Patch to combine GEPs

0001-Combine-GEPs.patch

From fe155df2fd7778c6602696055f7ebd0ddf433c3d Mon Sep 17 00:00:00 2001
From: Andrea Canciani <ranma42@gmail.com>
Date: Fri, 2 Dec 2016 16:21:02 +0100
Subject: [PATCH] Combine GEPs

---
 lib/Transforms/InstCombine/InstructionCombining.cpp | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/lib/Transforms/InstCombine/InstructionCombining.cpp b/lib/Transforms/InstCombine/InstructionCombining.cpp
index cdbc8eb..2339714 100644
--- a/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1567,12 +1567,18 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
         // normalized.
         if (SO1->getType() != GO1->getType())
           return nullptr;
-        // Only do the combine when GO1 and SO1 are both constants. Only in
-        // this case, we are sure the cost after the merge is never more than
-        // that before the merge.
-        if (!isa<Constant>(GO1) || !isa<Constant>(SO1))
+
+        // Only do the combine when we are sure the cost after the
+        // merge is never more than that before the merge.
+
+        // GO1 and SO1 are both constants, combine them
+        if (isa<Constant>(GO1) && isa<Constant>(SO1))
+          Sum = Builder->CreateAdd(SO1, GO1, PtrOp->getName()+".sum");
+        // GO1 = V - SO1, combine them
+        else if (match(GO1, m_Sub(m_Value(Sum), m_Specific(SO1))))
+          ;
+        else
           return nullptr;
-        Sum = Builder->CreateAdd(SO1, GO1, PtrOp->getName()+".sum");
       }
 
       // Update the GEP in place if possible.
-- 
2.7.4