* Bug report in GCC
@ 2016-10-18 12:16 Nicolas Le Scouarnec
0 siblings, 0 replies; only message in thread
From: Nicolas Le Scouarnec @ 2016-10-18 12:16 UTC (permalink / raw)
To: overseers
Dear administrator,
I request an account on gcc bugzilla to submit the following bug
report about incorrect code being generated by gcc when using gcc -O2
level.
--------
In the following code, the & is incorrectly eliminated for
optimization level 2 or on both gcc 5.3.1, gcc 6.2 with
march=haswell. It seems that it assumes that _tzcnt_u64 should always
return a value < 64, but the actual specification is that it returns
64 when l is 0.
#include <stdint.h>
#include <stdio.h>
#include <immintrin.h>
int main(int argc, char ** argv){
unsigned long l = atol(argv[1]);
uint32_t t = _tzcnt_u64(l) & (64u-1u);
printf("%u\n",t);
}
-------
--
Nicolas Le Scouarnec
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