From: Jin-Gu Kang <jaykang10@imrc.kist.re.kr>
Date: Sun, 13 Mar 2011 14:05:51 +0000 (+0000)
Subject: Add comment as following:
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=198e97c4238079f6af0ffd675c2339dcc46b9d8e;p=oota-llvm.git

Add comment as following:
load and store reference same memory location, the memory location
is represented by getelementptr with two uses (load and store) and
the getelementptr's base is alloca with single use. At this point,
instructions from alloca to store can be removed.
(this pattern is generated when bitfield is accessed.)
For example,
%u = alloca %struct.test, align 4               ; [#uses=1]
%0 = getelementptr inbounds %struct.test* %u, i32 0, i32 0;[#uses=2]
%1 = load i8* %0, align 4                       ; [#uses=1]
%2 = and i8 %1, -16                             ; [#uses=1]
%3 = or i8 %2, 5                                ; [#uses=1]
store i8 %3, i8* %0, align 4


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127565 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index f233ca6af1d..e5ff00fc1dc 100644
--- a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -433,6 +433,18 @@ Instruction *InstCombiner::visitStoreInst(StoreInst &SI) {
           !SI.isVolatile()) {
         if (LI == Val)
           return EraseInstFromFunction(SI);
+        // load and store reference same memory location, the memory location
+        // is represented by getelementptr with two uses (load and store) and
+        // the getelementptr's base is alloca with single use. At this point,
+        // instructions from alloca to store can be removed.
+        // (this pattern is generated when bitfield is accessed.)
+        // For example,
+        // %u = alloca %struct.test, align 4               ; [#uses=1]
+        // %0 = getelementptr inbounds %struct.test* %u, i32 0, i32 0;[#uses=2]
+        // %1 = load i8* %0, align 4                       ; [#uses=1]
+        // %2 = and i8 %1, -16                             ; [#uses=1]
+        // %3 = or i8 %2, 5                                ; [#uses=1]
+        // store i8 %3, i8* %0, align 4
         if (Ptr->hasNUses(2)) {
           if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr)) {
             if (isa<AllocaInst>(GEP->getOperand(0))) {