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From: "Simon.Thornington at tssecurities dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/101479] New: vectorized impossible conditional floating point operations still cause traps (-ffast-math, -O3) Date: Fri, 16 Jul 2021 20:48:56 +0000 [thread overview] Message-ID: <bug-101479-4@http.gcc.gnu.org/bugzilla/> (raw) 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
next reply other threads:[~2021-07-16 20:48 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-07-16 20:48 Simon.Thornington at tssecurities dot com [this message] 2021-07-16 20:52 ` [Bug c/101479] " Simon.Thornington at tssecurities dot com 2021-07-16 21:08 ` [Bug tree-optimization/101479] " pinskia at gcc dot gnu.org 2021-07-16 21:09 ` pinskia at gcc dot gnu.org 2021-07-17 0:37 ` Simon.Thornington at tssecurities dot com 2021-07-17 17:55 ` pinskia at gcc dot gnu.org
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