https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90838
--- Comment #14 from Jakub Jelinek <jakub at gcc dot gnu.org> --- The patch does: + bool zero_ok = CTZ_DEFINED_VALUE_AT_ZERO (TYPE_MODE (type), ctzval) == 2; + + /* Skip if there is no value defined at zero, or if we can't easily + return the correct value for zero. */ + if (!zero_ok) + return false; + if (zero_val != ctzval && !(zero_val == 0 && ctzval == type_size)) + return false; For CTZ_DEFINED_VALUE_AT_ZERO == 1 we could support it the same way but we'd need to emit into the IL an equivalent of val == 0 ? zero_val : .CTZ (val) (with GIMPLE_COND and a separate bb - not sure if anything in forwprop creates new basic blocks right now), where there is a high chance that RTL opts would turn it back into unconditional ctz. That still wouldn't help non--mbmi x86, because CTZ_DEFINED_VALUE_AT_ZERO is 0 there. We could handle even that case by doing the branches around, but those would stay there in the generated code, at which point I wonder whether it would be a win. The original code is branchless...
