[llvm-commits] PATCH: Initial patches for changing the semantics of llvm.cttz and llvm.ctlz

[Stephen Canon]

On Dec 1, 2011, at 2:20 PM, Dan Gohman wrote:

> The "old" semantics really are more desirable though, in general. The only
> reason I know of for the "new" semantics is to cater to x86's old bsf and
> bsr instructions. But x86 admits its own deficiency, and has since introduced
> the lzcnt and tzcnt instructions, which behave properly. It seems unfortunate
> to require people who want the sane semantics to use a branch (even if
> CodeGen is clever and can eliminate it).

Agreed.  The x86 ISA semantics of BSR/BSF are nuts (and not followed by any other mainstream architecture with which I am familiar).  Surely there's some way to give llvm the hint it needs to optimize this particular case on x86 without making the semantics of llvm ir similarly broken.

- Steve