The following C program gives incorrect results with vanilla gcc 4.1.1 (default
./configure options except --prefix, compiled on Fedora Core 5) and specific
compile options.
----------------------------
#include <stdio.h>
char buf[8] = {1,1,1,1,1,1,1,1};
char zero=0;
int main() {
int i;
*(long long*)buf = 0;
*(short*)buf ^= (zero) ^ (zero<<1);
for (i=0; i<8; i++) printf("%x,", buf[i]);
return 0;
}
----------------------------
The buffer should contain all zeros after the two assignments, but it doesn't:
$ ~/gcc-4.1.1/bin/gcc -W -Wall -Wno-long-long -pedantic \
-march=pentium-m -O2 -o gcc-bug gcc-bug.c
$ ./gcc-bug
1,1,0,0,0,0,0,0,
Looks like some sort of aliasing issue for long longs; I don't see it happening
with ints. Twiddling compile options and further code simplification tends to
make the problem disappear.
The above is a (painfully) reduced version of useful code which exhibited the
problem, so it's actually something you can bump into.
--
Summary: Code generation bug for aliased long long with -
mpentium-m
Product: gcc
Version: 4.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: gcc2eran at tromer dot org
GCC build triplet: 4.1.1
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29626
