From a4b9b9f42a514157169c39d0516aac2c4db77c4f Mon Sep 17 00:00:00 2001
From: Xin Tong <trent.xin.tong@gmail.com>
Date: Mon, 1 May 2017 17:15:37 +0000
Subject: [PATCH] Take indirect branch into account as well when folding.

We may not be able to rewrite indirect branch target, but we also want to take it into
account when folding, i.e. if it and all its successor's predecessors go to the same
destination, we can fold, i.e. no need to thread.

llvm-svn: 301816
---
 llvm/lib/Transforms/Scalar/JumpThreading.cpp       | 16 ++++---
 .../Transforms/JumpThreading/fold-not-thread.ll    | 54 ++++++++++++++++++++++
 2 files changed, 64 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 55b556bb..7dacaba 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -1253,16 +1253,12 @@ bool JumpThreadingPass::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
   Constant *OnlyVal = nullptr;
   Constant *MultipleVal = (Constant *)(intptr_t)~0ULL;
 
+  unsigned PredWithKnownDest = 0;
   for (const auto &PredValue : PredValues) {
     BasicBlock *Pred = PredValue.second;
     if (!SeenPreds.insert(Pred).second)
       continue;  // Duplicate predecessor entry.
 
-    // If the predecessor ends with an indirect goto, we can't change its
-    // destination.
-    if (isa<IndirectBrInst>(Pred->getTerminator()))
-      continue;
-
     Constant *Val = PredValue.first;
 
     BasicBlock *DestBB;
@@ -1294,6 +1290,14 @@ bool JumpThreadingPass::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
         OnlyVal = MultipleVal;
     }
 
+    // We know where this predecessor is going.
+    ++PredWithKnownDest;
+
+    // If the predecessor ends with an indirect goto, we can't change its
+    // destination.
+    if (isa<IndirectBrInst>(Pred->getTerminator()))
+      continue;
+
     PredToDestList.push_back(std::make_pair(Pred, DestBB));
   }
 
@@ -1305,7 +1309,7 @@ bool JumpThreadingPass::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
   // not thread. By doing so, we do not need to duplicate the current block and
   // also miss potential opportunities in case we dont/cant duplicate.
   if (OnlyDest && OnlyDest != MultipleDestSentinel) {
-    if (PredToDestList.size() ==
+    if (PredWithKnownDest ==
         (size_t)std::distance(pred_begin(BB), pred_end(BB))) {
       bool SeenFirstBranchToOnlyDest = false;
       for (BasicBlock *SuccBB : successors(BB)) {
diff --git a/llvm/test/Transforms/JumpThreading/fold-not-thread.ll b/llvm/test/Transforms/JumpThreading/fold-not-thread.ll
index 3bd7106..06ddc10 100644
--- a/llvm/test/Transforms/JumpThreading/fold-not-thread.ll
+++ b/llvm/test/Transforms/JumpThreading/fold-not-thread.ll
@@ -189,4 +189,58 @@ L3:
   ret void
 }
 
+; Make sure we can fold this branch ... We will not be able to thread it as
+; L0 is too big to duplicate.
+; We do not attempt to rewrite the indirectbr target here, but we still take
+; its target after L0 into account and that enables us to fold.
+;
+; L2 is the unreachable block here.
+; 
+; CHECK-LABEL: @test_br_folding_not_threading_indirect_branch(
+; CHECK: L1:
+; CHECK: call i32 @f2()
+; CHECK: call void @f3()
+; CHECK-NEXT: ret void
+; CHECK-NOT: br
+; CHECK: L3:
+define void @test_br_folding_not_threading_indirect_branch(i1 %condx, i1 %cond) nounwind {
+entry:
+  br i1 %condx, label %X0, label %X1
+
+X0:
+  br i1 %cond, label %L0, label %L3
+
+X1:
+  br i1 %cond, label %XX1, label %L3
+
+XX1:
+  indirectbr i8* blockaddress(@test_br_folding_not_threading_indirect_branch, %L0), [label %L0]
+
+L0:
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  br i1 %cond, label %L1, label %L2
 
+L1:
+  call void @f3()
+  ret void
+
+L2:
+  call void @f3()
+  ret void
+
+L3:
+  call void @f3()
+  ret void
+}
-- 
2.7.4