GCC issue with -Ofast?

I have a question about the latest GCC compilers (version >= 5) with this code:

#include <math.h>void test_nan (    const float * const __restrict__ in,    const int n,    char * const __restrict__ out ){    for (int i = 0; i < n; ++i)        out[i] = isnan(in[i]);}

The assembly listing from GCC:

test_nan:        movq    %rdx, %rdi        testl   %esi, %esi        jle     .L1        movslq  %esi, %rdx        xorl    %esi, %esi        jmp     memset.L1:        ret

This looks like memset(out, 0, n).Why does GCC assume that no entries can be NaN with -Ofast ?With the same compilation options, ICC does not show this issue.With GCC, the issue goes away with "-O3".

Note that with "-O3", this query gcc -c -Q -O3 --help=optimizers | egrep -i nan gives -fsignaling-nans [disabled].

I verified this both locally and on godbolt, with the additional option "-std=c99".

Edit: by following the helpful answers below I can confirm that -Ofast -std=c99 -fno-finite-math-only properly addresses this issue.