Prevent eliminating case constructs in block merging (#4976)

Fixes #4918

* Prevent block merging from producing an invalid case construct by
  merging a switch target/default with another construct's merge or
  continue block
* This is to satisfy the structural dominance requirement between the
  switch header and the case constructs

1 files changed, 20 insertions, 0 deletions

diff --git a/source/opt/block_merge_util.cpp b/source/opt/block_merge_util.cpp
index 8ae8020a5..83c702ca3 100644
--- a/source/opt/block_merge_util.cpp
+++ b/source/opt/block_merge_util.cpp
@@ -125,6 +125,26 @@ bool CanMergeWithSuccessor(IRContext* context, BasicBlock* block) {
       return false;
     }
   }
+
+  if (succ_is_merge || IsContinue(context, lab_id)) {
+    auto* struct_cfg = context->GetStructuredCFGAnalysis();
+    auto switch_block_id = struct_cfg->ContainingSwitch(block->id());
+    if (switch_block_id) {
+      auto switch_merge_id = struct_cfg->SwitchMergeBlock(switch_block_id);
+      const auto* switch_inst =
+          &*block->GetParent()->FindBlock(switch_block_id)->tail();
+      for (uint32_t i = 1; i < switch_inst->NumInOperands(); i += 2) {
+        auto target_id = switch_inst->GetSingleWordInOperand(i);
+        if (target_id == block->id() && target_id != switch_merge_id) {
+          // Case constructs must be structurally dominated by the OpSwitch.
+          // Since the successor is the merge/continue for another construct,
+          // merging the blocks would break that requirement.
+          return false;
+        }
+      }
+    }
+  }
+
   return true;
 }