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 f233ca6af1d22522fbb74a678f043cf7047b2a31..e5ff00fc1dc09f938d572a17289ee6a4f73ac2d0 100644 (file)
--- 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))) {