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

[amonakov at gcc dot gnu.org]

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

Alexander Monakov <amonakov at gcc dot gnu.org> changed:

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

--- Comment #23 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
In libcpp, search_line_fast implementations suffer from this, and what is
worse, attempts to workaround it by explicitly requesting zero extension don't
work:

char *foo(char *p, int x)
{
  return p + (unsigned)__builtin_ctz(x);
}

The above code is deliberately asking for zero extension, and yet various
optimizations in GCC transform it back to costlier form with sign extension.

(FWIW, LLVM gets this right)