From 46f00a25f91b2a5f744192ad8be3459bcbfbcfbb Mon Sep 17 00:00:00 2001
From: Dan Gohman <gohman@apple.com>
Date: Tue, 20 Jul 2010 16:53:00 +0000
Subject: [PATCH] Add a fast path for x - x.

llvm-svn: 108855
---
 llvm/lib/Analysis/ScalarEvolution.cpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index f0bc9360b16d..179aa8867d6d 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -2442,6 +2442,10 @@ const SCEV *ScalarEvolution::getNotSCEV(const SCEV *V) {
 ///
 const SCEV *ScalarEvolution::getMinusSCEV(const SCEV *LHS,
                                           const SCEV *RHS) {
+  // Fast path: X - X --> 0.
+  if (LHS == RHS)
+    return getConstant(LHS->getType(), 0);
+
   // X - Y --> X + -Y
   return getAddExpr(LHS, getNegativeSCEV(RHS));
 }