[Bug target/94413] New: auto-vectorization of isfinite raises FP exception

[kretz at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94413

            Bug ID: 94413
           Summary: auto-vectorization of isfinite raises FP exception
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kretz at kde dot org
  Target Milestone: ---
            Target: x86_64-*-*, i?86-*-*

Test case (`-O3`, cf. https://godbolt.org/z/jdfv3r):

#include <cfenv>
#include <cmath>
#include <limits>

using f4 [[gnu::vector_size(16)]] = float;

f4 isfinite(f4 x) {
  f4 r = {};
  for (int i = 0; i < 4; ++i) r[i] = std::isfinite(x[i]) ? 1.f : 0.f;
  return r;
}

using L = std::numeric_limits<float>;
f4 test = {1, 2, 3, L::quiet_NaN()};

int main() {
  std::feclearexcept(FE_ALL_EXCEPT);
  test = isfinite(test);
  if (std::fetestexcept(FE_ALL_EXCEPT) != 0) __builtin_abort();
  return 0;
}

This translates to `fabs(test)<=FLT_MAX` but incorrectly using a signaling
compare instruction.

Alterative vectorization of isfinite(x):
* interpret x as int vector
* return inf > x & inf

This works because if all exponent bits of x are set, x is either inf or NaN
(i.e. not finite).