Bug 594 – [llvm-gcc] llvm-gcc lays out bitfields backwards on big endian systems

Bug 594 - [llvm-gcc] llvm-gcc lays out bitfields backwards on big endian systems

Summary:

[llvm-gcc] llvm-gcc lays out bitfields backwards on big endian systems

Status:	RESOLVED FIXED

Product:	tools
Classification:	Unclassified
Component:	llvm-gcc
Version:	1.0
Platform:	Macintosh MacOS X

Importance:	P normal
Assigned To:	Chris Lattner

URL:
Keywords:	miscompilation

Depends on:
Blocks:
	Show dependency tree / graph

Reported:	2005-07-14 01:56 CDT by Chris Lattner
Modified:	2010-02-22 12:51 CST (History)
CC List:	1 user (show)

See Also:

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Description Chris Lattner 2005-07-14 01:56:09 CDT

This program is miscompiled by LLVM on powerpc.  It appears that bitfields are
being layed out backwards by the CFE.  This may be related to Bug 449.  It
probably also affects Sparc.  This testcase should be compiled with
-fno-strict-aliasing.

-Chris


struct X {
   int A : 4;
   int Z : 28;
};

void test(struct X *P, int A) {
   P->A = A;
}

int main() {
  int Y = ~0;
  test((struct X*)&Y, 0);
  printf("%x\n", Y);
}

Comment 1 Chris Lattner 2005-07-20 18:23:30 CDT

Fixed.  Patches here and here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20050711/027100.html
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20050718/027152.html

Fixing this fixes NAMD, 176.gcc and others.

-Chris