[InstCombine] don't form select from logic ops if it's unlikely that we'll eliminate any ops

llvm-svn: 274926

2 files changed, 23 insertions, 20 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index eadf0bf67a4..3865572e816 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -2222,23 +2222,28 @@ Instruction *InstCombiner::visitOr(BinaryOperator &I) {
       }
     }
 
-    // (Cond & C) | (~Cond & D) -> Cond ? C : D, and commuted variants.
-    if (Value *V = matchSelectFromAndOr(A, C, B, D, *Builder))
-      return replaceInstUsesWith(I, V);
-    if (Value *V = matchSelectFromAndOr(A, C, D, B, *Builder))
-      return replaceInstUsesWith(I, V);
-    if (Value *V = matchSelectFromAndOr(C, A, B, D, *Builder))
-      return replaceInstUsesWith(I, V);
-    if (Value *V = matchSelectFromAndOr(C, A, D, B, *Builder))
-      return replaceInstUsesWith(I, V);
-    if (Value *V = matchSelectFromAndOr(B, D, A, C, *Builder))
-      return replaceInstUsesWith(I, V);
-    if (Value *V = matchSelectFromAndOr(B, D, C, A, *Builder))
-      return replaceInstUsesWith(I, V);
-    if (Value *V = matchSelectFromAndOr(D, B, A, C, *Builder))
-      return replaceInstUsesWith(I, V);
-    if (Value *V = matchSelectFromAndOr(D, B, C, A, *Builder))
-      return replaceInstUsesWith(I, V);
+    // Don't try to form a select if it's unlikely that we'll get rid of at
+    // least one of the operands. A select is generally more expensive than the
+    // 'or' that it is replacing.
+    if (Op0->hasOneUse() || Op1->hasOneUse()) {
+      // (Cond & C) | (~Cond & D) -> Cond ? C : D, and commuted variants.
+      if (Value *V = matchSelectFromAndOr(A, C, B, D, *Builder))
+        return replaceInstUsesWith(I, V);
+      if (Value *V = matchSelectFromAndOr(A, C, D, B, *Builder))
+        return replaceInstUsesWith(I, V);
+      if (Value *V = matchSelectFromAndOr(C, A, B, D, *Builder))
+        return replaceInstUsesWith(I, V);
+      if (Value *V = matchSelectFromAndOr(C, A, D, B, *Builder))
+        return replaceInstUsesWith(I, V);
+      if (Value *V = matchSelectFromAndOr(B, D, A, C, *Builder))
+        return replaceInstUsesWith(I, V);
+      if (Value *V = matchSelectFromAndOr(B, D, C, A, *Builder))
+        return replaceInstUsesWith(I, V);
+      if (Value *V = matchSelectFromAndOr(D, B, A, C, *Builder))
+        return replaceInstUsesWith(I, V);
+      if (Value *V = matchSelectFromAndOr(D, B, C, A, *Builder))
+        return replaceInstUsesWith(I, V);
+    }
 
     // ((A&~B)|(~A&B)) -> A^B
     if ((match(C, m_Not(m_Specific(D))) &&
diff --git a/llvm/test/Transforms/InstCombine/logical-select.ll b/llvm/test/Transforms/InstCombine/logical-select.ll
index 86fa8f0fbcb..190cb653e2a 100644
--- a/llvm/test/Transforms/InstCombine/logical-select.ll
+++ b/llvm/test/Transforms/InstCombine/logical-select.ll
@@ -300,10 +300,8 @@ define i1 @bools_multi_uses2(i1 %a, i1 %b, i1 %c) {
 ; CHECK-NEXT:    [[NOT:%.*]] = xor i1 %c, true
 ; CHECK-NEXT:    [[AND1:%.*]] = and i1 [[NOT]], %a
 ; CHECK-NEXT:    [[AND2:%.*]] = and i1 %c, %b
-; CHECK-NEXT:    [[TMP1:%.*]] = select i1 %c, i1 %b, i1 %a
 ; CHECK-NEXT:    [[ADD:%.*]] = xor i1 [[AND1]], [[AND2]]
-; CHECK-NEXT:    [[AND3:%.*]] = and i1 [[TMP1]], [[ADD]]
-; CHECK-NEXT:    ret i1 [[AND3]]
+; CHECK-NEXT:    ret i1 [[ADD]]
 ;
   %not = xor i1 %c, -1
   %and1 = and i1 %not, %a