FoldBranchToCommonDest merges branches into a single branch with or/and of the condition. It has a heuristics for estimating when some of the dependencies are processed by out-of-order processors. This patch adds another rule to the heuristics that says that if the "BonusInstruction" that we speculatively execute is used by the condition of the second branch then it is okay to hoist it. This change exposes more opportunities for other passes to transform the code. It does not matter that much that we if-convert the code because the selectiondag builder splits or/and branches into multiple branches when profitable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194524 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 36 insertions, 2 deletions

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index d56bb32..ff50b12 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2090,8 +2090,13 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
     // Ensure that any values used in the bonus instruction are also used
     // by the terminator of the predecessor.  This means that those values
     // must already have been resolved, so we won't be inhibiting the
-    // out-of-order core by speculating them earlier.
-    if (BonusInst) {
+    // out-of-order core by speculating them earlier. We also allow
+    // instructions that are used by the terminator's condition because it
+    // exposes more merging opportunities.
+    bool UsedByBranch = (BonusInst && BonusInst->hasOneUse() &&
+                         *BonusInst->use_begin() == Cond);
+
+    if (BonusInst && !UsedByBranch) {
       // Collect the values used by the bonus inst
       SmallPtrSet<Value*, 4> UsedValues;
       for (Instruction::op_iterator OI = BonusInst->op_begin(),
diff --git a/test/Transforms/SimplifyCFG/common-dest-folding.ll b/test/Transforms/SimplifyCFG/common-dest-folding.ll
index 10fb7d4..0aa3b2c 100644
--- a/test/Transforms/SimplifyCFG/common-dest-folding.ll
+++ b/test/Transforms/SimplifyCFG/common-dest-folding.ll
@@ -25,4 +25,33 @@ define i32 @foo(i32 %k, i32 %c1, i32 %c2) {
   ret i32 undef
 }
 
+;CHECK: @conduse
+;CHECK: shl i32 1, %c1
+;CHECK-NEXT: shl i32 1, %c2
+;CHECK-NEXT: and i32
+;CHECK-NEXT: icmp eq i32
+;CHECK-NEXT: and i32
+;CHECK-NEXT: icmp eq i32
+;CHECK: ret
+define i32 @conduse(i32 %k, i32 %c1, i32 %c2) #0 {
+bb:
+  %tmp = shl i32 1, %c1
+  %tmp4 = shl i32 1, %c2
+  %tmp1 = and i32 %tmp, %k
+  %tmp2 = icmp eq i32 %tmp1, 0
+  br i1 %tmp2, label %bb9, label %bb3
+
+bb3:                                              ; preds = %bb
+  %tmp5 = and i32 %tmp4, %k
+  %tmp6 = icmp eq i32 %tmp5, 0
+  br i1 %tmp6, label %bb9, label %bb7
+
+bb7:                                              ; preds = %bb3
+  %tmp8 = tail call i32 (...)* @bar() #1
+  br label %bb9
+
+bb9:                                              ; preds = %bb7, %bb3, %bb
+  ret i32 undef
+}
+
 declare i32 @bar(...)