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

[Chris Lattner]

Changes in directory llvm/lib/Transforms/Scalar:

InstructionCombining.cpp updated: 1.251 -> 1.252
---
Log message:

shl is always zero extending, so always use a zero extending shift right.
This latent bug was exposed by recent changes, and is tested as:
llvm/test/Regression/Transforms/InstCombine/2004-09-28-BadShiftAndSetCC.llx



---
Diffs of the changes:  (+12 -5)

Index: llvm/lib/Transforms/Scalar/InstructionCombining.cpp
diff -u llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.251 llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.252
--- llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.251	Mon Sep 27 14:29:18 2004
+++ llvm/lib/Transforms/Scalar/InstructionCombining.cpp	Tue Sep 28 12:54:07 2004
@@ -1506,7 +1506,7 @@
           // rights, as they sign-extend.
           if (ShAmt) {
             bool CanFold = Shift->getOpcode() != Instruction::Shr ||
-                          Shift->getType()->isUnsigned();
+                           Shift->getType()->isUnsigned();
             if (!CanFold) {
               // To test for the bad case of the signed shr, see if any
               // of the bits shifted in could be tested after the mask.
@@ -1519,9 +1519,11 @@
             }
             
             if (CanFold) {
-              unsigned ShiftOp = Shift->getOpcode() == Instruction::Shl
-                ? Instruction::Shr : Instruction::Shl;
-              Constant *NewCst = ConstantExpr::get(ShiftOp, CI, ShAmt);
+              Constant *NewCst;
+              if (Shift->getOpcode() == Instruction::Shl)
+                NewCst = ConstantExpr::getUShr(CI, ShAmt);
+              else
+                NewCst = ConstantExpr::getShl(CI, ShAmt);
 
               // Check to see if we are shifting out any of the bits being
               // compared.
@@ -1535,7 +1537,12 @@
                   return ReplaceInstUsesWith(I, ConstantBool::True);
               } else {
                 I.setOperand(1, NewCst);
-                LHSI->setOperand(1, ConstantExpr::get(ShiftOp, AndCST,ShAmt));
+                Constant *NewAndCST;
+                if (Shift->getOpcode() == Instruction::Shl)
+                  NewAndCST = ConstantExpr::getUShr(AndCST, ShAmt);
+                else
+                  NewAndCST = ConstantExpr::getShl(AndCST, ShAmt);
+                LHSI->setOperand(1, NewAndCST);
                 LHSI->setOperand(0, Shift->getOperand(0));
                 WorkList.push_back(Shift); // Shift is dead.
                 AddUsesToWorkList(I);