nir/algebraic: better propagate constants up fadd chains

Make the optimization create more mad-friendly code if the order of the
fadd's operands is unlucky.

fossil-db (Navi):
Totals from 9259 (8.07% of 114665) affected shaders:
SGPRs: 615991 -> 616191 (+0.03%); split: -0.05%, +0.08%
VGPRs: 442184 -> 443568 (+0.31%); split: -0.10%, +0.41%
CodeSize: 32674876 -> 32625572 (-0.15%); split: -0.17%, +0.02%
MaxWaves: 108560 -> 108152 (-0.38%); split: +0.07%, -0.44%
Instrs: 6126473 -> 6120463 (-0.10%); split: -0.13%, +0.03%

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5631>

.gitlab-ci/traces-virgl.yml

@@ -21,7 +21,7 @@ traces:
   - path: gputest/pixmark-piano.trace
     expectations:
       - device: gl-virgl
-        checksum: 43b2c4db0d6810cca945071b9a645561
+        checksum: 8293e59b818715ddf1c23e9f60b17851
   - path: gputest/triangle.trace
     expectations:
       - device: gl-virgl

src/compiler/nir/nir_opt_algebraic.py

@@ -1241,6 +1241,8 @@ optimizations.extend([
    # Propagate constants up multiplication chains
    (('~fmul(is_used_once)', ('fmul(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), '#c'), ('fmul', ('fmul', a, c), b)),
    (('imul(is_used_once)', ('imul(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), '#c'), ('imul', ('imul', a, c), b)),
+   # Prefer moving out a multiplication for more MAD/FMA-friendly code
+   (('~fadd(is_used_once)', ('fadd(is_used_once)', 'a(is_not_const)', 'b(is_fmul)'), '#c'), ('fadd', ('fadd', a, c), b)),
    (('~fadd(is_used_once)', ('fadd(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), '#c'), ('fadd', ('fadd', a, c), b)),
    (('iadd(is_used_once)', ('iadd(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), '#c'), ('iadd', ('iadd', a, c), b)),
 

src/compiler/nir/nir_search_helpers.h

@@ -225,6 +225,22 @@ is_not_fmul(struct hash_table *ht, nir_alu_instr *instr, unsigned src,
    return src_alu->op != nir_op_fmul;
 }
 
+static inline bool
+is_fmul(struct hash_table *ht, nir_alu_instr *instr, unsigned src,
+        UNUSED unsigned num_components, UNUSED const uint8_t *swizzle)
+{
+   nir_alu_instr *src_alu =
+      nir_src_as_alu_instr(instr->src[src].src);
+
+   if (src_alu == NULL)
+      return false;
+
+   if (src_alu->op == nir_op_fneg)
+      return is_fmul(ht, src_alu, 0, 0, NULL);
+
+   return src_alu->op == nir_op_fmul;
+}
+
 static inline bool
 is_fsign(nir_alu_instr *instr, unsigned src,
          UNUSED unsigned num_components, UNUSED const uint8_t *swizzle)