[Bug tree-optimization/90838] Detect table-based ctz implementation

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

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...