[cfe-dev] [test-suite] making polybench/symm succeed with "-Ofast" and "-ffp-contract=on"
Sebastian Pop via cfe-dev
cfe-dev at lists.llvm.org
Fri Oct 14 15:05:01 PDT 2016
On Fri, Oct 14, 2016 at 10:50 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> ----- Original Message -----
>> From: "Renato Golin" <renato.golin at linaro.org>
>> To: "Sebastian Pop" <sebpop.llvm at gmail.com>
>> Cc: "Hal Finkel" <hfinkel at anl.gov>, "Sebastian Paul Pop" <s.pop at samsung.com>, "llvm-dev" <llvm-dev at lists.llvm.org>,
>> "Matthias Braun" <matze at braunis.de>, "Clang Dev" <cfe-dev at lists.llvm.org>, "nd" <nd at arm.com>, "Abe Skolnik"
>> <a.skolnik at samsung.com>
>> Sent: Friday, October 14, 2016 10:36:12 AM
>> Subject: Re: [test-suite] making polybench/symm succeed with "-Ofast" and "-ffp-contract=on"
>>
>> On 14 October 2016 at 15:50, Sebastian Pop <sebpop.llvm at gmail.com>
>> wrote:
>> > These 3 tests are passing with the following configurations:
>> > -O3 -ffp-contract=off
>> > -O3 -ffp-contract=on
>> > -O0 -ffp-contract=off
>> > -O0 -ffp-contract=on
>> >
>> > They are not passing at:
>> > -Ofast -ffp-contract=on
>> > -Ofast -ffp-contract=off
>>
>> Let's separate completely FP-contract and fast-math. They're
>> different
>> things and need different solutions.
>>
>>
>> > if(TEST_SUITE_USES_FAST_MATH)
>> > add_definitions(-DFP_ABSTOLERANCE=1e0)
>> > else()
>> > add_definitions(-DFP_ABSTOLERANCE=1e-5)
>> > endif()
>>
>> This doesn't make sense. If my program decreased precision by 5
>> orders
>> of magnitude with -ffast-math, I'd be *very* worried.
>
> Is this an absolute or a relative tolerance? If this is absolute, as the name implies, then I'm not sure how to feel. We should probably use a relative tolerance where possible.
>
This is absolute tolerance: here is the code I'm using to check.
double V1 = A[i][j];
double V2 = B[i][j];
double Diff = fabs(V1 - V2);
if (Diff > AbsTolerance) {
fprintf(stderr, "A[%d][%d] = %lf and B[%d][%d] = %lf differ more than"
" FP_ABSTOLERANCE = %lf\n", i, j, V1, i, j, V2, AbsTolerance);
return 0;
}
Do you want me to add the relative tolerance for the 2 tests?
polybench/linear-algebra/kernels/symm, FP_ABSTOLERANCE=1e1
polybench/linear-algebra/solvers/gramschmidt, FP_ABSTOLERANCE=1e0
What should be a good relative tolerance to set for these two tests?
Here is the code to compute the relative tolerance from fpcmp:
// Check to see if these are inside the absolute tolerance
if (AbsTolerance < fabs(V1-V2)) {
// Nope, check the relative tolerance...
double Diff;
if (V2)
Diff = fabs(V1/V2 - 1.0);
else if (V1)
Diff = fabs(V2/V1 - 1.0);
else
Diff = 0; // Both zero.
if (Diff > RelTolerance) {
fprintf(stderr, ("%s: Compared: %e and %e\n"
"abs. diff = %e rel.diff = %e\n"
"Out of tolerance: rel/abs: %e/%e\n"),
g_program, V1, V2, fabs(V1-V2), Diff, RelTolerance, AbsTolerance);
return true;
}
}
> -Hal
>
>>
>> I hope that fast-math in Clang isn't that broken, so that's probably
>> to do with the assumptions in the output reduction phase.
>>
>> But, as I said, let's do Ofast *later*. One thing at a time.
Agreed. I will update the patch, make sure all Polybench pass with
"-O3 -ffp-contract=on/off" and it will be ready to commit.
Thanks,
Sebastian
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