On Wed, Mar 3, 2021 at 2:02 AM Peter Zijlstra <[email protected]> wrote: > > 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?
https://bugs.llvm.org/show_bug.cgi?id=49406 filed
