Reapply 132348 with fixes. rdar://problem/6501862

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132402 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 15 insertions, 9 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index 94b619b203..2d29403097 100644
--- a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -136,19 +136,25 @@ Instruction *InstCombiner::visitMul(BinaryOperator &I) {
       }
     }
 
-    // (1 - X) * (-2) -> (x - 1) * 2, for all positive nonzero powers of 2
-    // The "* 2" thus becomes a potential shifting opportunity.
+    // (Y - X) * (-(2**n)) -> (X - Y) * (2**n), for positive nonzero n
+    // (Y + const) * (-(2**n)) -> (-constY) * (2**n), for positive nonzero n
+    // The "* (2**n)" thus becomes a potential shifting opportunity.
     {
       const APInt &   Val = CI->getValue();
       const APInt &PosVal = Val.abs();
       if (Val.isNegative() && PosVal.isPowerOf2()) {
-        Value *X = 0;
-        if (match(Op0, m_Sub(m_One(), m_Value(X)))) {
-          // ConstantInt::get(Op0->getType(), 2);
-          Value *Sub = Builder->CreateSub(X, ConstantInt::get(X->getType(), 1),
-                                          "dec1");
-          return BinaryOperator::CreateMul(Sub, ConstantInt::get(X->getType(),
-                                                                 PosVal));
+        Value *X = 0, *Y = 0;
+        if (Op0->hasOneUse()) {
+          ConstantInt *C1;
+          Value *Sub = 0;
+          if (match(Op0, m_Sub(m_Value(Y), m_Value(X))))
+            Sub = Builder->CreateSub(X, Y, "suba");
+          else if (match(Op0, m_Add(m_Value(Y), m_ConstantInt(C1))))
+            Sub = Builder->CreateSub(Builder->CreateNeg(C1), Y, "subc");
+          if (Sub)
+            return
+              BinaryOperator::CreateMul(Sub,
+                                        ConstantInt::get(Y->getType(), PosVal));
         }
       }
     }