Add a small transform: transform -(X<<Y) to (-X<<Y) when the shift has a single

use and X is free to negate.

llvm-svn: 94941

lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -676,6 +676,13 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
               return BinaryOperator::CreateSDiv(Op1I->getOperand(0),
                                           ConstantExpr::getNeg(DivRHS));
 
+      // 0 - (C << X)  -> (-C << X)
+      if (Op1I->getOpcode() == Instruction::Shl)
+        if (ConstantInt *CSI = dyn_cast<ConstantInt>(Op0))
+          if (CSI->isZero())
+            if (Value *ShlLHSNeg = dyn_castNegVal(Op1I->getOperand(0)))
+              return BinaryOperator::CreateShl(ShlLHSNeg, Op1I->getOperand(1));
+
       // X - X*C --> X * (1-C)
       ConstantInt *C2 = 0;
       if (dyn_castFoldableMul(Op1I, C2) == Op0) {

test/Transforms/InstCombine/sub.ll

@@ -272,4 +272,12 @@ define i64 @test25(i8* %P, i64 %A){
 ; CHECK-NEXT: ret i64 
 }
 
+define i32 @test26(i32 %x) {
+  %shl = shl i32 3, %x
+  %neg = sub i32 0, %shl
+  ret i32 %neg
+; CHECK: @test26
+; CHECK-NEXT: shl i32 -3
+; CHECK-NEXT: ret i32
+}