nir/regs_to_ssa: Handle regs in phi sources properly

Sources of phi instructions act as if they occur at the very end of the
predecessor block not the block in which the phi lives.  In order to
handle them correctly, we have to skip phi sources on the normal
instruction walk and handle them as a separate walk over the successor
phis.  While registers in phi instructions is a bit of an oddity it can
happen when we temporarily go out-of-SSA for control-flow manipulations.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111075
Cc: [email protected]
Reviewed-by: Caio Marcelo de Oliveira Filho <[email protected]>

1 files changed, 32 insertions, 2 deletions

diff --git a/src/compiler/nir/nir_lower_regs_to_ssa.c b/src/compiler/nir/nir_lower_regs_to_ssa.c
index 0db11ff1d1c..76ed1287379 100644
--- a/src/compiler/nir/nir_lower_regs_to_ssa.c
+++ b/src/compiler/nir/nir_lower_regs_to_ssa.c
@@ -251,9 +251,17 @@ nir_lower_regs_to_ssa_impl(nir_function_impl *impl)
 
    nir_foreach_block(block, impl) {
       nir_foreach_instr(instr, block) {
-         if (instr->type == nir_instr_type_alu) {
+         switch (instr->type) {
+         case nir_instr_type_alu:
             rewrite_alu_instr(nir_instr_as_alu(instr), &state);
-         } else {
+            break;
+
+         case nir_instr_type_phi:
+            /* We rewrite sources as a separate pass */
+            nir_foreach_dest(instr, rewrite_dest, &state);
+            break;
+
+         default:
             nir_foreach_src(instr, rewrite_src, &state);
             nir_foreach_dest(instr, rewrite_dest, &state);
          }
@@ -262,6 +270,28 @@ nir_lower_regs_to_ssa_impl(nir_function_impl *impl)
       nir_if *following_if = nir_block_get_following_if(block);
       if (following_if)
          rewrite_if_condition(following_if, &state);
+
+      /* Handle phi sources that source from this block.  We have to do this
+       * as a separate pass because the phi builder assumes that uses and
+       * defs are processed in an order that respects dominance.  When we have
+       * loops, a phi source may be a back-edge so we have to handle it as if
+       * it were one of the last instructions in the predecessor block.
+       */
+      for (unsigned i = 0; i < ARRAY_SIZE(block->successors); i++) {
+         if (block->successors[i] == NULL)
+            continue;
+
+         nir_foreach_instr(instr, block->successors[i]) {
+            if (instr->type != nir_instr_type_phi)
+               break;
+
+            nir_phi_instr *phi = nir_instr_as_phi(instr);
+            nir_foreach_phi_src(phi_src, phi) {
+               if (phi_src->pred == block)
+                  rewrite_src(&phi_src->src, &state);
+            }
+         }
+      }
    }
 
    nir_phi_builder_finish(phi_build);