nir/opt_algebraic: Add an option to lower uclz.

Signed-off-by: Georg Lehmann <dadschoorse@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18951>

src/compiler/nir/nir.h

@@ -3507,6 +3507,11 @@ typedef struct nir_shader_compiler_options {
     */
    bool lower_mul_32x16;
 
+   /**
+    * Set if uclz should be lowered to find_msb_rev.
+    */
+   bool lower_uclz;
+
    /**
     * Should IO be re-vectorized?  Some scalar ISAs still operate on vec4's
     * for IO purposes and would prefer loads/stores be vectorized.

src/compiler/nir/nir_opt_algebraic.py

@@ -1889,6 +1889,8 @@ optimizations.extend([
       ('ufind_msb_rev', 'value')),
      'options->lower_find_msb_to_reverse'),
 
+   (('uclz', a), ('umin', 32, ('ufind_msb_rev', a)), 'options->lower_uclz'),
+
    (('find_lsb', 'value'),
     ('ufind_msb', ('iand', 'value', ('ineg', 'value'))),
     'options->lower_find_lsb'),