LoopDeletion: Move EH pad check before the isLoopNeverExecuted Check (#78189)

This commit modifies `LoopDeletion::deleteLoopIfDead` to check if the
exit block of a loop is an EH pad before checking if the loop gets
executed. This handles the case where an unreachable loop has a
landingpad as an Exit block, and the loop gets deleted, leaving leaving
the landingpad without an edge from an unwind clause.

Fixes #76852.

Author: Nirhar

llvm/lib/Transforms/Scalar/LoopDeletion.cpp

@@ -452,6 +452,13 @@ static LoopDeletionResult deleteLoopIfDead(Loop *L, DominatorTree &DT,
 
   BasicBlock *ExitBlock = L->getUniqueExitBlock();
 
+  // We can't directly branch to an EH pad. Don't bother handling this edge
+  // case.
+  if (ExitBlock && ExitBlock->isEHPad()) {
+    LLVM_DEBUG(dbgs() << "Cannot delete loop exiting to EH pad.\n");
+    return LoopDeletionResult::Unmodified;
+  }
+
   if (ExitBlock && isLoopNeverExecuted(L)) {
     LLVM_DEBUG(dbgs() << "Loop is proven to never execute, delete it!\n");
     // We need to forget the loop before setting the incoming values of the exit
@@ -487,13 +494,6 @@ static LoopDeletionResult deleteLoopIfDead(Loop *L, DominatorTree &DT,
     return LoopDeletionResult::Unmodified;
   }
 
-  // We can't directly branch to an EH pad. Don't bother handling this edge
-  // case.
-  if (ExitBlock && ExitBlock->isEHPad()) {
-    LLVM_DEBUG(dbgs() << "Cannot delete loop exiting to EH pad.\n");
-    return LoopDeletionResult::Unmodified;
-  }
-
   // Finally, we have to check that the loop really is dead.
   bool Changed = false;
   if (!isLoopDead(L, SE, ExitingBlocks, ExitBlock, Changed, Preheader, LI)) {

llvm/test/Transforms/LoopDeletion/loop-with-ehpad-not-executed.ll

@@ -0,0 +1,37 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt %s -passes=loop-deletion -S | FileCheck %s
+
+define void @wombat() personality ptr null {
+; CHECK-LABEL: define void @wombat() personality ptr null {
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    br i1 false, label [[BB1:%.*]], label [[BB4:%.*]]
+; CHECK:       bb1:
+; CHECK-NEXT:    br label [[BB2:%.*]]
+; CHECK:       bb2:
+; CHECK-NEXT:    [[INVOKE:%.*]] = invoke double null()
+; CHECK-NEXT:            to label [[BB2]] unwind label [[BB3:%.*]]
+; CHECK:       bb3:
+; CHECK-NEXT:    [[LANDINGPAD:%.*]] = landingpad { ptr, i32 }
+; CHECK-NEXT:            cleanup
+; CHECK-NEXT:    ret void
+; CHECK:       bb4:
+; CHECK-NEXT:    ret void
+;
+bb:
+  br i1 false, label %bb1, label %bb4
+
+bb1:                                              ; preds = %bb
+  br label %bb2
+
+bb2:                                              ; preds = %bb1, %bb2
+  %invoke = invoke double null()
+  to label %bb2 unwind label %bb3
+
+bb3:                                              ; preds = %bb2
+  %landingpad = landingpad { ptr, i32 }
+  cleanup
+  ret void
+
+bb4:                                              ; preds = %bb
+  ret void
+}