[MemCpyOpt] Forward memcpy based on the actual copy memory locatio…

…n. (llvm#87190)

Fixes llvm#85560.

We can forward `memcpy` as long as the actual memory location being
copied have not been altered.

alive2: https://alive2.llvm.org/ce/z/q9JaHV

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -14,6 +14,7 @@
 #include "llvm/Transforms/Scalar/MemCpyOptimizer.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/iterator_range.h"
@@ -1124,28 +1125,79 @@ bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
 bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
                                                   MemCpyInst *MDep,
                                                   BatchAAResults &BAA) {
-  // We can only transforms memcpy's where the dest of one is the source of the
-  // other.
-  if (M->getSource() != MDep->getDest() || MDep->isVolatile())
-    return false;
-
   // If dep instruction is reading from our current input, then it is a noop
-  // transfer and substituting the input won't change this instruction.  Just
-  // ignore the input and let someone else zap MDep.  This handles cases like:
+  // transfer and substituting the input won't change this instruction. Just
+  // ignore the input and let someone else zap MDep. This handles cases like:
   //    memcpy(a <- a)
   //    memcpy(b <- a)
   if (M->getSource() == MDep->getSource())
     return false;
 
-  // Second, the length of the memcpy's must be the same, or the preceding one
+  // We can only optimize non-volatile memcpy's.
+  if (MDep->isVolatile())
+    return false;
+
+  int64_t MForwardOffset = 0;
+  const DataLayout &DL = M->getModule()->getDataLayout();
+  // We can only transforms memcpy's where the dest of one is the source of the
+  // other, or they have an offset in a range.
+  if (M->getSource() != MDep->getDest()) {
+    std::optional<int64_t> Offset =
+        M->getSource()->getPointerOffsetFrom(MDep->getDest(), DL);
+    if (!Offset || *Offset < 0)
+      return false;
+    MForwardOffset = *Offset;
+  }
+
+  // The length of the memcpy's must be the same, or the preceding one
   // must be larger than the following one.
-  if (MDep->getLength() != M->getLength()) {
+  if (MForwardOffset != 0 || MDep->getLength() != M->getLength()) {
     auto *MDepLen = dyn_cast<ConstantInt>(MDep->getLength());
     auto *MLen = dyn_cast<ConstantInt>(M->getLength());
-    if (!MDepLen || !MLen || MDepLen->getZExtValue() < MLen->getZExtValue())
+    if (!MDepLen || !MLen ||
+        MDepLen->getZExtValue() < MLen->getZExtValue() + MForwardOffset)
       return false;
   }
 
+  IRBuilder<> Builder(M);
+  auto *CopySource = MDep->getSource();
+  Instruction *NewCopySource = nullptr;
+  auto CleanupOnRet = llvm::make_scope_exit([&NewCopySource] {
+    if (NewCopySource && NewCopySource->use_empty())
+      // Safety: It's safe here because we will only allocate more instructions
+      // after finishing all BatchAA queries, but we have to be careful if we
+      // want to do something like this in another place. Then we'd probably
+      // have to delay instruction removal until all transforms on an
+      // instruction finished.
+      NewCopySource->eraseFromParent();
+  });
+  MaybeAlign CopySourceAlign = MDep->getSourceAlign();
+  // We just need to calculate the actual size of the copy.
+  auto MCopyLoc = MemoryLocation::getForSource(MDep).getWithNewSize(
+      MemoryLocation::getForSource(M).Size);
+
+  // When the forwarding offset is greater than 0, we transform
+  //    memcpy(d1 <- s1)
+  //    memcpy(d2 <- d1+o)
+  // to
+  //    memcpy(d2 <- s1+o)
+  if (MForwardOffset > 0) {
+    // The copy destination of `M` maybe can serve as the source of copying.
+    std::optional<int64_t> MDestOffset =
+        M->getRawDest()->getPointerOffsetFrom(MDep->getRawSource(), DL);
+    if (MDestOffset == MForwardOffset)
+      CopySource = M->getDest();
+    else {
+      NewCopySource = cast<Instruction>(Builder.CreateInBoundsPtrAdd(
+          CopySource, Builder.getInt64(MForwardOffset)));
+      CopySource = NewCopySource;
+    }
+    // We need to update `MCopyLoc` if an offset exists.
+    MCopyLoc = MCopyLoc.getWithNewPtr(CopySource);
+    if (CopySourceAlign)
+      CopySourceAlign = commonAlignment(*CopySourceAlign, MForwardOffset);
+  }
+
   // Verify that the copied-from memory doesn't change in between the two
   // transfers.  For example, in:
   //    memcpy(a <- b)
@@ -1155,14 +1207,12 @@ bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
   //
   // TODO: If the code between M and MDep is transparent to the destination "c",
   // then we could still perform the xform by moving M up to the first memcpy.
-  // TODO: It would be sufficient to check the MDep source up to the memcpy
-  // size of M, rather than MDep.
-  if (writtenBetween(MSSA, BAA, MemoryLocation::getForSource(MDep),
-                     MSSA->getMemoryAccess(MDep), MSSA->getMemoryAccess(M)))
+  if (writtenBetween(MSSA, BAA, MCopyLoc, MSSA->getMemoryAccess(MDep),
+                     MSSA->getMemoryAccess(M)))
     return false;
 
   // No need to create `memcpy(a <- a)`.
-  if (BAA.isMustAlias(M->getDest(), MDep->getSource())) {
+  if (BAA.isMustAlias(M->getDest(), CopySource)) {
     // Remove the instruction we're replacing.
     eraseInstruction(M);
     ++NumMemCpyInstr;
@@ -1191,23 +1241,22 @@ bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
 
   // TODO: Is this worth it if we're creating a less aligned memcpy? For
   // example we could be moving from movaps -> movq on x86.
-  IRBuilder<> Builder(M);
   Instruction *NewM;
   if (UseMemMove)
-    NewM = Builder.CreateMemMove(M->getRawDest(), M->getDestAlign(),
-                                 MDep->getRawSource(), MDep->getSourceAlign(),
-                                 M->getLength(), M->isVolatile());
+    NewM =
+        Builder.CreateMemMove(M->getDest(), M->getDestAlign(), CopySource,
+                              CopySourceAlign, M->getLength(), M->isVolatile());
   else if (isa<MemCpyInlineInst>(M)) {
     // llvm.memcpy may be promoted to llvm.memcpy.inline, but the converse is
     // never allowed since that would allow the latter to be lowered as a call
     // to an external function.
-    NewM = Builder.CreateMemCpyInline(
-        M->getRawDest(), M->getDestAlign(), MDep->getRawSource(),
-        MDep->getSourceAlign(), M->getLength(), M->isVolatile());
+    NewM = Builder.CreateMemCpyInline(M->getDest(), M->getDestAlign(),
+                                      CopySource, CopySourceAlign,
+                                      M->getLength(), M->isVolatile());
   } else
-    NewM = Builder.CreateMemCpy(M->getRawDest(), M->getDestAlign(),
-                                MDep->getRawSource(), MDep->getSourceAlign(),
-                                M->getLength(), M->isVolatile());
+    NewM =
+        Builder.CreateMemCpy(M->getDest(), M->getDestAlign(), CopySource,
+                             CopySourceAlign, M->getLength(), M->isVolatile());
   NewM->copyMetadata(*M, LLVMContext::MD_DIAssignID);
 
   assert(isa<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(M)));

llvm/test/Transforms/MemCpyOpt/lifetime.ll

@@ -124,7 +124,7 @@ define void @call_slot_lifetime_bitcast(ptr %ptr) {
 ; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 8 [[TMP2]], ptr align 4 [[PTR:%.*]], i64 4, i1 false)
 ; CHECK-NEXT:    [[TMP1_CAST:%.*]] = bitcast ptr [[TMP1]] to ptr
 ; CHECK-NEXT:    call void @llvm.lifetime.start.p0(i64 4, ptr nonnull [[TMP1_CAST]])
-; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 4 [[TMP1_CAST]], ptr align 4 [[PTR]], i64 4, i1 false)
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 4 [[TMP1]], ptr align 4 [[PTR]], i64 4, i1 false)
 ; CHECK-NEXT:    ret void
 ;
   %tmp1 = alloca i32

llvm/test/Transforms/MemCpyOpt/memcpy-memcpy-offset.ll

@@ -0,0 +1,203 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt < %s -passes=memcpyopt -S -verify-memoryssa | FileCheck %s
+
+%buf = type [9 x i8]
+
+; We can forward `memcpy` because the copy location are the same,
+define void @forward_offset(ptr %src, ptr %dest) {
+; CHECK-LABEL: define void @forward_offset(
+; CHECK-SAME: ptr [[SRC:%.*]], ptr [[DEST:%.*]]) {
+; CHECK-NEXT:    [[DEP_DEST:%.*]] = alloca [9 x i8], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[DEP_DEST]], ptr align 1 [[SRC]], i64 7, i1 false)
+; CHECK-NEXT:    [[SRC_OFFSET:%.*]] = getelementptr inbounds i8, ptr [[DEP_DEST]], i64 1
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i8, ptr [[SRC]], i64 1
+; CHECK-NEXT:    call void @llvm.memmove.p0.p0.i64(ptr align 1 [[DEST]], ptr align 1 [[TMP1]], i64 6, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %cpy_tmp = alloca %buf, align 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %cpy_tmp, ptr align 1 %src, i64 7, i1 false)
+  %cpy_tmp_offset = getelementptr inbounds i8, ptr %cpy_tmp, i64 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %dest, ptr align 1 %cpy_tmp_offset, i64 6, i1 false)
+  ret void
+}
+
+; We need to update the align value of the source of `memcpy` when forwarding.
+define void @forward_offset_align(ptr %src, ptr %dest) {
+; CHECK-LABEL: define void @forward_offset_align(
+; CHECK-SAME: ptr [[SRC:%.*]], ptr [[DEST:%.*]]) {
+; CHECK-NEXT:    [[DEP_DEST:%.*]] = alloca [9 x i8], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[DEP_DEST]], ptr align 4 [[SRC]], i64 9, i1 false)
+; CHECK-NEXT:    [[TMP_OFFSET:%.*]] = getelementptr inbounds i8, ptr [[DEP_DEST]], i64 3
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i8, ptr [[SRC]], i64 3
+; CHECK-NEXT:    call void @llvm.memmove.p0.p0.i64(ptr align 1 [[DEST]], ptr align 1 [[TMP1]], i64 5, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %cpy_tmp = alloca %buf, align 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %cpy_tmp, ptr align 4 %src, i64 9, i1 false)
+  %cpy_tmp_offset = getelementptr inbounds i8, ptr %cpy_tmp, i64 3
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %dest, ptr align 1 %cpy_tmp_offset, i64 5, i1 false)
+  ret void
+}
+
+; We can change the align value to 2 when forwarding.
+define void @forward_offset_align_2(ptr %src, ptr %dest) {
+; CHECK-LABEL: define void @forward_offset_align_2(
+; CHECK-SAME: ptr [[SRC:%.*]], ptr [[DEST:%.*]]) {
+; CHECK-NEXT:    [[DEP_DEST:%.*]] = alloca [9 x i8], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[DEP_DEST]], ptr align 4 [[SRC]], i64 9, i1 false)
+; CHECK-NEXT:    [[TMP_OFFSET:%.*]] = getelementptr inbounds i8, ptr [[DEP_DEST]], i64 2
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i8, ptr [[SRC]], i64 2
+; CHECK-NEXT:    call void @llvm.memmove.p0.p0.i64(ptr align 1 [[DEST]], ptr align 2 [[TMP1]], i64 6, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %cpy_tmp = alloca %buf, align 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %cpy_tmp, ptr align 4 %src, i64 9, i1 false)
+  %cpy_tmp_offset = getelementptr inbounds i8, ptr %cpy_tmp, i64 2
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %dest, ptr align 1 %cpy_tmp_offset, i64 6, i1 false)
+  ret void
+}
+
+; If the copy destination can be used as the copy source, we don't need to create a GEP instruction.
+define void @forward_offset_without_gep(ptr %src) {
+; CHECK-LABEL: define void @forward_offset_without_gep(
+; CHECK-SAME: ptr [[SRC:%.*]]) {
+; CHECK-NEXT:    [[TMP:%.*]] = alloca [9 x i8], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[TMP]], ptr align 1 [[SRC]], i64 7, i1 false)
+; CHECK-NEXT:    [[TMP_OFFSET:%.*]] = getelementptr inbounds i8, ptr [[TMP]], i64 1
+; CHECK-NEXT:    [[DEST:%.*]] = getelementptr inbounds i8, ptr [[SRC]], i64 1
+; CHECK-NEXT:    ret void
+;
+  %cpy_tmp = alloca %buf, align 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %cpy_tmp, ptr align 1 %src, i64 7, i1 false)
+  %cpy_tmp_offset = getelementptr inbounds i8, ptr %cpy_tmp, i64 1
+  %dest = getelementptr inbounds i8, ptr %src, i64 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %dest, ptr align 1 %cpy_tmp_offset, i64 6, i1 false)
+  ret void
+}
+
+; We need to create a GEP instruction when forwarding.
+define void @forward_offset_with_gep(ptr %src) {
+; CHECK-LABEL: define void @forward_offset_with_gep(
+; CHECK-SAME: ptr [[SRC:%.*]]) {
+; CHECK-NEXT:    [[DEP_DEST:%.*]] = alloca [9 x i8], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[DEP_DEST]], ptr align 1 [[SRC]], i64 7, i1 false)
+; CHECK-NEXT:    [[TMP_OFFSET:%.*]] = getelementptr inbounds i8, ptr [[DEP_DEST]], i64 1
+; CHECK-NEXT:    [[DEST:%.*]] = getelementptr inbounds i8, ptr [[SRC]], i64 2
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i8, ptr [[SRC]], i64 1
+; CHECK-NEXT:    call void @llvm.memmove.p0.p0.i64(ptr align 1 [[DEST]], ptr align 1 [[TMP1]], i64 6, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %cpy_tmp = alloca %buf, align 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %cpy_tmp, ptr align 1 %src, i64 7, i1 false)
+  %cpy_tmp_offset = getelementptr inbounds i8, ptr %cpy_tmp, i64 1
+  %dest = getelementptr inbounds i8, ptr %src, i64 2
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %dest, ptr align 1 %cpy_tmp_offset, i64 6, i1 false)
+  ret void
+}
+
+; Make sure we pass the right parameters when calling `memcpy`.
+define void @forward_offset_memcpy(ptr %src, ptr %dest) {
+; CHECK-LABEL: define void @forward_offset_memcpy(
+; CHECK-SAME: ptr [[SRC:%.*]], ptr [[DEST:%.*]]) {
+; CHECK-NEXT:    [[DEP_DEST:%.*]] = alloca [9 x i8], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[DEP_DEST]], ptr align 1 [[SRC]], i64 7, i1 false)
+; CHECK-NEXT:    [[TMP_OFFSET:%.*]] = getelementptr inbounds i8, ptr [[DEP_DEST]], i64 1
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i8, ptr [[SRC]], i64 1
+; CHECK-NEXT:    call void @llvm.memmove.p0.p0.i64(ptr align 1 [[DEST]], ptr align 1 [[TMP1]], i64 6, i1 false)
+; CHECK-NEXT:    call void @use(ptr [[DEST]])
+; CHECK-NEXT:    ret void
+;
+  %cpy_tmp = alloca %buf, align 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %cpy_tmp, ptr align 1 %src, i64 7, i1 false)
+  %cpy_tmp_offset = getelementptr inbounds i8, ptr %cpy_tmp, i64 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %dest, ptr align 1 %cpy_tmp_offset, i64 6, i1 false)
+  call void @use(ptr %dest)
+  ret void
+}
+
+; Make sure we pass the right parameters when calling `memcpy.inline`.
+define void @forward_offset_memcpy_inline(ptr %src, ptr %dest) {
+; CHECK-LABEL: define void @forward_offset_memcpy_inline(
+; CHECK-SAME: ptr [[SRC:%.*]], ptr [[DEST:%.*]]) {
+; CHECK-NEXT:    [[DEP_DEST:%.*]] = alloca [9 x i8], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[DEP_DEST]], ptr align 1 [[SRC]], i64 7, i1 false)
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i8, ptr [[DEP_DEST]], i64 1
+; CHECK-NEXT:    call void @llvm.memcpy.inline.p0.p0.i64(ptr align 1 [[DEST]], ptr align 1 [[TMP1]], i64 6, i1 false)
+; CHECK-NEXT:    call void @use(ptr [[DEST]])
+; CHECK-NEXT:    ret void
+;
+  %cpy_tmp = alloca %buf, align 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %cpy_tmp, ptr align 1 %src, i64 7, i1 false)
+  %cpy_tmp_offset = getelementptr inbounds i8, ptr %cpy_tmp, i64 1
+  call void @llvm.memcpy.inline.p0.p0.i64(ptr align 1 %dest, ptr align 1 %cpy_tmp_offset, i64 6, i1 false)
+  call void @use(ptr %dest)
+  ret void
+}
+
+; We cannot forward `memcpy` because it exceeds the size of `memcpy` it depends on.
+define void @do_not_forward_oversize_offset(ptr %src, ptr %dest) {
+; CHECK-LABEL: define void @do_not_forward_oversize_offset(
+; CHECK-SAME: ptr [[SRC:%.*]], ptr [[DEST:%.*]]) {
+; CHECK-NEXT:    [[DEP_DEST:%.*]] = alloca [9 x i8], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[DEP_DEST]], ptr align 1 [[SRC]], i64 6, i1 false)
+; CHECK-NEXT:    [[TMP_OFFSET:%.*]] = getelementptr inbounds i8, ptr [[DEP_DEST]], i64 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[DEST]], ptr align 1 [[TMP_OFFSET]], i64 6, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %cpy_tmp = alloca %buf, align 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %cpy_tmp, ptr align 1 %src, i64 6, i1 false)
+  %cpy_tmp_offset = getelementptr inbounds i8, ptr %cpy_tmp, i64 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %dest, ptr align 1 %cpy_tmp_offset, i64 6, i1 false)
+  ret void
+}
+
+; We can forward `memcpy` because the write operation does not corrupt the location to be copied.
+define void @forward_offset_and_store(ptr %src, ptr %dest) {
+; CHECK-LABEL: define void @forward_offset_and_store(
+; CHECK-SAME: ptr [[SRC:%.*]], ptr [[DEST:%.*]]) {
+; CHECK-NEXT:    [[DEP_DEST:%.*]] = alloca [9 x i8], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[DEP_DEST]], ptr align 1 [[SRC]], i64 7, i1 false)
+; CHECK-NEXT:    store i8 1, ptr [[SRC]], align 1
+; CHECK-NEXT:    [[DEP_SRC_END:%.*]] = getelementptr inbounds i8, ptr [[SRC]], i64 6
+; CHECK-NEXT:    store i8 1, ptr [[DEP_SRC_END]], align 1
+; CHECK-NEXT:    [[TMP_OFFSET:%.*]] = getelementptr inbounds i8, ptr [[DEP_DEST]], i64 1
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i8, ptr [[SRC]], i64 1
+; CHECK-NEXT:    call void @llvm.memmove.p0.p0.i64(ptr align 1 [[DEST]], ptr align 1 [[TMP1]], i64 5, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %cpy_tmp = alloca %buf, align 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %cpy_tmp, ptr align 1 %src, i64 7, i1 false)
+  store i8 1, ptr %src, align 1
+  %src_end = getelementptr inbounds i8, ptr %src, i64 6
+  store i8 1, ptr %src_end, align 1
+  %cpy_tmp_offset  = getelementptr inbounds i8, ptr %cpy_tmp, i64 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %dest, ptr align 1 %cpy_tmp_offset, i64 5, i1 false)
+  ret void
+}
+
+; We cannot forward `memcpy` because the write operation alters the location to be copied.
+; Also, make sure we have removed the GEP instruction that was created temporarily.
+define void @do_not_forward_offset_and_store(ptr %src, ptr %dest) {
+; CHECK-LABEL: define void @do_not_forward_offset_and_store(
+; CHECK-SAME: ptr [[SRC:%.*]], ptr [[DEST:%.*]]) {
+; CHECK-NEXT:    [[DEP_DEST:%.*]] = alloca [9 x i8], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[DEP_DEST]], ptr align 1 [[SRC]], i64 7, i1 false)
+; CHECK-NEXT:    [[DEP:%.*]] = getelementptr inbounds i8, ptr [[SRC]], i64 1
+; CHECK-NEXT:    store i8 1, ptr [[DEP]], align 1
+; CHECK-NEXT:    [[TMP_OFFSET:%.*]] = getelementptr inbounds i8, ptr [[DEP_DEST]], i64 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[DEST]], ptr align 1 [[TMP_OFFSET]], i64 5, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %cpy_tmp = alloca %buf, align 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %cpy_tmp, ptr align 1 %src, i64 7, i1 false)
+  %src_offset = getelementptr inbounds i8, ptr %src, i64 1
+  store i8 1, ptr %src_offset, align 1
+  %cpy_tmp_offset = getelementptr inbounds i8, ptr %cpy_tmp, i64 1
+  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %dest, ptr align 1 %cpy_tmp_offset, i64 5, i1 false)
+  ret void
+}
+
+declare void @use(ptr)
+
+declare void @llvm.memcpy.p0.p0.i64(ptr nocapture, ptr nocapture, i64, i1)
+declare void @llvm.memcpy.inline.p0.p0.i64(ptr nocapture, ptr nocapture, i64, i1)

llvm/test/Transforms/PhaseOrdering/memcpy-offset.ll

@@ -0,0 +1,28 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt < %s -passes=memcpyopt,instcombine -S -verify-memoryssa | FileCheck --check-prefix=CUSTOM %s
+; RUN: opt < %s -O2 -S | FileCheck --check-prefix=O2 %s
+
+; Check that we eliminate all `memcpy` calls in this function.
+define void @memcpy_forward_back_with_offset(ptr %arg) {
+; CUSTOM-LABEL: define void @memcpy_forward_back_with_offset(
+; CUSTOM-SAME: ptr [[ARG:%.*]]) {
+; CUSTOM-NEXT:    store i8 1, ptr [[ARG]], align 1
+; CUSTOM-NEXT:    ret void
+;
+; O2-LABEL: define void @memcpy_forward_back_with_offset(
+; O2-SAME: ptr nocapture writeonly [[ARG:%.*]]) local_unnamed_addr #[[ATTR0:[0-9]+]] {
+; O2-NEXT:    store i8 1, ptr [[ARG]], align 1
+; O2-NEXT:    ret void
+;
+  %i = alloca [753 x i8], align 1
+  %i1 = alloca [754 x i8], align 1
+  call void @llvm.memcpy.p0.p0.i64(ptr %i1, ptr %arg, i64 754, i1 false)
+  %i2 = getelementptr inbounds i8, ptr %i1, i64 1
+  call void @llvm.memcpy.p0.p0.i64(ptr %i, ptr %i2, i64 753, i1 false)
+  store i8 1, ptr %arg, align 1
+  %i3 = getelementptr inbounds i8, ptr %arg, i64 1
+  call void @llvm.memcpy.p0.p0.i64(ptr %i3, ptr %i, i64 753, i1 false)
+  ret void
+}
+
+declare void @llvm.memcpy.p0.p0.i64(ptr, ptr, i64, i1)