Mesa (staging/20.1): nir/opt_if: fix opt_if_merge when destination branch has a jump

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Module: Mesa
Branch: staging/20.1
Commit: 861fd03bd17beaf8527c5c7cdcce87891edf6369
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=861fd03bd17beaf8527c5c7cdcce87891edf6369

Author: Rhys Perry <pendingchaos02 at gmail.com>
Date:   Mon Sep  7 11:33:44 2020 +0100

nir/opt_if: fix opt_if_merge when destination branch has a jump

Fixes a case where opt_if_merge created code like:
if (...) {
   break;
   loop {
      ...
   }
}
which caused opt_peel_loop_initial_if to complain that the loop pre-header
wasn't a predecessor of the loop header. This patch prevents this
(invalid, I think) unreachable code from being created.

Signed-off-by: Rhys Perry <pendingchaos02 at gmail.com>
Reviewed-by: Timothy Arceri <tarceri at itsqueeze.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3496
Fixes: 4d3f6cb9739 ('nir: merge some basic consecutive ifs')
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6633>
(cherry picked from commit 6cef8040672e84393e59ed6efa9953c95f5f8c92)

---

 .pick_status.json             | 2 +-
 src/compiler/nir/nir_opt_if.c | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/.pick_status.json b/.pick_status.json
index be785fbf5f6..50f36527fd6 100644
--- a/.pick_status.json
+++ b/.pick_status.json
@@ -706,7 +706,7 @@
         "description": "nir/opt_if: fix opt_if_merge when destination branch has a jump",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "master_sha": null,
         "because_sha": "4d3f6cb9739dfeaf9605fcd2f5318e03acf5066e"
     },
diff --git a/src/compiler/nir/nir_opt_if.c b/src/compiler/nir/nir_opt_if.c
index d1be3518877..65f540745f7 100644
--- a/src/compiler/nir/nir_opt_if.c
+++ b/src/compiler/nir/nir_opt_if.c
@@ -1276,6 +1276,13 @@ opt_if_merge(nir_if *nif)
          if (nif->condition.ssa == next_if->condition.ssa &&
              exec_list_is_empty(&next_blk->instr_list)) {
 
+            /* This optimization isn't made to work in this case and
+             * opt_if_evaluate_condition_use will optimize it later.
+             */
+            if (nir_block_ends_in_jump(nir_if_last_then_block(nif)) ||
+                nir_block_ends_in_jump(nir_if_last_else_block(nif)))
+               return false;
+
             simple_merge_if(nif, next_if, true, true);
             simple_merge_if(nif, next_if, false, false);