Re: Should -ffp-contract=off the default on GCC?

[Qing Zhao <qing.zhao@oracle.com>]

> On Mar 24, 2023, at 3:42 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> 
> On Fri, Mar 24, 2023 at 1:14 AM Fangrui Song via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>> 
>> On Wed, Mar 22, 2023 at 8:52 AM Qing Zhao via Gcc-patches
>> <gcc-patches@gcc.gnu.org> wrote:
>>> 
>>> 
>>> 
>>>> On Mar 22, 2023, at 9:57 AM, Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>>>> 
>>>> On Wed, Mar 22, 2023 at 1:26 PM Alexander Monakov <amonakov@ispras.ru> wrote:
>>>>> 
>>>>> 
>>>>> On Wed, 22 Mar 2023, Richard Biener wrote:
>>>>> 
>>>>>> I think it's even less realistic to expect users to know the details of
>>>>>> floating-point math.  So I doubt any such sentence will be helpful
>>>>>> besides spreading some FUD?
>>>>> 
>>>>> I think it's closer to "fundamental notions" rather than "details". For
>>>>> users who bother to read the GCC manual there's a decent chance it wouldn't
>>>>> be for naught.
>>>>> 
>>>>> For documentation, I was thinking
>>>>> 
>>>>> Together with -fexcess-precision=standard, -ffp-contract=off
>>>>> is necessary to ensure that rounding of intermediate results to precision
>>>>> implied by the source code and the FLT_EVAL_METHOD macro is not
>>>>> omitted by the compiler.
>>>> 
>>>> that sounds good to me
>>> 
>>> Shall we add such clarification to our Gcc13 doc? That should be helpful if we keep the currently default.
>>> 
>>> Qing
>>>> 
>>>>> Alexander
>>> 
>> 
>> While updating the documentation, consider adding information that
>> #pragma STDC FP_CONTRACT OFF is ignored with -ffp-contract=fast.
>> 
>> This surprising behavior motivated Clang to add
>> -Xclang=-ffp-contract=fast-honor-pragmas
>> (https://discourse.llvm.org/t/fp-contract-fast-and-pragmas/58529).
> 
> `#pragma STDC FP_CONTRACT OFF` is not even implemented yet in GCC.
> Rather we should document that :).

Do we have any plan to implement this pragma?
Also, do we have any plan to implement -ffp-contract=on? 

I am very curious on why -ffp-contract=on has not been implemented for so long time?

There are some documentation on Floating Point implementation of GCC on the pragma STDC FP_CONTRACT
https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Floating-point-implementation.html#Floating-point-implementation

"
	• Whether and how floating expressions are contracted when not disallowed by the FP_CONTRACT pragma (C99 and C11 6.5).
Expressions are currently only contracted if -ffp-contract=fast, -funsafe-math-optimizations or -ffast-math are used. This is subject to change.

	• The default state for the FP_CONTRACT pragma (C99 and C11 7.12.2).
This pragma is not implemented. Expressions are currently only contracted if -ffp-contract=fast, -funsafe-math-optimizations or -ffast-math are used. This is subject to change.
“

So, looks like that we have documented this. But When the user read the documentation for —ffp-contract option, there is no such information. We might add a link in the documentation of -ffp-contract option to here to make it clear.

Qing




> It does not matter what clang does here really since GCC does not even
> implement the pragma.
> 
> Thanks,
> Andrew Pinski
> 
> 
>> 
>> 
>> 
>> --
>> 宋方睿