[LoopSimplifyCFG] Delete landing pads in dead exit blocks

[yrouban]

Summary

In addition to removing phi nodes this patch removes any landing pad the dead exit block might have.
Without this fix Verifier complains about a new switch instruction jumps to a block with a landing pad:

Block containing LandingPadInst must be jumped to only by the unwind edge of an invoke.
  %res = landingpad token
          cleanup
in function test_remove_lpad
LLVM ERROR: Broken function found, compilation aborted!

Regression tests provided.

[mkazantsev]

llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp

370

Why do we only delete phis and landingpad? If this block is proved dead, we can remove all its instructions and insert unreachable instead of its terminator.

[yrouban]

llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp

370

I think we do minimal changes for the sake of validity.

[mkazantsev]

LGTM. Please add a TODO that the whole block can be collapsed.

llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp

void handleDeadExits() {
 
     IRBuilder<> Builder(Preheader->getTerminator());
     SwitchInst *DummySwitch =
         Builder.CreateSwitch(Builder.getInt32(0), NewPreheader);
     Preheader->getTerminator()->eraseFromParent();
 
     unsigned DummyIdx = 1;
     for (BasicBlock *BB : DeadExitBlocks) {
-      SmallVector<Instruction *, 4> DeadPhis;
+      // Eliminate all Phis and LandingPads from dead exits.
+      // TODO: Consider removing all instructions in this dead block.
+      SmallVector<Instruction *, 4> DeadInstructions;
       for (auto &PN : BB->phis())
-        DeadPhis.push_back(&PN);
+        DeadInstructions.push_back(&PN);
 
-      // Eliminate all Phis from dead exits.
-      for (Instruction *PN : DeadPhis) {
-        PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
-        PN->eraseFromParent();
+      if (auto *LandingPad = dyn_cast<LandingPadInst>(BB->getFirstNonPHI()))
+        DeadInstructions.emplace_back(LandingPad);
+
+      for (Instruction *I : DeadInstructions) {
+        I->replaceAllUsesWith(UndefValue::get(I->getType()));
+        I->eraseFromParent();
       }
+
       assert(DummyIdx != 0 && "Too many dead exits!");
       DummySwitch->addCase(Builder.getInt32(DummyIdx++), BB);
       DTUpdates.push_back({DominatorTree::Insert, Preheader, BB});
       ++NumLoopExitsDeleted;
     }
 
     assert(L.getLoopPreheader() == NewPreheader && "Malformed CFG?");
     if (Loop *OuterLoop = LI.getLoopFor(Preheader)) {

llvm/test/Transforms/LoopSimplifyCFG/handle_dead_exits.ll

+; RUN: opt -S -enable-loop-simplifycfg-term-folding=true -loop-simplifycfg %s | FileCheck %s
+; RUN: opt -S -enable-loop-simplifycfg-term-folding=true -passes='require<domtree>,loop(simplify-cfg)' %s | FileCheck %s
+
+declare i32* @fake_personality_function()
+declare void @foo()
+
+define i32 @test_remove_lpad(i1 %exitcond) personality i32* ()* @fake_personality_function {
+; CHECK-LABEL: @test_remove_lpad(
+entry:
+  br label %for.body
+
+for.body:
+  br i1 0, label %never, label %next
+
+next:
+  br label %latch
+
+latch:
+  br i1 %exitcond, label %exit, label %for.body
+
+exit:
+  ret i32 0
+
+never:
+  invoke void @foo() to label %next unwind label %never-unwind
+
+never-unwind:
+; CHECK: never-unwind:
+; CHECK-NEXT: unreachable
+  %res = landingpad token cleanup
+  unreachable
+}
+
+define i32 @test_remove_phi_lpad(i1 %exitcond) personality i32* ()* @fake_personality_function {
+; CHECK-LABEL: @test_remove_phi_lpad(
+entry:
+  br label %for.body
+
+for.body:
+  br i1 0, label %never, label %next
+
+next:
+  br label %latch
+
+latch:
+  br i1 %exitcond, label %exit, label %for.body
+
+exit:
+  ret i32 0
+
+never:
+  invoke void @foo() to label %next unwind label %never-unwind
+
+never-unwind:
+; CHECK: never-unwind:
+; CHECK-NEXT: ret i32 undef
+  %p = phi i32 [1, %never]
+  %res = landingpad token cleanup
+  ret i32 %p
+}
+
+define i32 @test_split_remove_phi_lpad_(i1 %exitcond) personality i32* ()* @fake_personality_function {
+; CHECK-LABEL: @test_split_remove_phi_lpad_(
+entry:
+  invoke void @foo() to label %for.body unwind label %unwind-bb
+
+for.body:
+  br i1 0, label %never, label %next
+
+next:
+  br label %latch
+
+latch:
+  br i1 %exitcond, label %exit, label %for.body
+
+exit:
+  ret i32 0
+
+never:
+  invoke void @foo() to label %next unwind label %unwind-bb
+
+unwind-bb:
+; CHECK: unwind-bb.loopexit:
+; CHECK-NEXT: br label %unwind-bb
+  %p = phi i32 [1, %never], [2, %entry]
+  %res = landingpad token cleanup
+  ret i32 %p
+}