[LLVMbugs] [Bug 8974] New: dependency files absolutize paths such as -include ../path/foo.h on the command line

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=8974

           Summary: dependency files absolutize paths such as -include
                    ../path/foo.h on the command line
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: nlewycky at google.com
                CC: llvmbugs at cs.uiuc.edu


In a very small difference between clang and gcc behaviour, the command:

  $(CC) -M -MF hello.d -c hello.c -include emptydir/../hello.h

will produce a different hello.d for clang and for gcc. GCC always emits the
value of the -include argument verbatim, while clang emits the absolute path.
This breaks users who try to relocate their .d files.

The way this happens in clang is that InitPreprocessor.cpp's AddImplicitInclude
appends #include "..." to the file before we parse a file. Let me quote the
relevant comment (in NormalizeDashIncludePath):

  // Implicit include paths should be resolved relative to the current
  // working directory first, and then use the regular header search
  // mechanism. The proper way to handle this is to have the
  // predefines buffer located at the current working directory, but
  // it has not file entry. For now, workaround this by using an
  // absolute path if we find the file here, and otherwise letting
  // header search handle it.

so if it's found relative to '.' then we make the path absolute and then the
FileEntry takes on that name when it's #include'd, and that's what goes out to
dependency file generation.

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