvm-link', llvm_link]
> > +
> >  # NOTE: Add -Wno-template-id-cdtor for GCC versions >= 14.  This warning is
> >  # related to a change in the C++ standard in C++20, that also applies to C++14
> >  # for some reason. See defect report DR2237 and commit
> > -- 
> > 2.52.0
> 
> Reviewed-by: Alessandro Di Federico <ale@rev.ng>

Thanks

-- 
Anton Johansson
rev.ng Labs Srl.