nir/from_ssa: Move the loop bounds check in resolve_parallel_copy

We loop, effectively, over two stacks: ready and to_do and finish only
when both are empty.  In the case where ready is empty, we pull one off
of to_do, add a copy to a temporary, and push it onto the ready stack.
Previously, we assumed that we would never get to the temporary copy
case if to_do has exactly one entry because that would imply that there
was only one copy left which means there can't possibly be a cycle to
break.  This was true until c7fc44f9eb ("nir/from_ssa: Respect and
populate divergence information") which changed things such that
temporary copies sometimes get added in the case where a convergent
value is copied both to convergent and divergent destinations.

This patch adjusts our loop iteration to always attempt to clear the
ready stack before checking if there's anything left on the to_do stack.
I also added an assert to make the exit condition more clear.

Fixes: c7fc44f9eb ("nir/from_ssa: Respect and populate divergence information")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8037
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21315>
(cherry picked from commit 4e09d37f3b)

.pick_status.json

@@ -4,7 +4,7 @@
         "description": "nir/from_ssa: Move the loop bounds check in resolve_parallel_copy",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "c7fc44f9ebbe93f7aefc010aea4e13e29d0d67fd"
     },

src/compiler/nir/nir_from_ssa.c

@@ -767,7 +767,7 @@ resolve_parallel_copy(nir_parallel_copy_instr *pcopy,
          ready[++ready_idx] = i;
    }
 
-   while (to_do_idx >= 0) {
+   while (1) {
       while (ready_idx >= 0) {
          int b = ready[ready_idx--];
          int a = pred[b];
@@ -793,6 +793,11 @@ resolve_parallel_copy(nir_parallel_copy_instr *pcopy,
             }
          }
       }
+
+      assert(ready_idx < 0);
+      if (to_do_idx < 0)
+         break;
+
       int b = to_do[to_do_idx--];
       if (pred[b] == -1)
          continue;
@@ -805,6 +810,16 @@ resolve_parallel_copy(nir_parallel_copy_instr *pcopy,
        * allocation, so we would rather not create extra register
        * dependencies for the backend to deal with.  If it wants, the
        * backend can coalesce the (possibly multiple) temporaries.
+       *
+       * We can also get here in the case where there is no cycle but our
+       * source value is convergent, is also used as a destination by another
+       * element of the parallel copy, and all the destinations of the
+       * parallel copy which copy from it are divergent. In this case, the
+       * above loop cannot detect that the value has moved due to all the
+       * divergent destinations and we'll end up emitting a copy to a
+       * temporary which never gets used. We can avoid this with additional
+       * tracking or we can just trust the back-end to dead-code the unused
+       * temporary (which is trivial).
        */
       assert(num_vals < num_copies * 2);
       nir_register *reg = nir_local_reg_create(state->builder.impl);