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

Owen Anderson resistor at mac.com
Fri Dec 2 22:24:36 PST 2011

On Dec 1, 2011, at 3:06 PM, Chris Lattner wrote:

> On Dec 1, 2011, at 11:20 AM, Dan Gohman wrote:
>> On Dec 1, 2011, at 2:47 AM, Chandler Carruth wrote:
>>> 7) remove all support (other than auto-upgrade) for the old intrinsics
>> 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).
> I would be fine with keeping the existing intrinsics and adding two new ones with gcc-compatible semantics.

What's the point of having two?  Would any frontend actually generate the non-GCC compatible ones?


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