[SimplifyCFG] Merge together cleanuppads

[majnemer]

Summary

Cleanuppads may be merged together if one is the only predecessor of the
other in which case a simple transform can be performed: replace the
a cleanupret with a branch and remove an unnecessary cleanuppad.

[rnk]

lgtm

llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp

for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE;) {
     removeUnwindEdge(Pred);
   }
 
   // The landingpad is now unreachable.  Zap it.
   BB->eraseFromParent();
   return true;
 }
 
-bool SimplifyCFGOpt::SimplifyCleanupReturn(CleanupReturnInst *RI) {
+static bool removeEmptyCleanup(CleanupReturnInst *RI) {
   // If this is a trivial cleanup pad that executes no instructions, it can be
   // eliminated.  If the cleanup pad continues to the caller, any predecessor
   // that is an EH pad will be updated to continue to the caller and any
   // predecessor that terminates with an invoke instruction will have its invoke
   // instruction converted to a call instruction.  If the cleanup pad being
   // simplified does not continue to the caller, each predecessor will be
   // updated to continue to the unwind destination of the cleanup pad being
   // simplified.
for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE;) {
     }
   }
 
   // The cleanup pad is now unreachable.  Zap it.
   BB->eraseFromParent();
   return true;
 }
 
+// Try to merge two cleanuppads together.
+static bool mergeCleanupPad(CleanupReturnInst *RI) {
+  // Skip any cleanuprets which unwind to caller, there is nothing to merge
+  // with.
+  BasicBlock *UnwindDest = RI->getUnwindDest();
+  if (!UnwindDest)
+    return false;
+
+  // This cleanupret isn't the only predecessor of this cleanuppad, it wouldn't
+  // be safe to merge without code duplication.
+  if (UnwindDest->getSinglePredecessor() != RI->getParent())
+    return false;
+
+  // Verify that our cleanuppad's unwind destination is another cleanuppad.
+  auto *SuccessorCleanupPad = dyn_cast<CleanupPadInst>(&UnwindDest->front());
+  if (!SuccessorCleanupPad)
+    return false;
+
+  CleanupPadInst *PredecessorCleanupPad = RI->getCleanupPad();
+  // Replace any uses of the successor cleanupad with the predecessor pad
+  // The only cleanuppad uses should be this cleanupret, it's cleanupret and
+  // funclet bundle operands.
+  SuccessorCleanupPad->replaceAllUsesWith(PredecessorCleanupPad);
+  // Remove the old cleanuppad.
+  SuccessorCleanupPad->eraseFromParent();
+  // Now, we simply replace the cleanupret with a branch to the unwind
+  // destination.
+  BranchInst::Create(UnwindDest, RI->getParent());
+  RI->eraseFromParent();
+
+  return true;
+}
+
+bool SimplifyCFGOpt::SimplifyCleanupReturn(CleanupReturnInst *RI) {
+  if (removeEmptyCleanup(RI))
+    return true;
+
+  if (mergeCleanupPad(RI))
+    return true;
+
+  return false;
+}
+
 bool SimplifyCFGOpt::SimplifyReturn(ReturnInst *RI, IRBuilder<> &Builder) {
   BasicBlock *BB = RI->getParent();
   if (!BB->getFirstNonPHIOrDbg()->isTerminator()) return false;
 
   // Find predecessors that end with branches.
   SmallVector<BasicBlock*, 8> UncondBranchPreds;
   SmallVector<BranchInst*, 8> CondBranchPreds;
   for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
for (BasicBlock *OtherPred : predecessors(Succ)) {
     LandingPadInst *LPad2 = dyn_cast<LandingPadInst>(I);
     if (!LPad2 || !LPad2->isIdenticalTo(LPad))
       continue;
     for (++I; isa<DbgInfoIntrinsic>(I); ++I) {}
     BranchInst *BI2 = dyn_cast<BranchInst>(I);
     if (!BI2 || !BI2->isIdenticalTo(BI))
       continue;
 
-    // We've found an identical block.  Update our predeccessors to take that
+    // We've found an identical block.  Update our predecessors to take that
     // path instead and make ourselves dead.
     SmallSet<BasicBlock *, 16> Preds;
     Preds.insert(pred_begin(BB), pred_end(BB));
     for (BasicBlock *Pred : Preds) {
       InvokeInst *II = cast<InvokeInst>(Pred->getTerminator());
       assert(II->getNormalDest() != BB &&
              II->getUnwindDest() == BB && "unexpected successor");
       II->setUnwindDest(OtherPred);

llvm/trunk/test/Transforms/SimplifyCFG/merge-cleanuppads.ll

+; RUN: opt -S -simplifycfg < %s | FileCheck %s
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc18.0.0"
+
+; Function Attrs: uwtable
+define void @test1() #0 personality i32 (...)* @__CxxFrameHandler3 {
+entry:
+  invoke void @may_throw(i32 3)
+          to label %invoke.cont unwind label %ehcleanup
+
+invoke.cont:                                      ; preds = %entry
+  tail call void @may_throw(i32 2) #2
+  tail call void @may_throw(i32 1) #2
+  ret void
+
+ehcleanup:                                        ; preds = %entry
+  %cp = cleanuppad within none []
+  tail call void @may_throw(i32 2) #2 [ "funclet"(token %cp) ]
+  cleanupret from %cp unwind label %ehcleanup2
+
+ehcleanup2:
+  %cp2 = cleanuppad within none []
+  tail call void @may_throw(i32 1) #2 [ "funclet"(token %cp2) ]
+  cleanupret from %cp2 unwind to caller
+}
+
+; CHECK-LABEL: define void @test1(
+; CHECK: %[[cp:.*]] = cleanuppad within none []
+; CHECK: tail call void @may_throw(i32 2) #2 [ "funclet"(token %[[cp]]) ]
+; CHECK: tail call void @may_throw(i32 1) #2 [ "funclet"(token %[[cp]]) ]
+; CHECK: cleanupret from %[[cp]] unwind to caller
+
+declare void @may_throw(i32) #1
+
+declare i32 @__CxxFrameHandler3(...)
+
+attributes #0 = { uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #2 = { nounwind }