Index: llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -1302,9 +1302,13 @@
     return false;
 
   Value *TmpCast = MDep->getSource();
-  if (MDep->getSource()->getType() != ByValArg->getType())
-    TmpCast = new BitCastInst(MDep->getSource(), ByValArg->getType(),
-                              "tmpcast", CS.getInstruction());
+  if (MDep->getSource()->getType() != ByValArg->getType()) {
+    BitCastInst *TmpBitCast = new BitCastInst(MDep->getSource(), ByValArg->getType(),
+                                              "tmpcast", CS.getInstruction());
+    // Set the tmpcast's DebugLoc to MDep's
+    TmpBitCast->setDebugLoc(MDep->getDebugLoc());
+    TmpCast = TmpBitCast;
+  }
 
   LLVM_DEBUG(dbgs() << "MemCpyOptPass: Forwarding memcpy to byval:\n"
                     << "  " << *MDep << "\n"
Index: llvm/test/Transforms/MemCpyOpt/pr37967.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/MemCpyOpt/pr37967.ll
@@ -0,0 +1,23 @@
+; RUN: opt -debugify -memcpyopt -check-debugify -S < %s 2>&1 | FileCheck %s
+
+; CHECK: CheckModuleDebugify: PASS
+
+; CHECK-LABEL: define {{.*}} @_Z3bar3Foo
+; CHECK: [[target:%.*]] = load i8*, i8** bitcast (%struct.Foo** @a to i8**), align 8, !dbg
+; CHECK: %tmpcast = bitcast i8* [[target]] to %struct.Foo*, !dbg
+
+%struct.Foo = type { i64, i64, i64 }
+
+@a = dso_local global %struct.Foo* null, align 8
+
+define dso_local void @_Z3bar3Foo(%struct.Foo* byval(%struct.Foo) align 8 %0) {
+entry:
+  %agg.tmp = alloca %struct.Foo, align 8
+  %1 = load i8*, i8** bitcast (%struct.Foo** @a to i8**), align 8
+  %2 = bitcast %struct.Foo* %agg.tmp to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull align 8 dereferenceable(24) %2, i8* nonnull align 8 dereferenceable(24) %1, i64 24, i1 false)
+  call void @_Z3bar3Foo(%struct.Foo* nonnull byval(%struct.Foo) align 8 %agg.tmp)
+  ret void
+}
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nocapture writeonly, i8* noalias nocapture readonly, i64, i1 immarg) #0