LoopDeletion: Move EH pad check before the isLoopNeverExecuted Check

[Nirhar]

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 unreacheable 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

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[Nirhar]

@nikic Have a look at these changes, thanks!

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Manish Kausik H (Nirhar)

Changes

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 unreacheable 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

---

Full diff: https://github.com/llvm/llvm-project/pull/78189.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/LoopDeletion.cpp (+7-7)

diff --git a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp b/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
index c041e3621a16bd..bfe9374cf2f8c8 100644
--- a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
+++ b/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)) {

[nikic]

Please also add a test case in llvm/test/Transforms/LoopDeletion.

[Nirhar]

@nikic Have a look at the added test , thanks!

[nikic]

LGTM