Require an OpSelectionMerge before an OpSwitch (#4154)

Fixes #4153

* OpSwitch is required to be preceeded by OpSelectionMerge
* Update newly invalid tests

1 files changed, 7 insertions, 10 deletions

diff --git a/source/val/validate_cfg.cpp b/source/val/validate_cfg.cpp
index a5f6e6a1..36f632a8 100644
--- a/source/val/validate_cfg.cpp
+++ b/source/val/validate_cfg.cpp
@@ -654,18 +654,15 @@ spv_result_t ValidateStructuredSelections(
                << "Selection must be structured";
       }
     } else if (terminator->opcode() == SpvOpSwitch) {
-      uint32_t count = 0;
-      // Mark the targets as seen now, but only error out if this block was
-      // missing a merge instruction and there were multiple unseen labels.
+      if (!merge) {
+        return _.diag(SPV_ERROR_INVALID_CFG, terminator)
+               << "OpSwitch must be preceeded by an OpSelectionMerge "
+                  "instruction";
+      }
+      // Mark the targets as seen.
       for (uint32_t i = 1; i < terminator->operands().size(); i += 2) {
         const auto target = terminator->GetOperandAs<uint32_t>(i);
-        if (seen.insert(target).second) {
-          count++;
-        }
-      }
-      if (!merge && count > 1) {
-        return _.diag(SPV_ERROR_INVALID_CFG, terminator)
-               << "Selection must be structured";
+        seen.insert(target);
       }
     }
   }