[PATCH] D70350: [DWARF] Allow cross-CU references of subprogram definitions

[Vedant Kumar via Phabricator via llvm-commits]

vsk added a comment.

This is the problem: https://github.com/llvm/llvm-project/blob/master/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp#L954.

When llvm emits call site info, it creates the callee DIE if one doesn't exist. In an LTO build this only happens in CUs where a definition DISubprogram for the callee is available [1]. The issue with this is that the definition DIE is attached to the wrong parent/context DIE. The following assert catches the problem:

   DIE *DwarfUnit::getOrCreateContextDIE(const DIScope *Context) {
  -  if (!Context || isa<DIFile>(Context))
  +  if (!Context || isa<DIFile>(Context)) {
  +    assert((!Context || CUNode->getFile() == Context) &&
  +           "Can't find a suitable context DIE");
       return &getUnitDie();
  +  }

Once the definition is created, we do things like attach low/high pc ranges and/or inlined subroutines to it, assuming that it's nested within the correct CU when it's not. While this is a pre-existing issue, this patch surfaced the verifier failures because subprogram definitions were not shared pre-patch.

Some possible solutions:

1. Lazily get or create the //correct// context CU for definition subprograms, so that the definitions can be constructed properly. Not sure if it'd be a layering issue to call `DwarfDebug::getOrCreateDwarfCompileUnit` from within `DwarfCompileUnit`.
2. When a DIE for a subprogram is unavailable, insist on creating a fresh //declaration// DIE nested within the caller's CU (even if we have a definition DISubprogram at hand). Compared to (1) we'd emit a little extra debug info.
3. Maintain a list of unfinished call sites (pairs of `{DIE *, DISubprogram *}`) in DwarfDebug. Set DW_AT_call_origin within the unfinished call sites after all subprogram definition DIEs have been created.

Please let me know if you have alternate/preferred approaches. I think I'll try (3) tomorrow.

[1] When a //declaration// DIE for a callee is available, the call site info is emitted correctly today already. DIEs for declarations are created ahead of time in `DwarfDebug::getOrCreateDwarfCompileUnit`.


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https://reviews.llvm.org/D70350