diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -640,7 +640,7 @@
 
   // For C functions without prototypes, return false as their
   // names should not be mangled.
-  if (!FD->getType()->getAs<FunctionProtoType>())
+  if (!FD->hasPrototype())
     return false;
 
   if (isInternalLinkageDecl(ND))
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -3522,7 +3522,7 @@
                                            llvm::DIScope *&FDContext,
                                            llvm::DINodeArray &TParamsArray,
                                            llvm::DINode::DIFlags &Flags) {
-  const auto *FD = cast<FunctionDecl>(GD.getDecl());
+  const auto *FD = cast<FunctionDecl>(GD.getCanonicalDecl().getDecl());
   Name = getFunctionName(FD);
   // Use mangled name as linkage name for C/C++ functions.
   if (FD->hasPrototype()) {
diff --git a/clang/test/CodeGen/unique-internal-linkage-names-dwarf.c b/clang/test/CodeGen/unique-internal-linkage-names-dwarf.c
--- a/clang/test/CodeGen/unique-internal-linkage-names-dwarf.c
+++ b/clang/test/CodeGen/unique-internal-linkage-names-dwarf.c
@@ -8,21 +8,48 @@
 // RUN: %clang_cc1 -triple x86_64-unknown-linux -debug-info-kind=limited -dwarf-version=5 -funique-internal-linkage-names -emit-llvm -o -  %s | FileCheck %s --check-prefix=UNIQUE
 
 static int glob;
+// foo should be given a uniquefied name under -funique-internal-linkage-names.
 static int foo(void) {
   return glob;
 }
 
+// bar should not be given a uniquefied name under -funique-internal-linkage-names, 
+// since it doesn't come with valid prototype.
+static int bar(a) int a;
+{
+  return glob + a;
+}
+
+// go should be given a uniquefied name under -funique-internal-linkage-names, even 
+// if its definition doesn't come with a valid prototype, but the declaration here
+// has a prototype.
+static int go(int);
+
 void baz() {
   foo();
+  bar(1);
+  go(2);
 }
 
+static int go(a) int a;
+{
+  return glob + a;
+}
+
+
 // PLAIN: @glob = internal global i32
 // PLAIN: define internal i32 @foo()
+// PLAIN: define internal i32 @bar(i32 %a)
 // PLAIN: distinct !DIGlobalVariable(name: "glob"{{.*}})
 // PLAIN: distinct !DISubprogram(name: "foo"{{.*}})
+// PLAIN: distinct !DISubprogram(name: "bar"{{.*}})
+// PLAIN: distinct !DISubprogram(name: "go"{{.*}})
 // PLAIN-NOT: linkageName:
 //
 // UNIQUE: @glob = internal global i32
 // UNIQUE: define internal i32 @_ZL3foov.[[MODHASH:__uniq.[0-9]+]]()
+// UNIQUE: define internal i32 @bar(i32 %a)
+// UNIQUE: define internal i32 @_ZL2goi.[[MODHASH]](i32 %a)
 // UNIQUE: distinct !DIGlobalVariable(name: "glob"{{.*}})
 // UNIQUE: distinct !DISubprogram(name: "foo", linkageName: "_ZL3foov.[[MODHASH]]"{{.*}})
+// UNIQUE: distinct !DISubprogram(name: "go", linkageName: "_ZL2goi.[[MODHASH]]"{{.*}})