x86 denormal flag handling

[Michael Morrell <mmorrell@tachyum.com>]

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I've been following the recent changes to better support denormal handling and I don't think they are doing the right thing for x86.

I tried a simple program to convert a denormal float value (0x1.0p-127) to a double.  With the default of DAZ being 0 in MXCSR, x86 sets DE, but QEMU doesn't.  This is the opposite behavior of AARCH64 which sets their denormal input flag when it flushes a denormal input to 0.

Here's what I tried:

#include <stdio.h>
#include <immintrin.h>

volatile float f = 0x1.0p-127;

int main()
{
    double d = f;
    printf("Converting a denormal float to a double %s the DE bit in MXCSR\n",
           _mm_getcsr() & _MM_EXCEPT_DENORM ? "sets" : "does not set");
    return 0;
}

When run on a native machine, it prints:

Converting a denormal float to a double sets the DE bit in MXCSR

But when run using QEMU, it prints:

Converting a denormal float to a double does not set the DE bit in MXCSR

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