| Summary: | wrong code at -O2 and -O3 on x86_64-linux-gnu | ||
|---|---|---|---|
| Product: | gcc | Reporter: | Zhendong Su <su> |
| Component: | tree-optimization | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED DUPLICATE | ||
| Severity: | normal | CC: | jakub |
| Priority: | P3 | ||
| Version: | 4.7.4 | ||
| Target Milestone: | --- | ||
| Host: | Target: | ||
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | ||
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. |
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 $ a.out $ gcc-4.8.2 -O2 small.c $ a.out $ gcc-4.7.3 -O2 small.c $ a.out ^C $ gcc-4.7-branch -O2 small.c $ a.out ^C $ gcc-4.7-branch -v Using built-in specs. COLLECT_GCC=gcc-4.7-branch COLLECT_LTO_WRAPPER=/usr/local/gcc47-branch/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.7.4/lto-wrapper Target: x86_64-unknown-linux-gnu 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) $ ------------------- short a; int main() { for (; a >= 0; a++) ; return 0; }