https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101479
Bug ID: 101479
Summary: vectorized impossible conditional floating point
operations still cause traps (-ffast-math, -O3)
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: Simon.Thornington at tssecurities dot com
Target Milestone: ---
On all versions of gcc I could test, a vectorized operation of the form
y = could_be_zero ? 1.0 : (1.0 / x);
in a loop will still cause an FE_DIVBYZERO, even if x_not_zero is correctly set
to true whenever x could be 0.
This is with -ffast-math and -O3. With -fno-tree-vectorize it does not occur.
If I make could_be_zero volatile, it does not occur. If I insert a "compiler
fence" in the loop it also does not occur. At -O2 it doesn't happen either.
Reproduction:
#include <math.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <fenv.h>
#define FPCHECK_FORMAT(fmt, ...) \
do { \
int flags = fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW); \
if (flags) { \
printf("Floating point exception(s) detected:%s%s%s: %s (%s:%d) " fmt,
\
(flags & FE_INVALID) ? " FE_INVALID" : "", \
(flags & FE_DIVBYZERO) ? " FE_DIVBYZERO" : "", \
(flags & FE_OVERFLOW) ? " FE_OVERFLOW" : "", \
__func__, \
__FILE__, \
__LINE__, ## __VA_ARGS__); \
abort(); \
} \
} while(false) \
#define FPCHECK() FPCHECK_FORMAT("")
#define fpcheck(where) FPCHECK_FORMAT("%s", where)
#define compiler_fence() __asm__ __volatile__ ("" : : : "memory")
static bool close_to_zero(double x) {
return fabs(x) < 0.5;
}
void f(double *x, double *y, int n) {
fpcheck("before");
for (int i=0; i<n; i++) {
// making this volatile fixes it
bool smol = close_to_zero(x[i]);
// adding this compiler fence will fix it
//compiler_fence();
y[i] = smol ? 1.0 : (-1.0 / x[i]);
}
fpcheck("after");
}
int main(int argc, char *argv[]) {
double x[8] = { atof(argv[1]), atof(argv[2]), atof(argv[3]), atof(argv[4]),
atof(argv[5]), atof(argv[6]), atof(argv[7]), atof(argv[8]) };
double y[8];
printf("before\n");
f(x, y, 8);
printf("after\n");
return 0;
}
$ gcc -ffast-math -O3 -lm y.c && ./a.out 0 2 3 4 5 6 7 8
before
Floating point exception(s) detected: FE_DIVBYZERO: f (y.c:40) afterAborted
