[Reassociate] Convert shl by constant into multiply if it feeds a negation.

[mcrosier]

Summary

See the test case for an example of what this hits.

This is an alternative to D29777.

Chad

[mcrosier]

Abandon in favor of rL296003.

lib/Transforms/Scalar/Reassociate.cpp

 /// instructions is not allowed.
 void ReassociatePass::OptimizeInst(Instruction *I) {
   // Only consider operations that we understand.
   if (!isa<BinaryOperator>(I))
     return;
 
   if (I->getOpcode() == Instruction::Shl && isa<ConstantInt>(I->getOperand(1)))
     // If an operand of this shift is a reassociable multiply, or if the shift
-    // is used by a reassociable multiply or add, turn into a multiply.
+    // is used by a reassociable multiply, add or negate, turn into a multiply.
     if (isReassociableOp(I->getOperand(0), Instruction::Mul) ||
         (I->hasOneUse() &&
          (isReassociableOp(I->user_back(), Instruction::Mul) ||
-          isReassociableOp(I->user_back(), Instruction::Add)))) {
+          isReassociableOp(I->user_back(), Instruction::Add) ||
+          BinaryOperator::isNeg(I->user_back())))) {
       Instruction *NI = ConvertShiftToMul(I);
       RedoInsts.insert(I);
       MadeChange = true;
       I = NI;
     }
 
   // Canonicalize negative constants out of expressions.
   if (Instruction *Res = canonicalizeNegConstExpr(I))

test/Transforms/Reassociate/basictest.ll

define i32 @test15(i32 %X1, i32 %X2, i32 %X3) {
   %A = icmp ne i32 %X1, 0
   %B = icmp slt i32 %X2, %X3
   %C = and i1 %A, %B
   %D = select i1 %C, i32 %X1, i32 0
   ret i32 %D
 ; CHECK-LABEL: @test15
 ; CHECK: and i1 %A, %B
 }
+
+; -(a << 1)*2 + b --> -4a + b
+define i32 @test16(i1 %cmp, i32 %a, i32 %b) {
+entry:
+  %shl = shl i32 %a, 1
+  %shl.neg = sub i32 0, %shl
+  %add1 = mul i32 %shl.neg, 2
+  %add2 = add i32 %add1, %b
+  ret i32 %add2
+
+; CHECK-LABEL: @test16
+; CHECK: %[[MUL:.*]] = mul i32 %a, -4
+; CHECK: %[[ADD:.*]] = add i32 %[[MUL]], %b
+; CHECK: ret i32 %[[ADD]]
+}