[llvm] r301816 - Take indirect branch into account as well when folding.

[Xin Tong via llvm-commits]

Author: trentxintong
Date: Mon May  1 12:15:37 2017
New Revision: 301816

URL: http://llvm.org/viewvc/llvm-project?rev=301816&view=rev
Log:
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.

Modified:
    llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
    llvm/trunk/test/Transforms/JumpThreading/fold-not-thread.ll

Modified: llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp?rev=301816&r1=301815&r2=301816&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp Mon May  1 12:15:37 2017
@@ -1253,16 +1253,12 @@ bool JumpThreadingPass::ProcessThreadabl
   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::ProcessThreadabl
         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::ProcessThreadabl
   // 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)) {

Modified: llvm/trunk/test/Transforms/JumpThreading/fold-not-thread.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/JumpThreading/fold-not-thread.ll?rev=301816&r1=301815&r2=301816&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/JumpThreading/fold-not-thread.ll (original)
+++ llvm/trunk/test/Transforms/JumpThreading/fold-not-thread.ll Mon May  1 12:15:37 2017
@@ -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
+}