nir_algebraic: Add basic optimizations for umul_low and imadsh_mix16

For umul_low (al * bl), zero is returned if the low 16-bits word of either
source is zero.

for imadsh_mix16 (ah * bl << 16 + c), c is returned if either 'ah' or 'bl'
is zero.

A couple of nir_search_helpers are added:

is_upper_half_zero() returns true if the highest word of all components of
an integer NIR alu src are zero.

is_lower_half_zero() returns true if the lowest word of all components of
an integer nir alu src are zero.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/compiler/nir/nir_opt_algebraic.py

@@ -1105,6 +1105,15 @@ for op in ['fddx', 'fddx_fine', 'fddx_coarse',
       ((op, 'a'), 0.0, 'info->stage == MESA_SHADER_COMPUTE && info->cs.derivative_group == DERIVATIVE_GROUP_NONE')
 ]
 
+# Some optimizations for ir3-specific instructions.
+optimizations += [
+   # 'al * bl': If either 'al' or 'bl' is zero, return zero.
+   (('umul_low', '#a(is_lower_half_zero)', 'b'), (0)),
+   # '(ah * bl) << 16 + c': If either 'ah' or 'bl' is zero, return 'c'.
+   (('imadsh_mix16', '#a@32(is_upper_half_zero)', 'b@32', 'c@32'), ('c')),
+   (('imadsh_mix16', 'a@32', '#b@32(is_lower_half_zero)', 'c@32'), ('c')),
+]
+
 # This section contains "late" optimizations that should be run before
 # creating ffmas and calling regular optimizations for the final time.
 # Optimizations should go here if they help code generation and conflict

src/compiler/nir/nir_search_helpers.h

@@ -242,4 +242,50 @@ is_used_by_non_fsat(nir_alu_instr *instr)
    return false;
 }
 
+/**
+ * Returns true if a NIR ALU src represents a constant integer
+ * of either 32 or 64 bits, and the higher word (bit-size / 2)
+ * of all its components is zero.
+ */
+static inline bool
+is_upper_half_zero(nir_alu_instr *instr, unsigned src,
+                   unsigned num_components, const uint8_t *swizzle)
+{
+   if (nir_src_as_const_value(instr->src[src].src) == NULL)
+      return false;
+
+   for (unsigned i = 0; i < num_components; i++) {
+      unsigned half_bit_size = nir_src_bit_size(instr->src[src].src) / 2;
+      uint32_t high_bits = ((1 << half_bit_size) - 1) << half_bit_size;
+      if ((nir_src_comp_as_uint(instr->src[src].src,
+                                swizzle[i]) & high_bits) != 0) {
+         return false;
+      }
+   }
+
+   return true;
+}
+
+/**
+ * Returns true if a NIR ALU src represents a constant integer
+ * of either 32 or 64 bits, and the lower word (bit-size / 2)
+ * of all its components is zero.
+ */
+static inline bool
+is_lower_half_zero(nir_alu_instr *instr, unsigned src,
+                   unsigned num_components, const uint8_t *swizzle)
+{
+   if (nir_src_as_const_value(instr->src[src].src) == NULL)
+      return false;
+
+   for (unsigned i = 0; i < num_components; i++) {
+      uint32_t low_bits =
+         (1 << (nir_src_bit_size(instr->src[src].src) / 2)) - 1;
+      if ((nir_src_comp_as_int(instr->src[src].src, swizzle[i]) & low_bits) != 0)
+         return false;
+   }
+
+   return true;
+}
+
 #endif /* _NIR_SEARCH_ */