fft_accuracy test failure with Clang 11

m-hagelin commented 3 years ago

The testcase fft_accuracy fails quite badly after building with clang 11.

...
    type = float, size = 254; complex-inverse:
    [fail]    rms_diff_outofplace < min_prec | 6.27181 < 0.00140648
[----RUN----] dct...
[--SUCCESS--] 2 subtests of 2
===============================================================================
[--SUCCESS--] 145/146 tests 2677857/2678067 checks
[---ERROR---] 1/146 tests 210/2678067 checks
              fft_accuracy

My development machine is running Fedora 33, and I build with clang-11, cmake 3.18.4 and Ninja, the cpu is an i7-10510U, and the build uses avx2.
It worked with clang-10, and interestingly enough I have seen no issues when building in debug either.
Also get a lot of psABI warnings, but don't think the ABI would pertain to this issue seeing how the tests are all in the same application built with the same toolchain.

slarew commented 3 years ago

I see the same failures in the fft_accuracy test. I am running archlinux which moved to clang 11 not long ago. avx2 on an i7 8700k.

dancazarin commented 3 years ago

Investigating this. Likely vector optimizer bug in clang 11. Once a workaround will be found for this, patch for kfr will be released.

fcapozzi commented 3 years ago

I found same problem ... any news so far for kfr patch availability ?

slarew commented 2 years ago

I still see the failures in fft_accuracy test when compiling with clang 12.0.1 and clang 13 with CPU_ARCH=avx2. At the cost of a small performance loss, I found that disabling fast-math for libkfr_dft made all tests pass. Thus, it seems that one of the optimizations done by fast-math in newer clang/llvm versions is problematic.

dancazarin commented 2 years ago

Fixed in dev branch with no performance loss (spoiler: -ffp-contract=fast). @slarew, There was indeed a Clang bug in the code of optimizations enabled by -ffast-math switch but fortunately not -ffp-contract=fast. Newest Xcode 13 was also affected but clang-cl was not.

kfrlib / kfr

fft_accuracy test failure with Clang 11 #119