On Tue, Jan 12, 2016 at 1:43 PM, Jakub Jelinek <ja...@redhat.com> wrote:
> On Tue, Jan 12, 2016 at 01:32:05PM +0100, Uros Bizjak wrote:
>> 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.
>
> Well, for the -fexcess-precision=standard case (== -std=c99) FLT_EVAL_METHOD
> 2 doesn't hurt, that forces in the FE long double computation. While if it
> is 0 with -msse -mfpmath=sse, it means that the FE leaves computations as is
> and they are computed in float precision for floats and in long double
> precision for doubles. For -fexcess-precision=fast it is different, because
> the FE doesn't do anything, so in the end it is mixed in that case.
> So, for -msse -mfpmath=sse, I think either we need FLT_EVAL_METHOD 2 or -1
> or 2 for -fexcess-precision=standard and -1 for -fexcess-precision=fast.
I think that following definition describes -msse -mfpmath=sse
situation in the most elegant way. We can just declare that the
precision is not known in this case:
#define TARGET_FLT_EVAL_METHOD \
(TARGET_MIX_SSE_I387 ? -1 \
: (TARGET_80387 && !TARGET_SSE_MATH) ? 2 : TARGET_SSE2 ? 0 : -1)
Using this patch, the compiler will still generate SSE instructions
for the above test.
Joseph, what is your opinion on this approach?
Uros.
