https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101683
Bug ID: 101683
Summary: Floating point exception for double->unsigned
conversion on avx512 only
Product: gcc
Version: 10.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: bartoldeman at users dot sourceforge.net
Target Milestone: ---
Created attachment 51222
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51222&action=edit
File to reproduce
For this code:
#define _GNU_SOURCE
#include <fenv.h>
int main(int argc, char **argv) {
feenableexcept(FE_INVALID);
double argcm10 = argc / -0.1;
return (unsigned)(argcm10 < 0.0 ? 0 : argcm10);
}
$ gcc -O -march=skylake-avx512 fpexcept.c -lm
$ ./a.out
Floating point exception
the instructions
vcvttsd2usi %xmm0, %eax
vxorpd %xmm1, %xmm1, %xmm1
vucomisd %xmm0, %xmm1
movl $0, %edx
cmova %edx, %eax
are generated just after the division, so the conversion happens before the
comparison.
"If a converted result cannot be represented in the destination format, the
floating-point invalid exception is raised, and if this exception is masked,
the integer value 2^w – 1 is returned, where w represents the number of bits in
the destination format."
so when masked, for argcm10 = -10.0 the value 2^w-1 is discarded and all is
well, since it's < 0, but not when unmasked.
I can reproduce this issue with 9.3 as well, but not with 8.4 (the generated
code is correct for 8.4). I have not tried 11.1 yet.
Note: I found this issue with the UCX library when compiled with
-march=skylake-avx512, this example is stripped down from:
https://github.com/openucx/ucx/blob/f5362f5e6f80d930b88c44c63b4d8d71cf91d214/src/ucp/core/ucp_ep.c#L2699
