Bug: miscompiling i++ >= max

[Xi_W_]

icc 13.0.1 20121010, 64-bit Linux Test cases are from the Linux kernel. 1. Consider this simple program. icc miscompiles it. ------------------------------------------------------------------------ int count(int i) { if (i++ >= 0x7FFFFFFF) __builtin_trap(); return i; } #include #include int main(int argc, char **argv) { int x = atoi(argv[1]); printf("%d\n", count(x)); } ------------------------------------------------------------------------ This is the expected result, compiled with -O0. $ icc -O0 t.c $ ./a.out 0 1 This might be an icc bug with -O2. $ icc -O2 t.c $ ./a.out 0 Segmentation fault (core dumped) icc incorrectly folds (i++ >= 0x7FFFFFFF) into true. Do you happen to know why? 2. Consider a slightly different version. ------------------------------------------------------------------------ int count(int i, int max) { if (i++ >= max) __builtin_trap(); return i; } #include #include int main(int argc, char **argv) { int x = atoi(argv[1]); int max = atoi(argv[2]); printf("%d %d %d\n", x, max, count(x, max)); } ------------------------------------------------------------------------ The behavior changes from -O0 to O2. $ icc -O0 t.c $ ./a.out 2147483647 2147483647 Segmentation fault (core dumped) $ icc -O2 t.c $ ./a.out 2147483647 2147483647 2147483647 2147483647 -2147483648 icc turns (i++ >= max) into (++i > max). Strictly speaking, this is not a bug in icc as per the C standard, because 1) signed integer overflow is undefined behavior, and 2) icc is allowed to assume i++ never overflows. My question is, does icc provide anything like gcc's -fno-strict-overflow or -fwrapv to disable such optimizations based on signed integer overflow?

[Georg_Z_Intel]

Hello, thank you for reporting this! I think both cases are a bug (likely the same one) and filed a defect ticket for engineering (DPD200240103). I'm going to let you know about the progress. Regarding... - -fno-strict-overflow: That's already supported with Intel(R) Composer XE 2011 Update 10 and higher. - -fwrapv: We already have a feature request for it but no plan on the horizon yet. I've added you and am going to report here once it's available. Best regards, Georg Zitzlsberger

[Xi_W_]

Thanks for the update!

[Georg_Z_Intel]

Hello,

I've just seen the official validation of the ticket (DPD200240103) for this problem. Starting already with Intel(R) Composer XE 2013 SP1, this problem is fixed now.

Best regards,

Georg Zitzlsberger

[Ramith_H_]

Thanks!