[llvm-commits] CVS: llvm/lib/Transforms/Scalar/InstructionCombining.cpp

[Chris Lattner]

Changes in directory llvm/lib/Transforms/Scalar:

InstructionCombining.cpp updated: 1.437 -> 1.438
---
Log message:

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



---
Diffs of the changes:  (+18 -23)

 InstructionCombining.cpp |   41 ++++++++++++++++++-----------------------
 1 files changed, 18 insertions(+), 23 deletions(-)


Index: llvm/lib/Transforms/Scalar/InstructionCombining.cpp
diff -u llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.437 llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.438
--- llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.437	Fri Feb 17 21:20:33 2006
+++ llvm/lib/Transforms/Scalar/InstructionCombining.cpp	Fri Feb 24 12:05:58 2006
@@ -5074,30 +5074,25 @@
           }
 
           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);
-              }
-
-              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);
+            // 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");
+
+            NewSel = InsertNewInstBefore(NewSel, SI);
+            return BinaryOperator::createAdd(SubOp->getOperand(0), NewSel);
           }
         }
       }