[InstCombine] Don't aggressively replace xor with icmp

For some cases, InstCombine replaces the sequence of xor/sub instruction
followed by cmp instruction into a single cmp instruction.

However, this replacement may result suboptimal result especially when
the xor/sub has more than one use, as discussed in
bug 26465 (https://llvm.org/bugs/show_bug.cgi?id=26465).

This patch make the replacement happen only when xor/sub has only one
use.

Differential Revision: http://reviews.llvm.org/D16915

Patch by Taewook Oh!

llvm-svn: 260695

1 files changed, 20 insertions, 17 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 990305b1d8d..1e6e0727955 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2212,27 +2212,30 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
         }
         break;
       case Instruction::Xor:
-        // For the xor case, we can xor two constants together, eliminating
-        // the explicit xor.
-        if (Constant *BOC = dyn_cast<Constant>(BO->getOperand(1))) {
-          return new ICmpInst(ICI.getPredicate(), BO->getOperand(0),
-                              ConstantExpr::getXor(RHS, BOC));
-        } else if (RHSV == 0) {
-          // Replace ((xor A, B) != 0) with (A != B)
-          return new ICmpInst(ICI.getPredicate(), BO->getOperand(0),
-                              BO->getOperand(1));
+        if (BO->hasOneUse()) {
+          if (Constant *BOC = dyn_cast<Constant>(BO->getOperand(1))) {
+            // For the xor case, we can xor two constants together, eliminating
+            // the explicit xor.
+            return new ICmpInst(ICI.getPredicate(), BO->getOperand(0),
+                ConstantExpr::getXor(RHS, BOC));
+          } else if (RHSV == 0) {
+            // Replace ((xor A, B) != 0) with (A != B)
+            return new ICmpInst(ICI.getPredicate(), BO->getOperand(0),
+                BO->getOperand(1));
+          }
         }
         break;
       case Instruction::Sub:
-        // Replace ((sub A, B) != C) with (B != A-C) if A & C are constants.
-        if (ConstantInt *BOp0C = dyn_cast<ConstantInt>(BO->getOperand(0))) {
-          if (BO->hasOneUse())
+        if (BO->hasOneUse()) {
+          if (ConstantInt *BOp0C = dyn_cast<ConstantInt>(BO->getOperand(0))) {
+            // Replace ((sub A, B) != C) with (B != A-C) if A & C are constants.
             return new ICmpInst(ICI.getPredicate(), BO->getOperand(1),
-                                ConstantExpr::getSub(BOp0C, RHS));
-        } else if (RHSV == 0) {
-          // Replace ((sub A, B) != 0) with (A != B)
-          return new ICmpInst(ICI.getPredicate(), BO->getOperand(0),
-                              BO->getOperand(1));
+                ConstantExpr::getSub(BOp0C, RHS));
+          } else if (RHSV == 0) {
+            // Replace ((sub A, B) != 0) with (A != B)
+            return new ICmpInst(ICI.getPredicate(), BO->getOperand(0),
+                BO->getOperand(1));
+          }
         }
         break;
       case Instruction::Or: