Index: lib/Basic/Targets/X86.h
===================================================================
--- lib/Basic/Targets/X86.h
+++ lib/Basic/Targets/X86.h
@@ -264,6 +264,8 @@
     return checkCPUKind(getCPUKind(Name));
   }
 
+  void fillValidCPUList(SmallVectorImpl<StringRef> &Values) const override;
+
   bool setCPU(const std::string &Name) override {
     return checkCPUKind(CPU = getCPUKind(Name));
   }
Index: lib/Basic/Targets/X86.cpp
===================================================================
--- lib/Basic/Targets/X86.cpp
+++ lib/Basic/Targets/X86.cpp
@@ -15,6 +15,7 @@
 #include "clang/Basic/Builtins.h"
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/TargetBuiltins.h"
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/TargetParser.h"
@@ -1648,8 +1649,6 @@
 bool X86TargetInfo::checkCPUKind(CPUKind Kind) const {
   // Perform any per-CPU checks necessary to determine if this CPU is
   // acceptable.
-  // FIXME: This results in terrible diagnostics. Clang just says the CPU is
-  // invalid without explaining *why*.
   switch (Kind) {
   case CK_Generic:
     // No processor selected!
@@ -1662,6 +1661,18 @@
   llvm_unreachable("Unhandled CPU kind");
 }
 
+void X86TargetInfo::fillValidCPUList(SmallVectorImpl<StringRef> &Values) const {
+#define PROC(ENUM, STRING, IS64BIT)                                            \
+  if (IS64BIT || getTriple().getArch() == llvm::Triple::x86)                   \
+    Values.emplace_back(STRING);
+  // Go through CPUKind checking to ensure that the alias is de-aliased and 
+  // 64 bit-ness is checked.
+#define PROC_ALIAS(ENUM, ALIAS)                                                \
+  if (checkCPUKind(getCPUKind(ALIAS)))                                         \
+    Values.emplace_back(ALIAS);
+#include "clang/Basic/X86Target.def"
+}
+
 X86TargetInfo::CPUKind X86TargetInfo::getCPUKind(StringRef CPU) const {
   return llvm::StringSwitch<CPUKind>(CPU)
 #define PROC(ENUM, STRING, IS64BIT) .Case(STRING, CK_##ENUM)
Index: test/Misc/target-invalid-cpu-note.c
===================================================================
--- test/Misc/target-invalid-cpu-note.c
+++ test/Misc/target-invalid-cpu-note.c
@@ -7,3 +7,27 @@
 // AARCH64: error: unknown target CPU 'not-a-cpu'
 // AARCH64: note: valid target CPU values are:
 // AARCH64-SAME: cortex-a35
+
+// RUN: not %clang_cc1 -triple i386--- -target-cpu not-a-cpu -fsyntax-only %s 2>&1 | FileCheck %s --check-prefix X86
+// X86: error: unknown target CPU 'not-a-cpu'
+// X86: note: valid target CPU values are: i386, i486, winchip-c6, winchip2, c3,
+// X86-SAME: i586, pentium, pentium-mmx, pentiumpro, i686, pentium2, pentium3,
+// X86-SAME: pentium3m, pentium-m, c3-2, yonah, pentium4, pentium4m, prescott,
+// X86-SAME: nocona, core2, penryn, bonnell, atom, silvermont, slm, goldmont,
+// X86-SAME: nehalem, corei7, westmere, sandybridge, corei7-avx, ivybridge,
+// X86-SAME: core-avx-i, haswell, core-avx2, broadwell, skylake, skylake-avx512,
+// X86-SAME: skx, cannonlake, icelake, knl, knm, lakemont, k6, k6-2, k6-3,
+// X86-SAME: athlon, athlon-tbird, athlon-xp, athlon-mp, athlon-4, k8, athlon64,
+// X86-SAME: athlon-fx, opteron, k8-sse3, athlon64-sse3, opteron-sse3, amdfam10,
+// X86-SAME: barcelona, btver1, btver2, bdver1, bdver2, bdver3, bdver4, znver1,
+// X86-SAME: x86-64, geode
+
+// RUN: not %clang_cc1 -triple x86_64--- -target-cpu not-a-cpu -fsyntax-only %s 2>&1 | FileCheck %s --check-prefix X86_64
+// X86_64: error: unknown target CPU 'not-a-cpu'
+// X86_64: note: valid target CPU values are: nocona, core2, penryn, bonnell,
+// X86_64-SAME: atom, silvermont, slm, goldmont, nehalem, corei7, westmere,
+// X86_64-SAME: sandybridge, corei7-avx, ivybridge, core-avx-i, haswell,
+// X86_64-SAME: core-avx2, broadwell, skylake, skylake-avx512, skx, cannonlake,
+// X86_64-SAME: icelake, knl, knm, k8, athlon64, athlon-fx, opteron, k8-sse3,
+// X86_64-SAME: athlon64-sse3, opteron-sse3, amdfam10, barcelona, btver1,
+// X86_64-SAME: btver2, bdver1, bdver2, bdver3, bdver4, znver1, x86-64