Bug#491653: gcc-4.3: misoptimization of 64-bit bitfield when not byte aligned

[Nick Lewycky <nicholas@mxc.ca>]

Package: gcc-4.3
Version: 4.3.1-6
Severity: normal

This testcase produces different output depending on whether -O1 or -O2 is
specified. Correct:

  # gcc PR1386.c -o pr1386 -O1
  PR1386.c: In function ‘main’:
  PR1386.c:15: warning: large integer implicitly truncated to unsigned type
  PR1386.c:17: warning: large integer implicitly truncated to unsigned type
  # ./pr1386
  bad bits: 0

Wrong:

  # gcc PR1386.c -o pr1386 -O2
  PR1386.c: In function ‘main’:
  PR1386.c:15: warning: large integer implicitly truncated to unsigned type
  PR1386.c:17: warning: large integer implicitly truncated to unsigned type
  # ./pr1386 
  bad bits: 268237f7fb6a30

Testcase taken from llvm.org/PR1386 (it shows up as a flaw in llvm testing,
but it's actually gcc getting it wrong. Doh!)

-- System Information:
Debian Release: lenny/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (500, 'testing'), (1, 'experimental')
Architecture: i386 (x86_64)

Kernel: Linux 2.6.25.9 (PREEMPT)
Locale: LANG=en_CA.UTF-8, LC_CTYPE=en_CA.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash

Versions of packages gcc-4.3 depends on:
hi  binutils              2.18.50.20080507-1 The GNU assembler, linker and bina
ii  cpp-4.3               4.3.1-6            The GNU C preprocessor
ii  gcc-4.3-base          4.3.1-6            The GNU Compiler Collection (base 
ii  libc6                 2.7-12             GNU C Library: Shared libraries
ii  libgcc1               1:4.3.1-6          GCC support library
ii  libgomp1              4.3.1-6            GCC OpenMP (GOMP) support library

Versions of packages gcc-4.3 recommends:
ii  libc6-dev                     2.7-12     GNU C Library: Development Librari

Versions of packages gcc-4.3 suggests:
pn  gcc-4.3-doc                   <none>     (no description available)
pn  gcc-4.3-locales               <none>     (no description available)
pn  gcc-4.3-multilib              <none>     (no description available)
ii  libgcc1-dbg                   1:4.3.1-6  GCC support library (debug symbols
pn  libgomp1-dbg                  <none>     (no description available)
pn  libmudflap0-4.3-dev           <none>     (no description available)
pn  libmudflap0-dbg               <none>     (no description available)

-- no debconf information

#include <stdio.h>
#include <stdint.h>

struct X {
  unsigned char pad : 4;
  uint64_t a : 64;
  uint64_t b : 60;
} __attribute__((packed));

int main (void)
{
  struct X x;
  uint64_t bad_bits;

  x.pad = 255;
  x.a = -1ULL;
  x.b = -1ULL;

  bad_bits = ((uint64_t)-1ULL) ^ *(1+(uint64_t *) &x);
  printf("bad bits: %llx\n", bad_bits);
  return bad_bits != 0;
}