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Jit stencils on Windows contain debug data in 3.15 #142050
Description
Bug report
Bug description:
E.g.
void
emit__NOP(
unsigned char *code, unsigned char *data, _PyExecutorObject *executor,
const _PyUOpInstruction *instruction, jit_state *state)
{
//
// _NOP.o: file format coff-x86-64
// 0: '\x00\x00\x00\x00\x04\x00\x00\x00\xf1\x00\x00\x00<\x00\x00\x00\n\x00\x01\x11\x00\x00\x00\x00\x00\x00\x00\x00.\x00<\x11\x00\x00\x00\x00\xd0\x00\x15\x00\x01\x00\x04\x00\x00\x00\x16R\x00\x00\x00\x00\x00\x00clang version 21.1.4\x00\x00'
// 4c: 00 00 00 00
const unsigned char data_body[80] = {
0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00,
0xf1, 0x00, 0x00, 0x00, 0x3c, 0x00, 0x00, 0x00,
0x0a, 0x00, 0x01, 0x11, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x2e, 0x00, 0x3c, 0x11,
0x00, 0x00, 0x00, 0x00, 0xd0, 0x00, 0x15, 0x00,
0x01, 0x00, 0x04, 0x00, 0x00, 0x00, 0x16, 0x52,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x63, 0x6c,
0x61, 0x6e, 0x67, 0x20, 0x76, 0x65, 0x72, 0x73,
0x69, 0x6f, 0x6e, 0x20, 0x32, 0x31, 0x2e, 0x31,
0x2e, 0x34,
};
memcpy(data, data_body, sizeof(data_body));
}which is in fact empty on main for Linux (at least for me on WSL) or Windows on 3.14.
The above block is contained in every stencil :(
Clang now generates debug info into the object files. Don't know when it happened, maybe one of the clang updates. Doesn't matter anyway, the fix is easy, just ignore .debug$S COFF sections, which brings us back to
void
emit__NOP(
unsigned char *code, unsigned char *data, _PyExecutorObject *executor,
const _PyUOpInstruction *instruction, jit_state *state)
{
//
// _NOP.o: file format coff-x86-64
// 0: '\x00\x00\x00\x00'
// 4: 00 00 00 00
}CPython versions tested on:
CPython main branch
Operating systems tested on:
Windows
Activity
[-]Jit stencils on Windows contain debug data on Windows[/-][+]Jit stencils on Windows contain debug data[/+]chris-eibl commentedon Nov 30, 2025
I've done some further investigation. Just changing back to
in
cpython/Tools/jit/_llvm.py
Lines 14 to 15 in cd4d0ae
fixes this. So it must be related to the update of clang to 21.1.4.0 (#140973).
Looking closer at the data
reveals
clang version 21.1.4. Thus, I had high hopes that using -fno-ident might fix it. But unfortunately, this is not enough, still more metadata is written. So searching in the clang command line reference formetadataI found -fno-stack-size-section which doesn't change anything.Thus, I think just ignoring
.debug$SCOFF sections is the easiest way to go and I have verified usingthat with this fix for all three configurations this metadata is no longer contained in the stencils (and was present in all of them before).
I've checked again in WSL Ubuntu 24.04.3 LTS for clang 21.1.3 and on native Ubuntu 24.04.3 LTS for clang 21.1.7 that this does not happen for
x86_64-pc-linux-gnu. I don't have a setup for cross builds, so unfortunately I cannot comment onaarch64-unknown-linux-gnufor Linux and I have no Mac.I have no idea why clang changed that only for Windows.
chris-eibl commentedon Nov 30, 2025
As written in #142052 (comment), I am unsure about a backport, because
aarch64-pc-windows-msvcx86_64-pc-windows-msvci686-pc-windows-msvc._llmv.py) be changed. Make LLVM_VERSION configurable with env variable #138497 supports that only for configure based builds. And it is discouraged to use a different clang version, anyway. And the jit is experimental in 3.14 ...[-]Jit stencils on Windows contain debug data[/-][+]Jit stencils on Windows contain debug data in 3.15[/+]chris-eibl commentedon Dec 1, 2025
Ha, I found it, even though it is not part of the 21.1.0 release notes:
llvm/llvm-project#142970: [CodeGen][COFF] Always emit CodeView compiler info on Windows targets
It is contained in clang since 21.1.0-rc1.
Unfortunately, this is done unconditionally (at first it was planned to do it only when hotpatching is enabled) and there seems to be no switch to disable it.
GH-142050: Jit stencils on Windows contain debug data (#142052)
chris-eibl commentedon Dec 4, 2025
PR is merged, I think this can be closed.
pythonGH-142050: Jit stencils on Windows contain debug data (python#1…