[SCEV] Add one more case in computeConstantDifference

This patch teaches `computeConstantDifference` handle calculation of constant
difference between `(X + C1)` and `(X + C2)` which is `(C2 - C1)`.

Differential Revision: https://reviews.llvm.org/D43759
Reviewed By: anna

llvm-svn: 328609

1 files changed, 18 insertions, 10 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 004eac7f555..b44107d9a89 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -9449,17 +9449,25 @@ Optional<APInt> ScalarEvolution::computeConstantDifference(const SCEV *More,
     return M - L;
   }
 
-  const SCEV *L, *R;
   SCEV::NoWrapFlags Flags;
-  if (splitBinaryAdd(Less, L, R, Flags))
-    if (const auto *LC = dyn_cast<SCEVConstant>(L))
-      if (R == More)
-        return -(LC->getAPInt());
-
-  if (splitBinaryAdd(More, L, R, Flags))
-    if (const auto *LC = dyn_cast<SCEVConstant>(L))
-      if (R == Less)
-        return LC->getAPInt();
+  const SCEV *LLess = nullptr, *RLess = nullptr;
+  const SCEV *LMore = nullptr, *RMore = nullptr;
+  const SCEVConstant *C1 = nullptr, *C2 = nullptr;
+  // Compare (X + C1) vs X.
+  if (splitBinaryAdd(Less, LLess, RLess, Flags))
+    if ((C1 = dyn_cast<SCEVConstant>(LLess)))
+      if (RLess == More)
+        return -(C1->getAPInt());
+
+  // Compare X vs (X + C2).
+  if (splitBinaryAdd(More, LMore, RMore, Flags))
+    if ((C2 = dyn_cast<SCEVConstant>(LMore)))
+      if (RMore == Less)
+        return C2->getAPInt();
+
+  // Compare (X + C1) vs (X + C2).
+  if (C1 && C2 && RLess == RMore)
+    return C2->getAPInt() - C1->getAPInt();
 
   return None;
 }