nir: propagates if condition evaluation down some alu chains

v2:
 - only allow nir_op_inot or nir_op_b2i when alu input is 1.
 - use some helpers as suggested by Jason.

v3:
 - evaluate alu op for single input alu ops
 - add helper function to decide if to propagate through alu
 - make use of nir_before_src in another spot

shader-db IVB results:

total instructions in shared programs: 9993483 -> 9993472 (-0.00%)
instructions in affected programs: 1300 -> 1289 (-0.85%)
helped: 11
HURT: 0

total cycles in shared programs: 219476091 -> 219476059 (-0.00%)
cycles in affected programs: 7675 -> 7643 (-0.42%)
helped: 10
HURT: 1

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir_opt_if.c

@@ -23,6 +23,7 @@
 
 #include "nir.h"
 #include "nir/nir_builder.h"
+#include "nir_constant_expressions.h"
 #include "nir_control_flow.h"
 #include "nir_loop_analyze.h"
 
@@ -400,6 +401,119 @@ evaluate_if_condition(nir_if *nif, nir_cursor cursor, uint32_t *value)
    }
 }
 
+/*
+ * This propagates if condition evaluation down the chain of some alu
+ * instructions. For example by checking the use of some of the following alu
+ * instruction we can eventually replace ssa_107 with NIR_TRUE.
+ *
+ *   loop {
+ *      block block_1:
+ *      vec1 32 ssa_85 = load_const (0x00000002)
+ *      vec1 32 ssa_86 = ieq ssa_48, ssa_85
+ *      vec1 32 ssa_87 = load_const (0x00000001)
+ *      vec1 32 ssa_88 = ieq ssa_48, ssa_87
+ *      vec1 32 ssa_89 = ior ssa_86, ssa_88
+ *      vec1 32 ssa_90 = ieq ssa_48, ssa_0
+ *      vec1 32 ssa_91 = ior ssa_89, ssa_90
+ *      if ssa_86 {
+ *         block block_2:
+ *             ...
+ *            break
+ *      } else {
+ *            block block_3:
+ *      }
+ *      block block_4:
+ *      if ssa_88 {
+ *            block block_5:
+ *             ...
+ *            break
+ *      } else {
+ *            block block_6:
+ *      }
+ *      block block_7:
+ *      if ssa_90 {
+ *            block block_8:
+ *             ...
+ *            break
+ *      } else {
+ *            block block_9:
+ *      }
+ *      block block_10:
+ *      vec1 32 ssa_107 = inot ssa_91
+ *      if ssa_107 {
+ *            block block_11:
+ *            break
+ *      } else {
+ *            block block_12:
+ *      }
+ *   }
+ */
+static bool
+propagate_condition_eval(nir_builder *b, nir_if *nif, nir_src *use_src,
+                         nir_src *alu_use, nir_alu_instr *alu,
+                         bool is_if_condition)
+{
+   bool progress = false;
+
+   nir_const_value bool_value;
+   b->cursor = nir_before_src(alu_use, is_if_condition);
+   if (nir_op_infos[alu->op].num_inputs == 1) {
+      assert(alu->op == nir_op_inot || alu->op == nir_op_b2i);
+
+      if (evaluate_if_condition(nif, b->cursor, &bool_value.u32[0])) {
+         assert(nir_src_bit_size(alu->src[0].src) == 32);
+
+         nir_const_value result =
+            nir_eval_const_opcode(alu->op, 1, 32, &bool_value);
+
+         replace_if_condition_use_with_const(b, alu_use, result,
+                                             is_if_condition);
+         progress = true;
+      }
+   } else {
+      assert(alu->op == nir_op_ior || alu->op == nir_op_iand);
+
+      if (evaluate_if_condition(nif, b->cursor, &bool_value.u32[0])) {
+         nir_ssa_def *def[2];
+         for (unsigned i = 0; i < 2; i++) {
+            if (alu->src[i].src.ssa == use_src->ssa) {
+               def[i] = nir_build_imm(b, 1, 32, bool_value);
+            } else {
+               def[i] = alu->src[i].src.ssa;
+            }
+         }
+
+         nir_ssa_def *nalu =
+            nir_build_alu(b, alu->op, def[0], def[1], NULL, NULL);
+
+         /* Rewrite use to use new alu instruction */
+         nir_src new_src = nir_src_for_ssa(nalu);
+
+         if (is_if_condition)
+            nir_if_rewrite_condition(alu_use->parent_if, new_src);
+         else
+            nir_instr_rewrite_src(alu_use->parent_instr, alu_use, new_src);
+
+         progress = true;
+      }
+   }
+
+   return progress;
+}
+
+static bool
+can_propagate_through_alu(nir_src *src)
+{
+   if (src->parent_instr->type == nir_instr_type_alu &&
+       (nir_instr_as_alu(src->parent_instr)->op == nir_op_ior ||
+        nir_instr_as_alu(src->parent_instr)->op == nir_op_iand ||
+        nir_instr_as_alu(src->parent_instr)->op == nir_op_inot ||
+        nir_instr_as_alu(src->parent_instr)->op == nir_op_b2i))
+      return true;
+
+   return false;
+}
+
 static bool
 evaluate_condition_use(nir_builder *b, nir_if *nif, nir_src *use_src,
                        bool is_if_condition)
@@ -414,6 +528,20 @@ evaluate_condition_use(nir_builder *b, nir_if *nif, nir_src *use_src,
       progress = true;
    }
 
+   if (!is_if_condition && can_propagate_through_alu(use_src)) {
+      nir_alu_instr *alu = nir_instr_as_alu(use_src->parent_instr);
+
+      nir_foreach_use_safe(alu_use, &alu->dest.dest.ssa) {
+         progress |= propagate_condition_eval(b, nif, use_src, alu_use, alu,
+                                              false);
+      }
+
+      nir_foreach_if_use_safe(alu_use, &alu->dest.dest.ssa) {
+         progress |= propagate_condition_eval(b, nif, use_src, alu_use, alu,
+                                              true);
+      }
+   }
+
    return progress;
 }