Fix a problem that Nate noticed that boils down to an over conservative check

in the code that does "select C, (X+Y), (X-Y) --> (X+(select C, Y, (-Y)))".
We now compile this loop:

LBB1_1: ; no_exit
        add r6, r2, r3
        subf r3, r2, r3
        cmpwi cr0, r2, 0
        addi r7, r5, 4
        lwz r2, 0(r5)
        addi r4, r4, 1
        blt cr0, LBB1_4 ; no_exit
LBB1_3: ; no_exit
        mr r3, r6
LBB1_4: ; no_exit
        cmpwi cr0, r4, 16
        mr r5, r7
        bne cr0, LBB1_1 ; no_exit

into this instead:

LBB1_1: ; no_exit
        srawi r6, r2, 31
        add r2, r2, r6
        xor r6, r2, r6
        addi r7, r5, 4
        lwz r2, 0(r5)
        addi r4, r4, 1
        add r3, r3, r6
        cmpwi cr0, r4, 16
        mr r5, r7
        bne cr0, LBB1_1 ; no_exit


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26356 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 17 insertions, 22 deletions

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 7b01531fb2..b98228c092 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -5074,30 +5074,25 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
           }
 
           if (OtherAddOp) {
-            // So at this point we know we have:
-            //        select C, (add X, Y), (sub X, ?)
-            // We can do the transform profitably if either 'Y' = '?' or '?' is
-            // a constant.
-            if (SubOp->getOperand(1) == AddOp ||
-                isa<Constant>(SubOp->getOperand(1))) {
-              Value *NegVal;
-              if (Constant *C = dyn_cast<Constant>(SubOp->getOperand(1))) {
-                NegVal = ConstantExpr::getNeg(C);
-              } else {
-                NegVal = InsertNewInstBefore(
-                           BinaryOperator::createNeg(SubOp->getOperand(1)), SI);
-              }
+            // So at this point we know we have (Y -> OtherAddOp):
+            //        select C, (add X, Y), (sub X, Z)
+            Value *NegVal;  // Compute -Z
+            if (Constant *C = dyn_cast<Constant>(SubOp->getOperand(1))) {
+              NegVal = ConstantExpr::getNeg(C);
+            } else {
+              NegVal = InsertNewInstBefore(
+                    BinaryOperator::createNeg(SubOp->getOperand(1), "tmp"), SI);
+            }
 
-              Value *NewTrueOp = OtherAddOp;
-              Value *NewFalseOp = NegVal;
-              if (AddOp != TI)
-                std::swap(NewTrueOp, NewFalseOp);
-              Instruction *NewSel =
-                new SelectInst(CondVal, NewTrueOp,NewFalseOp,SI.getName()+".p");
+            Value *NewTrueOp = OtherAddOp;
+            Value *NewFalseOp = NegVal;
+            if (AddOp != TI)
+              std::swap(NewTrueOp, NewFalseOp);
+            Instruction *NewSel =
+              new SelectInst(CondVal, NewTrueOp,NewFalseOp,SI.getName()+".p");
 
-              NewSel = InsertNewInstBefore(NewSel, SI);
-              return BinaryOperator::createAdd(SubOp->getOperand(0), NewSel);
-            }
+            NewSel = InsertNewInstBefore(NewSel, SI);
+            return BinaryOperator::createAdd(SubOp->getOperand(0), NewSel);
           }
         }
       }