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

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

> On Mar 16, 2023, at 12:31 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> 
> On Thu, Mar 16, 2023 at 9:25 AM Qing Zhao via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>> 
>> Hi,
>> 
>> Recently, we discovered some floating point precision diffs when using GCC8 to build our
>> application on arm64: After some investigation, it turns out that this is due to the
>> -ffp-contract=fast option that is on by default. Therefore, we have to explicitly add
>> -ffp-contract=off and do a full-rebuild.
>> 
>> GCC by default turns -ffp-contract=fast on.
>> https://gcc.gnu.org/onlinedocs/gcc-8.5.0/gcc/Optimize-Options.html#Optimize-Options
>> https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Optimize-Options.html#Optimize-Options
>> 
>> "
>> -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.
>> "
>> 
>> This can be shown by a small example for arm64 with gcc8.5 in https://godbolt.org/z/MxYfnG8TE.
>> Only when adding -std=c89 explicitly, this transformaton is off.
>> 
>> another exmaple also shows that Clang and MSVC only allow this transformation when speifiying
>> ffast-math and fp:fast:  https://godbolt.org/z/o54bYfPbP
>> 
>> When searching online, we found that there were similar discussions recently on the exact same issue:
>> https://github.com/dotnet/runtime/issues/64604
>> https://github.com/dotnet/runtime/issues/64591
>> 
>> a summary of these discussions is:
>> 
>> 1. "fmadd" is a fused operation and will return a different result for many inputs;
>> 2. therefore, -ffp-contract=fast is not a safe optimization to be on by default;
>> 3. Clang and MSVC only allow this when specifying ffast-math and fp:fast since this is not an
>>  IEEE754 compliant optimization;
>> 4. The reasons why GCC turns on this option by default are:
>>  A. GNU C language spec allows such transformation.
>>  B. this did not expose real problem for most X86/X64 apps previously since FMA instructions
>>     didn't exist until 2013 when the FMA3 instruction set was added, and also these instructions
>>     were not always available..
>> 5. Arm64 has fused multiply-add instructions as "baseline" and are always available. therefore
>>  -ffp-contract=fast exposed more serious problems on Arm64 platforms.
> 
> This summary ignores x87 and even ignores PowerPC in GCC having FMA
> even before clang/LLVM was around.

Okay. 
> 
>> 
>> our major question:
>> 
>> Should GCC turn off -ffp-contract=fast by default since it's not IEEE754 compliant and more
>>  modern processors have the FMA instructions available by default?
> 
> 
> NO. We have this debate every few years and such.

So, what’s the major reason we keep the default  that is not IEEE754 compliant from the beginning? 

thanks.

Qing
> 
> Thanks,
> Andrew Pinski
> 
>> 
>> Thanks.
>> 
>> Qing