[Bug c/124838] __builtin_clzl(0) overoptimization even on targets with native lzcnt instruction

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124838

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu.org
         Resolution|WONTFIX                     |---
           Keywords|                            |missed-optimization
             Status|RESOLVED                    |NEW
   Last reconfirmed|                            |2026-04-10
     Ever confirmed|0                           |1

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
int bar3(unsigned long x) { return x ? __builtin_clzl(x) : 64; }

works, the pattern detection is confused by the conversion to unsinged long
which moved inside the conditional early:

  return x != 0 ? (long unsigned int) __builtin_clzl (x) : 64;

without that phiopt1 could

phiopt match-simplify trying:
        x_2(D) != 0 ? _4 : 64
Applying pattern match.pd:10864, gimple-match-9.cc:6415

phiopt match-simplify back:
_3 = .CLZ (x_2(D), 64);
result: _3
rejected because early

but phiopt2 then performs the transform.  Not with the conversion though
as we match

(for func (CLZ) 
 (simplify
  (cond (ne @0 integer_zerop@1) (func (convert?@3 @0)) INTEGER_CST@2)


I think we want to fix both, not reject early and add another
convert? around func and adjust phiopt to then consider the folding,
which it does not.