Re: [PATCH] PR target/69225: Set FLT_EVAL_METHOD to 2 only if 387 FPU is used

[Uros Bizjak <ubizjak at gmail dot com>]

On Tue, Jan 12, 2016 at 1:12 PM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Tue, Jan 12, 2016 at 12:18 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>> On Tue, Jan 12, 2016 at 12:10:20PM +0100, Uros Bizjak wrote:
>>> On Tue, Jan 12, 2016 at 1:15 AM, Joseph Myers <joseph@codesourcery.com> wrote:
>>> > On Mon, 11 Jan 2016, H.J. Lu wrote:
>>> >
>>> >> Here is the updated patch.  Joseph, is this OK?
>>> >
>>> > I have no objections to this patch.
>>>
>>> Thinking some more, it looks to me that we also have to return 2 when
>>> SSE2 (SSE doubles) is not enabled.
>>>
>>> I'm testing following patch:
>>
>> That looks weird.  If TARGET_80387 and !TARGET_SSE_MATH, then no matter
>> whether sse2 is enabled or not, normal floating point operations will be
>> performed in 387 stack and thus FLT_EVAL_METHOD should be 2, not 0.
>> Do you want to do this because some instructions might be vectorized and
>> therefore end up in sse registers?  For -std=c99 that shouldn't happen,
>> already the C FE would promote all the arithmetics to be done in long
>> doubles, and for -std=gnu99 it is acceptable if non-vectorized computations
>> honor FLT_EVAL_METHOD and vectorized ones don't.
>
> Eh, today is just not the day for science.
>
> Hopefully, the logic in the patch below is correct:
>
> diff --git a/gcc/config/i386/i386.h b/gcc/config/i386/i386.h
> index 6c63871..5b42e89 100644
> --- a/gcc/config/i386/i386.h
> +++ b/gcc/config/i386/i386.h
> @@ -693,8 +693,9 @@ extern const char *host_detect_local_cpu (int
> argc, const char **argv);
>     only SSE, rounding is correct; when using both SSE and the FPU,
>     the rounding precision is indeterminate, since either may be chosen
>     apparently at random.  */
> -#define TARGET_FLT_EVAL_METHOD \
> -  (TARGET_MIX_SSE_I387 ? -1 : (TARGET_80387 && !TARGET_SSE_MATH) ? 2 : 0)
> +#define TARGET_FLT_EVAL_METHOD                                         \
> +  (TARGET_MIX_SSE_I387 ? -1                                            \
> +   : TARGET_80387 && !(TARGET_SSE2 && TARGET_SSE_MATH) ? 2 : 0)
>
>  /* Whether to allow x87 floating-point arithmetic on MODE (one of
>     SFmode, DFmode and XFmode) in the current excess precision

Using this patch, SSE math won't be emitted for a simple testcase
using " -O2 -msse -m32 -std=c99 -mfpmath=sse" compile flags:

float test (float a, float b)
{
  return a + b;
}

since we start with:

test (float a, float b)
{
  long double _2;
  long double _4;
  long double _5;
  float _6;

  <bb 2>:
  _2 = (long double) a_1(D);
  _4 = (long double) b_3(D);
  _5 = _2 + _4;
  _6 = (float) _5;
  return _6;
}

This is counter-intuitive, so I'd say we leave things as they are. The
situation where only floats are evaluated as floats and doubles are
evaluated as long doubles is not covered in the FLT_EVAL_METHOD spec.

Uros.