[Bug target/29776] result of ffs/clz/ctz/popcount/parity are already sign-extended

[ubizjak at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29776

--- Comment #4 from Uros Bizjak <ubizjak at gmail dot com> 2012-07-30 11:36:23 UTC ---
> Perhaps REE can be taught about ctz giving a non-negative result.

Maybe we need some VRP pass to remove these extensions. Please note an example
from (duplicate) PR54115, where we generate:

#include <stdint.h>
uint64_t foo(long x){
 return __builtin_ctzl(x);
}

foo:
  bsfq  %rdi, %rax
  cltq

this is DImode -> DImode operation, followed by sign-extend from SImode partial
reg.

Your examples in comment #3:

  bsfl %esi, %esi
  movslq  %esi, %rsi

can be "fixed" by slapping any_extend:DI to CTZ pattern, to consume either
ZERO_EXTEND or SIGN_EXTEND of the value (it doesn't matter for ranges [0..(some
small number)]).

The DImode example above can be "fixed" by adding SUBREG to all patterns. But,
I think that there is more optimal implementation than exploding the number of
bit manipulating patterns.