diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -1201,13 +1201,18 @@
 /// In other words, transform:
 /// \code
 ///   memset(dst, c, dst_size);
+///   ...
 ///   memcpy(dst, src, src_size);
 /// \endcode
 /// into:
 /// \code
-///   memcpy(dst, src, src_size);
+///   ...
 ///   memset(dst + src_size, c, dst_size <= src_size ? 0 : dst_size - src_size);
+///   memcpy(dst, src, src_size);
 /// \endcode
+///
+/// The memset is sunk to just before the memcpy to ensure that src_size is
+/// present when emitting the simplified memset.
 bool MemCpyOptPass::processMemSetMemCpyDependence(MemCpyInst *MemCpy,
                                                   MemSetInst *MemSet,
                                                   BatchAAResults &BAA) {
@@ -1255,6 +1260,15 @@
 
   IRBuilder<> Builder(MemCpy);
 
+  // Preserve the debug location of the old memset for the code emitted here
+  // related to the new memset. This is correct according to the rules in
+  // https://llvm.org/docs/HowToUpdateDebugInfo.html about "when to preserve an
+  // instruction location", given that we move the memset within the basic
+  // block.
+  assert(MemSet->getParent() == MemCpy->getParent() &&
+         "Preserving debug location based on moving memset within BB.");
+  Builder.SetCurrentDebugLocation(MemSet->getDebugLoc());
+
   // If the sizes have different types, zext the smaller one.
   if (DestSize->getType() != SrcSize->getType()) {
     if (DestSize->getType()->getIntegerBitWidth() >
@@ -1278,9 +1292,8 @@
 
   assert(isa<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(MemCpy)) &&
          "MemCpy must be a MemoryDef");
-  // The new memset is inserted after the memcpy, but it is known that its
-  // defining access is the memset about to be removed which immediately
-  // precedes the memcpy.
+  // The new memset is inserted before the memcpy, and it is known that the
+  // memcpy's defining access is the memset about to be removed.
   auto *LastDef =
       cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(MemCpy));
   auto *NewAccess = MSSAU->createMemoryAccessBefore(
@@ -1439,8 +1452,8 @@
       MSSA->getWalker()->getClobberingMemoryAccess(AnyClobber, DestLoc, BAA);
 
   // Try to turn a partially redundant memset + memcpy into
-  // memcpy + smaller memset.  We don't need the memcpy size for this.
-  // The memcpy most post-dom the memset, so limit this to the same basic
+  // smaller memset + memcpy.  We don't need the memcpy size for this.
+  // The memcpy must post-dom the memset, so limit this to the same basic
   // block. A non-local generalization is likely not worthwhile.
   if (auto *MD = dyn_cast<MemoryDef>(DestClobber))
     if (auto *MDep = dyn_cast_or_null<MemSetInst>(MD->getMemoryInst()))
diff --git a/llvm/test/Transforms/MemCpyOpt/memset-memcpy-dbgloc.ll b/llvm/test/Transforms/MemCpyOpt/memset-memcpy-dbgloc.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/MemCpyOpt/memset-memcpy-dbgloc.ll
@@ -0,0 +1,47 @@
+; RUN: opt -passes=memcpyopt -S %s -verify-memoryssa | FileCheck %s
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+@C = external constant [0 x i8]
+
+declare void @llvm.memset.p0.i64(ptr nocapture, i8, i64, i1)
+declare void @llvm.memcpy.p0.p0.i64(ptr nocapture, ptr nocapture readonly, i64, i1)
+
+define void @test_constant(i64 %src_size, ptr %dst, i64 %dst_size, i8 %c) !dbg !5 {
+; CHECK-LABEL: @test_constant(
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ule i64 [[DST_SIZE:%.*]], [[SRC_SIZE:%.*]], !dbg [[DBG11:![0-9]+]]
+; CHECK-NEXT:    [[TMP2:%.*]] = sub i64 [[DST_SIZE]], [[SRC_SIZE]], !dbg [[DBG11]]
+; CHECK-NEXT:    [[TMP3:%.*]] = select i1 [[TMP1]], i64 0, i64 [[TMP2]], !dbg [[DBG11]]
+; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr i8, ptr [[DST:%.*]], i64 [[SRC_SIZE]], !dbg [[DBG11]]
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 1 [[TMP4]], i8 [[C:%.*]], i64 [[TMP3]], i1 false), !dbg [[DBG11]]
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr [[DST]], ptr @C, i64 [[SRC_SIZE]], i1 false), !dbg [[DBG12:![0-9]+]]
+; CHECK-NEXT:    ret void, !dbg [[DBG13:![0-9]+]]
+;
+  call void @llvm.memset.p0.i64(ptr %dst, i8 %c, i64 %dst_size, i1 false), !dbg !11
+  call void @llvm.memcpy.p0.p0.i64(ptr %dst, ptr @C, i64 %src_size, i1 false), !dbg !12
+  ret void, !dbg !13
+}
+
+; Validate that the memset is mapped to DILocation for the original memset.
+; CHECK: [[DBG11]] = !DILocation(line: 1,
+; CHECK: [[DBG12]] = !DILocation(line: 2,
+; CHECK: [[DBG13]] = !DILocation(line: 3,
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!2, !3}
+!llvm.module.flags = !{!4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "memset-memcpy-dbgloc.ll", directory: "/")
+!2 = !{i32 3}
+!3 = !{i32 1}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "test_constant", linkageName: "test_constant", scope: null, file: !1, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !8)
+!6 = !DISubroutineType(types: !7)
+!7 = !{}
+!8 = !{!9}
+!9 = !DILocalVariable(name: "1", scope: !5, file: !1, line: 3, type: !10)
+!10 = !DIBasicType(name: "ty32", size: 32, encoding: DW_ATE_unsigned)
+!11 = !DILocation(line: 1, column: 1, scope: !5)
+!12 = !DILocation(line: 2, column: 1, scope: !5)
+!13 = !DILocation(line: 3, column: 1, scope: !5)