clang gets numerical underflow wrong, please fix.

[Steve Kargl]

On Mon, Mar 14, 2016 at 01:02:20AM +0100, Dimitry Andric wrote:
> 
> $ gcc -O overflow-iter.c -o overflow-iter-gcc -lm
> $ ./overflow-iter-gcc
> FE_OVERFLOW: x = inf after 1024 iterations
> $ gcc -O2 overflow-iter.c -o overflow-iter-gcc -lm
> $ ./overflow-iter-gcc
> FE_OVERFLOW: x = inf after 16384 iterations
> 

Change the program to 

#include <fenv.h>
#include <stdio.h>

int
main(void)
{
   int i;
   float x = 1.f;
   i = 0;
   feclearexcept(FE_ALL_EXCEPT);
   do {
      x *= 2;
      i++;
      printf("%d %e\n", i, x);
   } while(!fetestexcept(FE_OVERFLOW));
   if (fetestexcept(FE_OVERFLOW)) printf("FE_UNDERFLOW: ");
   printf("x = %e after %d iterations\n", x, i);

   return 0;
}

You'll get a bunch of invalid output before the OVERFLOW.

% cc -O -o z b.c -lm && ./z | tail
1016 7.022239e+305  <-- not a valid float
1017 1.404448e+306  <-- not a valid float
1018 2.808896e+306  <-- not a valid float
1019 5.617791e+306  <-- not a valid float
1020 1.123558e+307  <-- not a valid float
1021 2.247116e+307  <-- not a valid float
1022 4.494233e+307  <-- not a valid float
1023 8.988466e+307  <-- not a valid float
1024 inf
FE_UNDERFLOW: x = inf after 1024 iterations

Clang is broken with or without #pragma FENV_ACCESS "on".

-- 
Steve