[PATCH] D136762: [LLD][COFF] Survive empty and invalid PCH signature

[Alexandre Ganea via Phabricator via llvm-commits]

aganea updated this revision to Diff 471621.
aganea retitled this revision from "[LLD][COFF] Survive empty PCH signature" to "[LLD][COFF] Survive empty and invalid PCH signature".
aganea edited the summary of this revision.
aganea added a comment.
Herald added subscribers: MaskRay, hiraditya.
Herald added a reviewer: jhenderson.

I finally found some time to look at this. The issue is more complicated than I thought, and the solution is not the same for GHASH than non-GHASH.

Another issue was that `.PCH.OBJ`s are non-deterministic by default. Example:

  >cat precomp.cpp
  #include "precomp.h"
  
  >cl /Yc /Z7 /c precomp.cpp /nologo
  precomp.cpp
  
  >llvm-pdbutil dump --all precomp.obj > dump.txt
  
  >cl /Yc /Z7 /c precomp.cpp /nologo
  precomp.cpp
  
  >llvm-pdbutil dump --all precomp.obj > dump2.txt
  
  >diff dump.txt dump2.txt
  17991c17991
  < 0x2382 | LF_ENDPRECOMP [size = 8] signature = 0x522700AA
  ---
  > 0x2382 | LF_ENDPRECOMP [size = 8] signature = 0x2E1E4F09
  18020c18020
  <        0 | S_OBJNAME [size = 40] sig=1378287786, `C:\git\llvm-project\precomp.obj`
  ---
  >        0 | S_OBJNAME [size = 40] sig=773738249, `C:\git\llvm-project\precomp.obj`

There was a situation where our `.PCH.OBJ`s were always recompiled on a clean rebuild, but not the `.OBJ`s (they were cached). MSVC link.exe was able to link without warnings or errors. When I first implemented the PCH support, I initially thought the PCH signature was a hard requirement, but it seems it's just a hint for the runtime. I relaxed that requirement in LLD, but now we'd still fail if the `LF_ENDPRECOMP` index doesn't line up with the data in `LF_PRECOMP`.

Also worth noting that using MSVC's `cl /Yc /experimental:deterministic ..` makes the PCH signature deterministic.

Would you mind taking a second look please?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D136762/new/

https://reviews.llvm.org/D136762

Files:
  lld/COFF/DebugTypes.cpp
  lld/COFF/DebugTypes.h
  lld/COFF/InputFiles.cpp
  lld/test/COFF/precomp-link.test
  llvm/include/llvm/DebugInfo/CodeView/TypeStreamMerger.h
  llvm/lib/DebugInfo/CodeView/TypeStreamMerger.cpp
  llvm/tools/llvm-readobj/COFFDumper.cpp

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