Correct lzcnt results

[Alex Guteniev (AlexGuteniev)]

Fixes #1103

[Stephan T. Lavavej (StephanTLavavej)]

Looks great, just a couple of questions about naming. If you or Igor Zhukov (@fsb4000) can confirm that the tests for countl_zero fail on the affected processors before your change, and pass after, I think that should be sufficient for validation. (Note that you can compile a test program on one machine and just copy it to another if you statically link.)

[Igor Zhukov (fsb4000)]

If you or Igor Zhukov (@fsb4000) can confirm that the tests for countl_zero fail on the affected processors before your change, and pass after, I think that should be sufficient for validation.

On my CPU(AMD FX8300) both versions(current master and the pull request) print same(correct) result:

[Alex Guteniev (AlexGuteniev)]

I confirm the fix on pre AMB-LZCNT-POPCNT machine

[Stephan T. Lavavej (StephanTLavavej)]

Thanks Igor Zhukov (@fsb4000) and Alex Guteniev (@AlexGuteniev)! I think this all makes sense - the FX-8300 worked because it physically supported Advanced Bit Manipulation, so the previous code said "oh, this doesn't support AVX2, we need to special-case 0" but otherwise used lzcnt which works. The test previously failed on Alex's even older processor which doesn't physically support lzcnt.

[Charlie Barto (barcharcraz)]

it sucks that we can't test this by modifying __isa_available before running the functions.

[Charlie Barto (barcharcraz)]

approved with a testing suggestion, I don't feel that suggestion should block merging this though.

[Stephan T. Lavavej (StephanTLavavej)]

Thanks statementreply for noticing this bug, Alex Guteniev (@AlexGuteniev) for fixing it, and Igor Zhukov (@fsb4000) for helping to validate the fix! 😺 😸 🐈