Index: clang/include/clang/Basic/DebugInfoOptions.h
===================================================================
--- clang/include/clang/Basic/DebugInfoOptions.h
+++ clang/include/clang/Basic/DebugInfoOptions.h
@@ -13,6 +13,11 @@
 namespace clang {
 namespace codegenoptions {
 
+enum DebugInfoFormat {
+  DIF_DWARF,
+  DIF_CodeView,
+};
+
 enum DebugInfoKind {
   NoDebugInfo,         /// Don't generate debug info.
   LocTrackingOnly,     /// Emit location information but do not generate
Index: clang/include/clang/Driver/ToolChain.h
===================================================================
--- clang/include/clang/Driver/ToolChain.h
+++ clang/include/clang/Driver/ToolChain.h
@@ -12,6 +12,7 @@
 
 #include "clang/Basic/LLVM.h"
 #include "clang/Basic/Sanitizers.h"
+#include "clang/Basic/DebugInfoOptions.h"
 #include "clang/Driver/Action.h"
 #include "clang/Driver/Multilib.h"
 #include "clang/Driver/Types.h"
@@ -402,6 +403,11 @@
   /// Complain if this tool chain doesn't support Objective-C ARC.
   virtual void CheckObjCARC() const {}
 
+  /// Get the default debug info format. Typically, this is DWARF.
+  virtual codegenoptions::DebugInfoFormat getDefaultDebugFormat() const {
+    return codegenoptions::DIF_DWARF;
+  }
+
   /// UseDwarfDebugFlags - Embed the compile options to clang into the Dwarf
   /// compile unit information.
   virtual bool UseDwarfDebugFlags() const { return false; }
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -3075,21 +3075,24 @@
     if (checkDebugInfoOption(A, Args, D, TC))
       DWARFVersion = DwarfVersionNum(A->getSpelling());
 
-  // Forward -gcodeview. EmitCodeView might have been set by CL-compatibility
-  // argument parsing.
-  if (EmitCodeView) {
-    if (const Arg *A = Args.getLastArg(options::OPT_gcodeview)) {
-      EmitCodeView = checkDebugInfoOption(A, Args, D, TC);
-      if (EmitCodeView) {
-        // DWARFVersion remains at 0 if no explicit choice was made.
-        CmdArgs.push_back("-gcodeview");
-      }
-    }
+  if (const Arg *A = Args.getLastArg(options::OPT_gcodeview)) {
+    if (checkDebugInfoOption(A, Args, D, TC))
+      EmitCodeView = true;
   }
 
+  // If the user asked for debug info but did not explicitly specify -gcodeview
+  // or -gdwarf, ask the toolchain for the default format.
   if (!EmitCodeView && DWARFVersion == 0 &&
-      DebugInfoKind != codegenoptions::NoDebugInfo)
-    DWARFVersion = TC.GetDefaultDwarfVersion();
+      DebugInfoKind != codegenoptions::NoDebugInfo) {
+    switch (TC.getDefaultDebugFormat()) {
+    case codegenoptions::DIF_CodeView:
+      EmitCodeView = true;
+      break;
+    case codegenoptions::DIF_DWARF:
+      DWARFVersion = TC.GetDefaultDwarfVersion();
+      break;
+    }
+  }
 
   // -gline-directives-only supported only for the DWARF debug info.
   if (DWARFVersion == 0 && DebugInfoKind == codegenoptions::DebugDirectivesOnly)
@@ -3164,6 +3167,9 @@
       CmdArgs.push_back("-gembed-source");
   }
 
+  if (EmitCodeView)
+    CmdArgs.push_back("-gcodeview");
+
   RenderDebugEnablingArgs(Args, CmdArgs, DebugInfoKind, DWARFVersion,
                           DebuggerTuning);
 
@@ -3884,8 +3890,6 @@
   types::ID InputType = Input.getType();
   if (D.IsCLMode())
     AddClangCLArgs(Args, InputType, CmdArgs, &DebugInfoKind, &EmitCodeView);
-  else
-    EmitCodeView = Args.hasArg(options::OPT_gcodeview);
 
   const Arg *SplitDWARFArg = nullptr;
   RenderDebugOptions(TC, D, RawTriple, Args, EmitCodeView, IsWindowsMSVC,
@@ -5471,7 +5475,6 @@
       *DebugInfoKind = codegenoptions::LimitedDebugInfo;
     else
       *DebugInfoKind = codegenoptions::DebugLineTablesOnly;
-    CmdArgs.push_back("-gcodeview");
   } else {
     *EmitCodeView = false;
   }
Index: clang/lib/Driver/ToolChains/MSVC.h
===================================================================
--- clang/lib/Driver/ToolChains/MSVC.h
+++ clang/lib/Driver/ToolChains/MSVC.h
@@ -11,6 +11,7 @@
 #define LLVM_CLANG_LIB_DRIVER_TOOLCHAINS_MSVC_H
 
 #include "Cuda.h"
+#include "clang/Basic/DebugInfoOptions.h"
 #include "clang/Driver/Compilation.h"
 #include "clang/Driver/Tool.h"
 #include "clang/Driver/ToolChain.h"
@@ -78,6 +79,18 @@
   bool isPIEDefault() const override;
   bool isPICDefaultForced() const override;
 
+  /// Set CodeView as the default debug info format. Users can use -gcodeview
+  /// and -gdwarf to override the default.
+  codegenoptions::DebugInfoFormat getDefaultDebugFormat() const override {
+    return codegenoptions::DIF_CodeView;
+  }
+
+  /// Set the debugger tuning to "default", since we're definitely not tuning
+  /// for GDB.
+  llvm::DebuggerKind getDefaultDebuggerTuning() const override {
+    return llvm::DebuggerKind::Default;
+  }
+
   enum class SubDirectoryType {
     Bin,
     Include,
Index: clang/test/CodeGen/dwarf-version.c
===================================================================
--- clang/test/CodeGen/dwarf-version.c
+++ clang/test/CodeGen/dwarf-version.c
@@ -14,14 +14,29 @@
 // RUN: %clang -target powerpc-unknown-openbsd -g -S -emit-llvm -o - %s | FileCheck %s --check-prefix=VER2
 // RUN: %clang -target powerpc-unknown-freebsd -g -S -emit-llvm -o - %s | FileCheck %s --check-prefix=VER2
 // RUN: %clang -target i386-pc-solaris -g -S -emit-llvm -o - %s | FileCheck %s --check-prefix=VER2
-//
-// Test what -gcodeview and -gdwarf do on Windows.
-// RUN: %clang -target i686-pc-windows-msvc -gcodeview -S -emit-llvm -o - %s | FileCheck %s --check-prefix=NODWARF --check-prefix=CODEVIEW
-// RUN: %clang -target i686-pc-windows-msvc -gdwarf -gcodeview -S -emit-llvm -o - %s | FileCheck %s --check-prefix=VER4 --check-prefix=CODEVIEW
+
+// Check which debug info formats we use on Windows. By default, in an MSVC
+// environment, we should use codeview. You can enable dwarf, which implicitly
+// disables codeview, of you can explicitly ask for both if you don't know how
+// the app will be debugged.
+//     Default is codeview.
+// RUN: %clang -target i686-pc-windows-msvc -g -S -emit-llvm -o - %s \
+// RUN:     | FileCheck %s --check-prefixes=NODWARF,CODEVIEW
+//     Explicitly request codeview.
+// RUN: %clang -target i686-pc-windows-msvc -gcodeview -S -emit-llvm -o - %s \
+// RUN:     | FileCheck %s --check-prefixes=NODWARF,CODEVIEW
+//     Explicitly request DWARF.
+// RUN: %clang -target i686-pc-windows-msvc -gdwarf -S -emit-llvm -o - %s \
+// RUN:     | FileCheck %s --check-prefixes=VER4,NOCODEVIEW
+//     Explicitly request both.
+// RUN: %clang -target i686-pc-windows-msvc -gdwarf -gcodeview -S -emit-llvm -o - %s \
+// RUN:     | FileCheck %s --check-prefixes=VER4,CODEVIEW
 int main (void) {
   return 0;
 }
 
+// NOCODEVIEW-NOT: !"CodeView"
+
 // VER2: !{i32 2, !"Dwarf Version", i32 2}
 // VER3: !{i32 2, !"Dwarf Version", i32 3}
 // VER4: !{i32 2, !"Dwarf Version", i32 4}
@@ -29,4 +44,5 @@
 
 // NODWARF-NOT: !"Dwarf Version"
 // CODEVIEW: !{i32 2, !"CodeView", i32 1}
+// NOCODEVIEW-NOT: !"CodeView"
 // NODWARF-NOT: !"Dwarf Version"
Index: clang/test/CodeGenCXX/debug-info-byval.cpp
===================================================================
--- clang/test/CodeGenCXX/debug-info-byval.cpp
+++ clang/test/CodeGenCXX/debug-info-byval.cpp
@@ -1,5 +1,5 @@
 // FIXME: Check IR rather than asm, then triple is not needed.
-// RUN: %clang -Xclang -triple=%itanium_abi_triple -g -S %s -o - | FileCheck %s
+// RUN: %clang --target=%itanium_abi_triple -g -S %s -o - | FileCheck %s
 // Test to check presence of debug info for byval parameter.
 // Radar 8350436.
 class DAG {
Index: clang/test/CodeGenCXX/debug-info-ctor2.cpp
===================================================================
--- clang/test/CodeGenCXX/debug-info-ctor2.cpp
+++ clang/test/CodeGenCXX/debug-info-ctor2.cpp
@@ -1,5 +1,5 @@
 // FIXME: Check IR rather than asm, then triple is not needed.
-// RUN: %clang -Xclang -triple=%itanium_abi_triple -fverbose-asm -g -S %s -o - | grep AT_explicit
+// RUN: %clang --target=%itanium_abi_triple -fverbose-asm -g -S %s -o - | grep AT_explicit
 
 
 class MyClass
Index: clang/test/CodeGenCXX/debug-info-member.cpp
===================================================================
--- clang/test/CodeGenCXX/debug-info-member.cpp
+++ clang/test/CodeGenCXX/debug-info-member.cpp
@@ -1,5 +1,5 @@
 // FIXME: Check IR rather than asm, then triple is not needed.
-// RUN: %clang -Xclang -triple=%itanium_abi_triple -fverbose-asm -g -S %s -o - | grep DW_ACCESS_public
+// RUN: %clang --target=%itanium_abi_triple -fverbose-asm -g -S %s -o - | grep DW_ACCESS_public
 class A {
 public:
   int x;
Index: clang/test/CodeGenCXX/debug-info-method-spec.cpp
===================================================================
--- clang/test/CodeGenCXX/debug-info-method-spec.cpp
+++ clang/test/CodeGenCXX/debug-info-method-spec.cpp
@@ -1,5 +1,5 @@
 // FIXME: Check IR rather than asm, then triple is not needed.
-// RUN: %clang -Xclang -triple=%itanium_abi_triple -fverbose-asm -g -S %s -o - | grep DW_AT_specification
+// RUN: %clang --target=%itanium_abi_triple -fverbose-asm -g -S %s -o - | grep DW_AT_specification
 // Radar 9254491
 class A {
 public:
Index: clang/test/Driver/debug-options.c
===================================================================
--- clang/test/Driver/debug-options.c
+++ clang/test/Driver/debug-options.c
@@ -64,6 +64,14 @@
 // RUN: %clang -### -c -g %s -target x86_64-pc-freebsd10.0 2>&1 \
 // RUN:             | FileCheck -check-prefix=G_GDB %s
 
+// Windows.
+// RUN: %clang -### -c -g %s -target x86_64-w64-windows-gnu 2>&1 \
+// RUN:             | FileCheck -check-prefix=G_GDB %s
+// RUN: %clang -### -c -g %s -target x86_64-windows-msvc 2>&1 \
+// RUN:             | FileCheck -check-prefix=G_NOTUNING %s
+// RUN: %clang_cl -### -c -Z7 -target x86_64-windows-msvc -- %s 2>&1 \
+// RUN:             | FileCheck -check-prefix=G_NOTUNING %s
+
 // On the PS4, -g defaults to -gno-column-info, and we always generate the
 // arange section.
 // RUN: %clang -### -c %s -target x86_64-scei-ps4 2>&1 \
@@ -204,7 +212,7 @@
 // RUN: %clang -### -gmodules -gline-tables-only %s 2>&1 \
 // RUN:        | FileCheck -check-prefix=GLTO_ONLY %s
 //
-// RUN: %clang -### -gmodules -gline-directives-only %s 2>&1 \
+// RUN: %clang -### -target %itanium_abi_triple -gmodules -gline-directives-only %s 2>&1 \
 // RUN:        | FileCheck -check-prefix=GLIO_ONLY %s
 //
 // G: "-cc1"
@@ -259,6 +267,9 @@
 // G_LLDB: "-debugger-tuning=lldb"
 // G_SCE:  "-debugger-tuning=sce"
 //
+// G_NOTUNING: "-cc1"
+// G_NOTUNING-NOT: "-debugger-tuning="
+//
 // This tests asserts that "-gline-tables-only" "-g0" disables debug info.
 // GLTO_NO: "-cc1"
 // GLTO_NO-NOT: -debug-info-kind=
@@ -301,7 +312,8 @@
 //
 // NOCI-NOT: "-dwarf-column-info"
 //
-// GEXTREFS: "-dwarf-ext-refs" "-fmodule-format=obj" "-debug-info-kind={{standalone|limited}}"
+// GEXTREFS: "-dwarf-ext-refs" "-fmodule-format=obj"
+// GEXTREFS: "-debug-info-kind={{standalone|limited}}"
 
 // RUN: not %clang -cc1 -debug-info-kind=watkind 2>&1 | FileCheck -check-prefix=BADSTRING1 %s
 // BADSTRING1: error: invalid value 'watkind' in '-debug-info-kind=watkind'