Re: Tests of gcc development beyond its testsuite (in this case, for gfortran)

[Toon Moene <toon@moene.org>]

On 5/7/24 20:44, Toon Moene wrote:

> On 5/7/24 20:35, Andrew Pinski wrote:
> 
>> On Tue, May 7, 2024 at 11:31 AM Toon Moene <toon@moene.org> wrote:
>>>
>>> On 5/7/24 00:02, Toon Moene wrote:
>>>
>>>> OK, perhaps on the aarch64 I need the following option to make the
>>>> comparison fair:
>>>>
>>>> ‘rdma’
>>>>
>>>>       Enable Round Double Multiply Accumulate instructions. This is 
>>>> on by
>>>> default for -march=armv8.1-a.
>>>>
>>>> I.e., -mno-rdma
>>>>
>>>> (I hope that's correct - I'll will try that when the Sun rises again 
>>>> and
>>>> I have some power to run the AArch64 machine ...).
>>>
>>> Well, I did two independent runs with gfortran-13.2 and the following
>>> options:
>>>
>>> -O3 -march=armv8.1-a+rdma
>>>
>>> and
>>>
>>> -O3 -march=armv8.1-a+nordma
>>>
>>> No difference in the number of error runs exceeding the prescribed
>>> thresholds.
>>>
>>> So, unless I made a mistake in the option specification (or the compiler
>>> silently ignored them because they were not applicable to my machine -
>>> ugh), the cause of the problem lies elsewhere.
>>
>>
>> AARCH64 armv8-a has FMA as part of its base ISA.
>> So you want to try with `-ffp-contract=off` instead.
>> RDMA turns on/off instructions which are not used by the
>> auto-vectorizer (yet) and used by intrinsics for them (If I read the
>> code correctly).
> 
> Ah, thanks - I'll try that tomorrow.

Yep, that did it:


			-->   LAPACK TESTING SUMMARY  <--
		Processing LAPACK Testing output found in the TESTING directory
SUMMARY             	nb test run 	numerical error   	other error
================   	===========	=================	================
REAL             	1327023		0	(0.000%)	0	(0.000%)	
DOUBLE PRECISION	1327845		0	(0.000%)	0	(0.000%)	
COMPLEX          	786775		0	(0.000%)	0	(0.000%)	
COMPLEX16         	787842		0	(0.000%)	0	(0.000%)	

--> ALL PRECISIONS	4229485		0	(0.000%)	0	(0.000%)	

So, obviously, the threshold values for these tests were derived on a 
machine without fused-multiply-add, or without using them if present.

This is perhaps not surprising, as the default build-and-test setup 
(make.inc.example) of the LAPACK package as distributed from netlib.org 
lists as the compiler choice:

FC = gfortran
FFLAGS = -O2 -frecursive
FFLAGS_DRV = $(FFLAGS)
FFLAGS_NOOPT = -O0 -frecursive

which means that the choice of architecture on x86-64 would be "generic" 
and wouldn't include FMA instructions. If the authors had used that 
setup in deriving the thresholds, it is not surprising that you need 
-ffp-contract=off on architectures that include FMA instructions by default.

Thanks for helping me out with this !

-- 
Toon Moene - e-mail: toon@moene.org - phone: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands