[PATCH] D131179: [StripDeadDebugInfo] Drop dead CUs for const global expression

[Alexey Bader via Phabricator via llvm-commits]

bader added a comment.

In D131179#3784293 <https://reviews.llvm.org/D131179#3784293>, @aprantl wrote:

> I don't think this would end up matching the user expectation. If I'm in the debugger I would expect to be able to access global constants in the expression evaluator.

That sounds reasonable.

> Why would optimizing them out be the desired behavior?

I'm following up on the discussion in this thread - https://discourse.llvm.org/t/questions-about-the-work-of-stripdeaddebuginfo-pass/60278. The situation we have is following. There an LLVM module (A), which was extracted from a bigger LLVM module (B). Let's imagine that original module B is a library of functions and we are trying to extract a sub-set (i.e. module A), which satisfy certain requirements (e.g. don't use particular HW features). We observe that module A includes a lot of debug information not relevant to extracted functions. If I understand correctly, this patch removes compile unit debug information if it's referenced by "dead" constant expressions ("dead" here means they are not used in extracted module A). 
I think we have following case in our code:

  //-------------------------------------
  // a.cpp
  int shared(int);
  const int A = 8 * 10;
  int foo() { return A; }
  
  int bar() { return shared(A); }
  
  //-------------------------------------
  // b.cpp
  const int B = 2 * 20;
  int baz() { return B; }
  
  int shared (int x) { return x; }
  
  //-------------------------------------
  // c.cpp
  const int C = 3 + 2;
  
  int foobar() { return C; }

We compile and link these files.

  clang++ -c -S -emit-llvm a.cpp -g
  clang++ -c -S -emit-llvm b.cpp -g
  clang++ -c -S -emit-llvm c.cpp -g
  llvm-link a.ll b.ll c.ll -S -o linked.ll

Then let's say we extract only function `foo` and I see following debug info in extracted module:

  !7 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !8, producer: "clang version 16.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, globals: !9, splitDebugInlining: false, nameTableKind: None)
  !8 = !DIFile(filename: "b.cpp", directory: "bader/tmp", checksumkind: CSK_MD5, checksum: "0c078350ba11ff83d6f23257d234280f")
  !9 = !{!10}
  !10 = !DIGlobalVariableExpression(var: !11, expr: !DIExpression(DW_OP_constu, 40, DW_OP_stack_value))
  !11 = distinct !DIGlobalVariable(name: "B", scope: !7, file: !8, line: 2, type: !5, isLocal: true, isDefinition: true)
  !12 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !13, producer: "clang version 16.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, globals: !14, splitDebugInlining: false, nameTableKind: None)
  !13 = !DIFile(filename: "c.cpp", directory: "bader/tmp", checksumkind: CSK_MD5, checksum: "ae01ec03776b2faa3f5f8d470bac1874")
  !14 = !{!15}
  !15 = !DIGlobalVariableExpression(var: !16, expr: !DIExpression(DW_OP_constu, 5, DW_OP_stack_value))
  !16 = distinct !DIGlobalVariable(name: "C", scope: !12, file: !13, line: 3, type: !5, isLocal: true, isDefinition: true)

Does it make sense to keep debug metadata for constant C in this case?


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