[PATCH] D96931: [LTO] Discard non-prevailing defined symbols in module-level assembly

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a subscriber: nickdesaulniers.
MaskRay added a comment.

The regular LTO precodegen bitcode file `%to1.0.5.precodegen.bc` is

  module asm ".weak foo"
  module asm "\09 .equ foo,bar"
  
  @llvm.compiler.used = appending global [1 x i8*] [i8* bitcast (i32 (i32)* @bar to i8*)], section "llvm.metadata"
  
  ; Function Attrs: norecurse nounwind readnone willreturn
  define dso_local i32 @foo(i32 %0) local_unnamed_addr #0 {
    %2 = add nsw i32 %0, 2
    ret i32 %2
  }
  
  ; Function Attrs: norecurse nounwind readnone willreturn
  define internal i32 @bar(i32 %0) #0 {
    %2 = add nsw i32 %0, 1
    ret i32 %2
  }

The module-level inline asm is emitted before the functions. The call tree is roughly:

  AsmPrinter::doInitialization
    emitInlineAsm (AsmPrinter.cpp:319)
      (void)Parser->Run(/*NoInitialTextSection*/ true, (AsmPrinterInlineAsm.cpp:124)
        ...
          MCELFStreamer::emitSymbolAttribute (set binding to STB_WEAK)
  AsmPrinter::emitFunctionBody
    AsmPrinter::emitFunctionHeader
      AsmPrinter::emitLinkage
        MCELFStreamer::emitSymbolAttribute (`error: ... changed binding to STB_GLOBAL`)

A more elegant solution is to let the `MCELFStreamer` ignore non-prevailing definitions during `emitInlineAsm`.

If that is complex to implement, @pcc having this short-term solution may be useful depending on how urgent @nickdesaulniers wants https://github.com/ClangBuiltLinux/linux/issues/1269 to be fixed.

In https://github.com/ClangBuiltLinux/linux/issues/1269#issuecomment-786963047 , @nickdesaulniers mentioned the binding can change from STB_GLOBAL to STB_WEAK, then STB_WEAK to STB_GLOBAL.
I am curious how that can happen:

    LTO     vmlinux.o
  __ia32_compat_sys_sysctl: binding: 1
  __ia32_compat_sys_sysctl: binding: 2
  __ia32_compat_sys_sysctl: binding: 2
  <unknown>:0: error: __ia32_compat_sys_sysctl changed binding to STB_GLOBAL
  __ia32_compat_sys_sysctl: binding: 1
  LLVM ERROR: Error parsing inline asm
  
  PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D96931/new/

https://reviews.llvm.org/D96931