[Bug c/50168] __builtin_ctz() and intrinsics __bsr(), __bsf() generate suboptimal code on x86_64

["jakub at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50168

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |uros at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-08-23 17:58:52 UTC ---
Those aren't equivalent unfortunately, because bsf and bsr insns on x86 have
undefined value if the source is zero.  While __builtin_c[lt]z* documentation
says that the result is undefined in that case, I wonder if it would be fine
even if long l = (int) __builtin_c[lt]z* (x); gave a value that wasn't actually
sign-extended to 64 bits.
The combiner already simplifies zero or sign extension of popcount/parity/ffs
and, if ctz or clz value is defined at zero, also those, but if it is undefined
it assumes anything in any of the bits and thus can't optimize the sign/zero
extension away.  With -mbmi it will be optimized just fine, because for tzcnt
(and lzcnt for -mlzcnt) insns are well defined even for source operand zero.