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

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

> On Mar 22, 2023, at 8:33 AM, Alexander Monakov <amonakov@ispras.ru> wrote:
> 
> 
> On Mon, 20 Mar 2023, Jakub Jelinek via Gcc-patches wrote:
> 
>> On Mon, Mar 20, 2023 at 10:05:57PM +0000, Qing Zhao via Gcc-patches wrote:
>>> My question: is the above section the place in C standard “explicitly allows contractions”? If not, where it is in C standard?
>> 
>> http://port70.net/%7Ensz/c/c99/n1256.html#6.5p8
>> http://port70.net/%7Ensz/c/c99/n1256.html#note78
>> http://port70.net/%7Ensz/c/c99/n1256.html#F.6
> 
> C only allows contractions within expressions, not across statements (i.e.
> either -ffp-contract=on or -ffp-contract=off would be compliant, but not
> our default -ffp-contract=fast).

Oh, thanks for the info.

Just read the documentation of -fp-contract=style again: -:)

"
-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is currently not implemented and treated equal to -ffp-contract=off.

The default is -ffp-contract=fast.”

I was a little confused about the difference between -ffp-contract=fast and -ffp-contract=on previously,  now I understand.  

So, looks like that it's -ffp-contract=on that is compliant with C standard, but not -ffp-contract=fast?  (However, my understanding from the above doc and also from the GCC source code is, currently, -ffp-contract=on is not implemented and is equal to -fp-contract=off).

Therefore, the default value of -fp-contract=fast is NOT compliant with the language standard? 

Do I miss anything here?

> 
> Unrestricted contraction across statements together with other optimizations
> gives rise to difficult-to-debug issues such as PR 106902.

Just read this bug’s comments, yes, I agree.

Qing
> 
> Alexander