The latest gcc 4.7 branch miscompiles the following code on x86_64-linux at -O2 and -O3 in both 32-bit and 64-bit modes.
It also affects 4.6.x, but not 4.8.x and the current gcc trunk.
The issue seems to be considering signed short and signed char for overflows, but neither overflows.
$ gcc-trunk -O2 small.c
$ gcc-4.8.2 -O2 small.c
$ gcc-4.7.3 -O2 small.c
$ gcc-4.7-branch -O2 small.c
$ gcc-4.7-branch -v
Using built-in specs.
Configured with: ../gcc-4.7/configure --enable-languages=c,c++
Thread model: posix
gcc version 4.7.4 20140209 (prerelease) [gcc-4_7-branch revision 207641] (GCC)
for (; a >= 0; a++)
Dup of bug 35634.
*** This bug has been marked as a duplicate of bug 35634 ***
Note there is no overflow happening here at all. In that a++ is the same as a = a + 1 which is the same as a = (short)(((int)a) + 1) due to promotion rules in C.
Technically there is an overflow there. But GCC defines conversion to a smaller signed integer type, when the value cannot be represented in that smaller type, as a non-signalling truncation. Still, portable code mustn't rely on that.
Well, the conversion is implementation-defined behavior, and GCC documents what it does in that case (does it?) and thus you can rely on it, and given that other compilers also have simimilar implementation-defined behavior choice for that case, you can portably assume it unless you are targetting extinct architectures.