[LLVMbugs] [Bug 9219] New: Possible false positive with -Wcast-align and #pragma pack(push, 2)

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=9219

           Summary: Possible false positive with -Wcast-align and #pragma
                    pack(push, 2)
           Product: new-bugs
           Version: trunk
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: sean at rogue-research.com
                CC: llvmbugs at cs.uiuc.edu


Consider this C code:

--------
#include <CoreServices/CoreServices.h>

int main(void)
{
    FSCatalogInfo catalogInfo = {};
    FileInfo* fileInfo = (FileInfo*)(&catalogInfo.finderInfo);
    (void)fileInfo;

    return 0;
}
--------

With -Wcast-align clang says:

test.c:6:23: warning: cast from 'UInt8 (*)[16]' to 'FileInfo *' (aka
'struct FileInfo *')
      increases required alignment from 1 to 2 [-Wcast-align]
        FileInfo* fileInfo = (FileInfo*)(&catalogInfo.finderInfo);
                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I think this is maybe a false positive, though I'm not sure.

a) The docs for the FSCatalogInfo struct say:

  /*
   * kFSCatInfoFinderInfo / file type, creator, flags, location. 
   * Coerce to a File/FolderInfo to use.
   */
  UInt8               finderInfo[16];

Implying that the cast should be safe.

b) Files.h starts with #pragma pack(push, 2).  I'm thinking maybe clang does
not take that into account.


If clang is indeed correct here, then I'll file a Radar against
CarbonCore/Files.h.

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