Make jump threading eliminate blocks that just contain phi nodes,

debug intrinsics, and an unconditional branch when possible.  This
reuses the TryToSimplifyUncondBranchFromEmptyBlock function split
out of simplifycfg.

llvm-svn: 86722

2 files changed, 34 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 9c9461fa382..1d89399e741 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -115,6 +115,7 @@ bool JumpThreading::runOnFunction(Function &F) {
     bool Changed = false;
     for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
       BasicBlock *BB = I;
+      // Thread all of the branches we can over this block. 
       while (ProcessBlock(BB))
         Changed = true;
       
@@ -129,6 +130,26 @@ bool JumpThreading::runOnFunction(Function &F) {
         LoopHeaders.erase(BB);
         DeleteDeadBlock(BB);
         Changed = true;
+      } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
+        // Can't thread an unconditional jump, but if the block is "almost
+        // empty", we can replace uses of it with uses of the successor and make
+        // this dead.
+        if (BI->isUnconditional() && 
+            BB != &BB->getParent()->getEntryBlock()) {
+          BasicBlock::iterator BBI = BB->getFirstNonPHI();
+          // Ignore dbg intrinsics.
+          while (isa<DbgInfoIntrinsic>(BBI))
+            ++BBI;
+          // If the terminator is the only non-phi instruction, try to nuke it.
+          if (BBI->isTerminator()) {
+            bool Erased = LoopHeaders.erase(BB);
+            
+            if (TryToSimplifyUncondBranchFromEmptyBlock(BB))
+              Changed = true;
+            else if (Erased)
+              LoopHeaders.insert(BB);
+          }
+        }
       }
     }
     AnotherIteration = Changed;
diff --git a/llvm/test/Transforms/JumpThreading/basic.ll b/llvm/test/Transforms/JumpThreading/basic.ll
index 1be086b645d..ccb2a3f8330 100644
--- a/llvm/test/Transforms/JumpThreading/basic.ll
+++ b/llvm/test/Transforms/JumpThreading/basic.ll
@@ -211,19 +211,29 @@ define i32 @test8b(i1 %cond, i1 %cond2) {
 T0:
         %A = call i1 @test8a()
 	br i1 %A, label %T1, label %F1
+        
+; CHECK: T0:
+; CHECK-NEXT: call
+; CHECK-NEXT: br i1 %A, label %T1, label %Y
+
 T1:
         %B = call i1 @test8a()
 	br i1 %B, label %T2, label %F1
+
+; CHECK: T1:
+; CHECK-NEXT: call
+; CHECK-NEXT: br i1 %B, label %T2, label %Y
 T2:
         %C = call i1 @test8a()
 	br i1 %cond, label %T3, label %F1
+
+; CHECK: T2:
+; CHECK-NEXT: call
+; CHECK-NEXT: br i1 %cond, label %T3, label %Y
 T3:
         ret i32 0
 
 F1:
-; TODO: F1 uncond branch block should be removed, T2 should jump directly to Y.
-; CHECK: F1:
-; CHECK-NEXT br label %Y
         %D = phi i32 [0, %T0], [0, %T1], [1, %T2]
         %E = icmp eq i32 %D, 1
         %F = and i1 %E, %cond