1 files changed, 46 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 18a08725039..29de6f7b889 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2037,14 +2037,59 @@ static APInt DemandedBitsLHSMask(ICmpInst &I,
 
 }
 
+/// \brief Check if the order of \p Op0 and \p Op1 as operand in an ICmpInst
+/// should be swapped.
+/// The descision is based on how many times these two operands are reused
+/// as subtract operands and their positions in those instructions.
+/// The rational is that several architectures use the same instruction for
+/// both subtract and cmp, thus it is better if the order of those operands
+/// match.
+/// \return true if Op0 and Op1 should be swapped.
+static bool swapMayExposeCSEOpportunities(const Value * Op0,
+                                          const Value * Op1) {
+  // Filter out pointer value as those cannot appears directly in subtract.
+  // FIXME: we may want to go through inttoptrs or bitcasts.
+  if (Op0->getType()->isPointerTy())
+    return false;
+  // Count every uses of both Op0 and Op1 in a subtract.
+  // Each time Op0 is the first operand, count -1: swapping is bad, the
+  // subtract has already the same layout as the compare.
+  // Each time Op0 is the second operand, count +1: swapping is good, the
+  // subtract has a diffrent layout as the compare.
+  // At the end, if the benefit is greater than 0, Op0 should come second to
+  // expose more CSE opportunities.
+  int GlobalSwapBenefits = 0;
+  for (Value::const_use_iterator UI = Op0->use_begin(), UIEnd = Op0->use_end(); UI != UIEnd; ++UI) {
+    const BinaryOperator *BinOp = dyn_cast<BinaryOperator>(*UI);
+    if (!BinOp || BinOp->getOpcode() != Instruction::Sub)
+      continue;
+    // If Op0 is the first argument, this is not beneficial to swap the
+    // arguments.
+    int LocalSwapBenefits = -1;
+    unsigned Op1Idx = 1;
+    if (BinOp->getOperand(Op1Idx) == Op0) {
+      Op1Idx = 0;
+      LocalSwapBenefits = 1;
+    }
+    if (BinOp->getOperand(Op1Idx) != Op1)
+      continue;
+    GlobalSwapBenefits += LocalSwapBenefits;
+  }
+  return GlobalSwapBenefits > 0;
+}
+
 Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
   bool Changed = false;
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
+  unsigned Op0Cplxity = getComplexity(Op0);
+  unsigned Op1Cplxity = getComplexity(Op1);
 
   /// Orders the operands of the compare so that they are listed from most
   /// complex to least complex.  This puts constants before unary operators,
   /// before binary operators.
-  if (getComplexity(Op0) < getComplexity(Op1)) {
+  if (Op0Cplxity < Op1Cplxity ||
+        (Op0Cplxity == Op1Cplxity &&
+         swapMayExposeCSEOpportunities(Op0, Op1))) {
     I.swapOperands();
     std::swap(Op0, Op1);
     Changed = true;