[all-commits] [llvm/llvm-project] 7f482a: [clang modules] Setting `DebugCompilationDir` when...

[Qiongsi Wu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 7f482aa848c5f136d2b32431f91f88492c78c709
      https://github.com/llvm/llvm-project/commit/7f482aa848c5f136d2b32431f91f88492c78c709
  Author: Qiongsi Wu <qiongsiwu at gmail.com>
  Date:   2025-02-26 (Wed, 26 Feb 2025)

  Changed paths:
    M clang/include/clang/Tooling/DependencyScanning/ModuleDepCollector.h
    M clang/lib/Tooling/DependencyScanning/ModuleDepCollector.cpp
    A clang/test/ClangScanDeps/modules-debug-dir.c

  Log Message:
  -----------
  [clang modules] Setting `DebugCompilationDir` when it is safe to ignore current working directory (#128446)

This PR explicitly sets `DebugCompilationDir` to the system's root
directory if it is safe to ignore the current working directory.

This fixes a problem where a PCM file's embedded debug information can
lead to compilation failure. The compiler may have decided it is indeed
safe to ignore the current working directory. In this case, the PCM
file's content is functionally correct regardless of the current working
directory because no inputs use relative paths (see
https://github.com/llvm/llvm-project/pull/124786). However, a PCM may
contain debug info. If debug info is requested, the compiler uses the
current working directory value to set `DW_AT_comp_dir`. This may lead
to the following situation:
1. Two different compilations need the same PCM file. 
2. The PCM file is compiled assuming a working directory, which is
embedded in the debug info, but otherwise has no effect.
3. The second compilation assumes a different working directory, and
expects an identically-sized pcm file. However, it cannot find such a
PCM, because the existing PCM file has been compiled assuming a
different `DW_AT_comp_dir `, which is embedded in the debug info.

This PR resets the `DebugCompilationDir` if it is functionally safe to
ignore the working directory so the above situation is avoided, since
all debug information will share the same working directory.

rdar://145249881



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