[InstCombine]Avoid introduction of unaligned mem access

InstCombine is able to transform mem transfer instrinsic to alone store or store/load pair.
It might result in generation of unaligned atomic load/store which later in backend
will be transformed to libcall. It is not an evident gain and it is better to keep intrinsic as is
and handle it at backend.

Reviewers: reames, anna, apilipenko, mkazantsev
Reviewed By: reames
Subscribers: t.p.northover, jfb, llvm-commits
Differential Revision: https://reviews.llvm.org/D56582

llvm-svn: 351295

Author: Serguei Katkov

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -136,6 +136,14 @@ Instruction *InstCombiner::SimplifyAnyMemTransfer(AnyMemTransferInst *MI) {
   if (Size > 8 || (Size&(Size-1)))
     return nullptr;  // If not 1/2/4/8 bytes, exit.
 
+  // If it is an atomic and alignment is less than the size then we will
+  // introduce the unaligned memory access which will be later transformed
+  // into libcall in CodeGen. This is not evident performance gain so disable
+  // it now.
+  if (isa<AtomicMemTransferInst>(MI))
+    if (CopyDstAlign < Size || CopySrcAlign < Size)
+      return nullptr;
+
   // Use an integer load+store unless we can find something better.
   unsigned SrcAddrSp =
     cast<PointerType>(MI->getArgOperand(1)->getType())->getAddressSpace();
@@ -220,6 +228,18 @@ Instruction *InstCombiner::SimplifyAnyMemSet(AnyMemSetInst *MI) {
   Alignment = MI->getDestAlignment();
   assert(Len && "0-sized memory setting should be removed already.");
 
+  // Alignment 0 is identity for alignment 1 for memset, but not store.
+  if (Alignment == 0)
+    Alignment = 1;
+
+  // If it is an atomic and alignment is less than the size then we will
+  // introduce the unaligned memory access which will be later transformed
+  // into libcall in CodeGen. This is not evident performance gain so disable
+  // it now.
+  if (isa<AtomicMemSetInst>(MI))
+    if (Alignment < Len)
+      return nullptr;
+
   // memset(s,c,n) -> store s, c (for n=1,2,4,8)
   if (Len <= 8 && isPowerOf2_32((uint32_t)Len)) {
     Type *ITy = IntegerType::get(MI->getContext(), Len*8);  // n=1 -> i8.
@@ -229,9 +249,6 @@ Instruction *InstCombiner::SimplifyAnyMemSet(AnyMemSetInst *MI) {
     Type *NewDstPtrTy = PointerType::get(ITy, DstAddrSp);
     Dest = Builder.CreateBitCast(Dest, NewDstPtrTy);
 
-    // Alignment 0 is identity for alignment 1 for memset, but not store.
-    if (Alignment == 0) Alignment = 1;
-
     // Extract the fill value and store.
     uint64_t Fill = FillC->getZExtValue()*0x0101010101010101ULL;
     StoreInst *S = Builder.CreateStore(ConstantInt::get(ITy, Fill), Dest,