On 12/18/2023 1:34 PM, Rémi Denis-Courmont wrote:
Le sunnuntaina 17. joulukuuta 2023, 23.57.50 EET Martin Storsjö a écrit :
Rounding errors would not cause a constant gap across the different test
cases. This is most likely an off-by-one in the x86 code. I don't know if
this is a bug in the x86 code, or the test case being a little loose with
input parameters, and I have neither time, nor motivation not to mention
skills to figure that out, so there will be no test cases for this
function form me afterall.
FWIW, we've had these situations elsewhere before as well, in swscale,
where the existing x86 assembly mismatches the C code in nontrivial ways,
and we have new assembly (aarch64 in that case) that is missing a test
(even if one was written) due to this.
First I considered if we should collect these extra checkasm tests in some
branch somewhere, so they aren't lost, as they are useful when working on
assembly on other architectures.
But rather than having the code rot, forgotten in a stray branch
somewhere, I wonder if we should just go ahead and merge it with an #if
!ARCH_X86 or something, together with a notable FIXME comment.
I'd certainly welcome more checkasm that literally anyone other than me wrote.
If the divergence in the X86 code is simply due to optimising an inexact
algorithm differently, that seems fine.
But if it is a case that the X86 code is demonstrably buggy, I think that it
should be commented out or removed. That would not only fix a bug, but also put
stronger incentives for X68 fanboys to actually fix it. Worst case, the
optimisation has become meaningless and we have actually fixed a bug.
I looked at the sse2 implementation briefly and it may in fact be buggy.
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