Index: include/llvm/Analysis/TargetLibraryInfo.h
===================================================================
--- include/llvm/Analysis/TargetLibraryInfo.h
+++ include/llvm/Analysis/TargetLibraryInfo.h
@@ -193,13 +193,9 @@
     ShouldSignExtI32Param = Val;
   }
 
-  /// Returns the size of the wchar_t type in bytes.
+  /// Returns the size of the wchar_t type in bytes or 0 if the size is unknown.
+  /// This queries the 'wchar_size' metadata.
   unsigned getWCharSize(const Module &M) const;
-
-  /// Returns size of the default wchar_t type on target \p T. This is mostly
-  /// intended to verify that the size in the frontend matches LLVM. All other
-  /// queries should use getWCharSize() instead.
-  static unsigned getTargetWCharSize(const Triple &T);
 };
 
 /// Provides information about what library functions are available for
Index: lib/Analysis/TargetLibraryInfo.cpp
===================================================================
--- lib/Analysis/TargetLibraryInfo.cpp
+++ lib/Analysis/TargetLibraryInfo.cpp
@@ -1519,20 +1519,11 @@
   return *Impl;
 }
 
-unsigned TargetLibraryInfoImpl::getTargetWCharSize(const Triple &T) {
-  // See also clang/lib/Basic/Targets.cpp.
-  if (T.isPS4() || T.isOSWindows() || T.isArch16Bit())
-    return 2;
-  if (T.getArch() == Triple::xcore)
-    return 1;
-  return 4;
-}
-
 unsigned TargetLibraryInfoImpl::getWCharSize(const Module &M) const {
   if (auto *ShortWChar = cast_or_null<ConstantAsMetadata>(
       M.getModuleFlag("wchar_size")))
     return cast<ConstantInt>(ShortWChar->getValue())->getZExtValue();
-  return getTargetWCharSize(Triple(M.getTargetTriple()));
+  return 0;
 }
 
 TargetLibraryInfoWrapperPass::TargetLibraryInfoWrapperPass()
Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -508,6 +508,9 @@
 Value *LibCallSimplifier::optimizeWcslen(CallInst *CI, IRBuilder<> &B) {
   Module &M = *CI->getParent()->getParent()->getParent();
   unsigned WCharSize = TLI->getWCharSize(M) * 8;
+  // We cannot perform this optimization without wchar_size metadata.
+  if (WCharSize == 0)
+    return nullptr;
 
   return optimizeStringLength(CI, B, WCharSize);
 }
Index: test/Transforms/InstCombine/wcslen-1.ll
===================================================================
--- test/Transforms/InstCombine/wcslen-1.ll
+++ test/Transforms/InstCombine/wcslen-1.ll
@@ -7,6 +7,9 @@
 
 declare i64 @wcslen(i32*)
 
+!0 = !{i32 1, !"wchar_size", i32 4}
+!llvm.module.flags = !{!0}
+
 @hello = constant [6 x i32] [i32 104, i32 101, i32 108, i32 108, i32 111, i32 0]
 @longer = constant [7 x i32] [i32 108, i32 111, i32 110, i32 103, i32 101, i32 114, i32 0]
 @null = constant [1 x i32] zeroinitializer
Index: test/Transforms/InstCombine/wcslen-2.ll
===================================================================
--- test/Transforms/InstCombine/wcslen-2.ll
+++ test/Transforms/InstCombine/wcslen-2.ll
@@ -4,6 +4,9 @@
 
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 
+!0 = !{i32 1, !"wchar_size", i32 4}
+!llvm.module.flags = !{!0}
+
 @hello = constant [6 x i32] [i32 104, i32 101, i32 108, i32 108, i32 111, i32 0]
 
 declare i64 @wcslen(i32*, i32)
Index: test/Transforms/InstCombine/wcslen-4.ll
===================================================================
--- test/Transforms/InstCombine/wcslen-4.ll
+++ test/Transforms/InstCombine/wcslen-4.ll
@@ -4,15 +4,17 @@
 
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 
+; Without the wchar_size metadata we should see no optimization happening.
+
 @hello = constant [6 x i32] [i32 104, i32 101, i32 108, i32 108, i32 111, i32 0]
 
-declare i64 @wcslen(i32*, i32)
+declare i64 @wcslen(i32*)
 
 define i64 @test_no_simplify1() {
 ; CHECK-LABEL: @test_no_simplify1(
+; CHECK-NEXT: %hello_l = call i64 @wcslen(i32* getelementptr inbounds ([6 x i32], [6 x i32]* @hello, i64 0, i64 0))
+; CHECK-NEXT: ret i64 %hello_l
   %hello_p = getelementptr [6 x i32], [6 x i32]* @hello, i64 0, i64 0
-  %hello_l = call i64 @wcslen(i32* %hello_p, i32 187)
-; CHECK-NEXT: %hello_l = call i64 @wcslen
+  %hello_l = call i64 @wcslen(i32* %hello_p)
   ret i64 %hello_l
-; CHECK-NEXT: ret i64 %hello_l
 }