Re: [PATCH] sched: Optimize __calc_delta.

On Tue, Mar 02, 2021 at 12:57:37PM -0800, Josh Don wrote:

On gcc, the asm versions of `fls` are about the same speed as the
builtin. On clang, the versions that use fls (fls,fls64) are more than
twice as slow as the builtin. This is because the way the `fls` function
is written, clang puts the value in memory:
https://godbolt.org/z/EfMbYe. This can be fixed in a separate patch.

Is this because clang gets the asm constraints wrong? ISTR that
happening before, surely the right thing is to fix clang?

On Wed, Mar 3, 2021 at 2:02 AM Peter Zijlstra [off-list ref] wrote:

On Tue, Mar 02, 2021 at 12:57:37PM -0800, Josh Don wrote:

quoted

On gcc, the asm versions of `fls` are about the same speed as the
builtin. On clang, the versions that use fls (fls,fls64) are more than
twice as slow as the builtin. This is because the way the `fls` function
is written, clang puts the value in memory:
https://godbolt.org/z/EfMbYe. This can be fixed in a separate patch.

Is this because clang gets the asm constraints wrong? ISTR that
happening before, surely the right thing is to fix clang?

https://bugs.llvm.org/show_bug.cgi?id=49406 filed