From 03edcd68dd739cce53256f8573a141b09ea49ae3 Mon Sep 17 00:00:00 2001
From: Nikolai Bozhenov <nikolai.bozhenov@intel.com>
Date: Wed, 7 Aug 2019 17:38:38 +0000
Subject: [SCEV] Return zero from computeConstantDifference(X, X)

Without this patch computeConstantDifference returns None for cases like
these:

  computeConstantDifference(%x, %x)
  computeConstantDifference({%x,+,16}, {%x,+,16})

Differential Revision: https://reviews.llvm.org/D65474

llvm-svn: 368193
---
 llvm/lib/Analysis/ScalarEvolution.cpp | 4 ++++
 1 file changed, 4 insertions(+)

(limited to 'llvm/lib/Analysis/ScalarEvolution.cpp')

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index bc2cfd6fcc4..8552d784c43 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -9833,6 +9833,10 @@ Optional<APInt> ScalarEvolution::computeConstantDifference(const SCEV *More,
   // We avoid subtracting expressions here because this function is usually
   // fairly deep in the call stack (i.e. is called many times).
 
+  // X - X = 0.
+  if (More == Less)
+    return APInt(getTypeSizeInBits(More->getType()), 0);
+
   if (isa<SCEVAddRecExpr>(Less) && isa<SCEVAddRecExpr>(More)) {
     const auto *LAR = cast<SCEVAddRecExpr>(Less);
     const auto *MAR = cast<SCEVAddRecExpr>(More);
-- 
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