Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -1952,6 +1952,7 @@
   Flags<[CC1Option]>, HelpText<"Provide minimal debug info in the object/executable to facilitate online symbolication/stack traces in the absence of .dwo/.dwp files when using Split DWARF">;
 def fno_split_dwarf_inlining: Flag<["-"], "fno-split-dwarf-inlining">, Group<f_Group>,
   Flags<[CC1Option]>;
+def fdebug_default_version: Joined<["-"], "fdebug-default-version=">, Group<f_Group>, HelpText<"Default DWARF version to use, if a -g option caused DWARF debug info to be produced.">;
 def fdebug_prefix_map_EQ
   : Joined<["-"], "fdebug-prefix-map=">, Group<f_Group>,
     Flags<[CC1Option,CC1AsOption]>,
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -3145,7 +3145,6 @@
   // This avoids having to monkey around further in cc1 other than to disable
   // codeview if not running in a Windows environment. Perhaps even that
   // decision should be made in the driver as well though.
-  unsigned DWARFVersion = 0;
   llvm::DebuggerKind DebuggerTuning = TC.getDefaultDebuggerTuning();
 
   bool SplitDWARFInlining =
@@ -3228,11 +3227,18 @@
     }
   }
 
+  unsigned DWARFVersion = 0;
+  unsigned DefaultDWARFVersion = ParseDebugDefaultVersion(TC, Args);
   if (EmitDwarf) {
     // Start with the platform default DWARF version
     DWARFVersion = TC.GetDefaultDwarfVersion();
     assert(DWARFVersion && "toolchain default DWARF version must be nonzero");
 
+    // If the user specified a default DWARF version, that takes precedence
+    // over the platform default.
+    if (DefaultDWARFVersion)
+      DWARFVersion = DefaultDWARFVersion;
+
     // Override with a user-specified DWARF version
     if (GDwarfN)
       if (auto ExplicitVersion = DwarfVersionNum(GDwarfN->getSpelling()))
@@ -6161,6 +6167,12 @@
     if (WantDebug)
       DwarfVersion = DwarfVersionNum(A->getSpelling());
   }
+
+  unsigned DefaultDwarfVersion = ParseDebugDefaultVersion(getToolChain(),
+                                                          Args);
+  if (DwarfVersion == 0)
+    DwarfVersion = DefaultDwarfVersion;
+
   if (DwarfVersion == 0)
     DwarfVersion = getToolChain().GetDefaultDwarfVersion();
 
Index: clang/lib/Driver/ToolChains/CommonArgs.h
===================================================================
--- clang/lib/Driver/ToolChains/CommonArgs.h
+++ clang/lib/Driver/ToolChains/CommonArgs.h
@@ -68,6 +68,9 @@
 unsigned ParseFunctionAlignment(const ToolChain &TC,
                                 const llvm::opt::ArgList &Args);
 
+unsigned ParseDebugDefaultVersion(const ToolChain &TC,
+                                  const llvm::opt::ArgList &Args);
+
 void AddAssemblerKPIC(const ToolChain &ToolChain,
                       const llvm::opt::ArgList &Args,
                       llvm::opt::ArgStringList &CmdArgs);
Index: clang/lib/Driver/ToolChains/CommonArgs.cpp
===================================================================
--- clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -1138,6 +1138,22 @@
   return Value ? llvm::Log2_32_Ceil(std::min(Value, 65536u)) : Value;
 }
 
+unsigned tools::ParseDebugDefaultVersion(const ToolChain &TC,
+                                         const ArgList &Args) {
+  const Arg *A = Args.getLastArg(options::OPT_fdebug_default_version);
+
+  if (!A)
+    return 0;
+
+  unsigned Value = 0;
+  if (StringRef(A->getValue()).getAsInteger(10, Value)
+      || Value > 5
+      || Value < 2)
+    TC.getDriver().Diag(diag::err_drv_invalid_int_value)
+        << A->getAsString(Args) << A->getValue();
+  return Value;
+}
+
 void tools::AddAssemblerKPIC(const ToolChain &ToolChain, const ArgList &Args,
                              ArgStringList &CmdArgs) {
   llvm::Reloc::Model RelocationModel;
Index: clang/test/Driver/debug-default-version.c
===================================================================
--- /dev/null
+++ clang/test/Driver/debug-default-version.c
@@ -0,0 +1,67 @@
+// RUN: %clang -### -c -target x86_64-linux-gnu -fdebug-default-version=4 -gdwarf-2 -S -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF2
+// RUN: %clang -### -target x86_64-linux-gnu -gdwarf-3 -fdebug-default-version=4 -S -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF3
+// RUN: %clang -### -target x86_64-linux-gnu -gdwarf-4 -fdebug-default-version=2 -S -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF4
+// RUN: %clang -### -target x86_64-linux-gnu -gdwarf-5 -S -fdebug-default-version=2 -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF5
+// RUN: %clang -### -target x86_64-linux-gnu -fdebug-default-version=5 -g -S -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF5
+// RUN: %clang -### -target x86_64-linux-gnu -gdwarf -fdebug-default-version=2 -S -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF2
+// RUN: %clang -target x86_64-linux-gnu -fdebug-default-version=4 -S -o - %s | FileCheck %s --check-prefixes=NODEBUGINFO,NODWARF
+
+// The -isysroot is used as a hack to avoid LIT messing with the SDKROOT
+// environment variable which indirecty overrides the version in the target
+// triple used here.
+// RUN: %clang -### -target x86_64-apple-macosx10.11 -fdebug-default-version=5 -g -S -o - %s -isysroot %t 2>&1 | FileCheck %s --check-prefix=DWARF5
+// RUN: %clang -### -target x86_64-apple-darwin14 -g -fdebug-default-version=4 -S -o - %s -isysroot %t 2>&1 | FileCheck %s --check-prefix=DWARF4
+
+// RUN: %clang -### -target powerpc-unknown-openbsd -fdebug-default-version=4 -g -S -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF4
+// RUN: %clang -### -target powerpc-unknown-freebsd -g -fdebug-default-version=4 -S -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF4
+// RUN: %clang -### -target i386-pc-solaris -fdebug-default-version=4 -g -S -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF4
+
+// 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.
+// RUN: %clang -### -target i686-pc-windows-msvc -fdebug-default-version=2 -gdwarf -S -o - %s 2>&1 \
+// RUN:     | FileCheck %s --check-prefixes=DWARF2,NOCODEVIEW
+//     Explicitly request both.
+// RUN: %clang -### -target i686-pc-windows-msvc -fdebug-default-version=4 -gdwarf -gcodeview -S -o - %s 2>&1 \
+// RUN:     | FileCheck %s --check-prefixes=DWARF4,CODEVIEW
+
+
+// Do Assembler testing most of the same test cases as those above.
+
+// RUN: %clang -### -target x86_64-linux-gnu -fdebug-default-version=5 -g -x assembler -c -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF5
+
+// RUN: %clang -### -c -target x86_64-linux-gnu -fdebug-default-version=4 -gdwarf-2 -x assembler -c -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF2
+// RUN: %clang -### -target x86_64-linux-gnu -gdwarf-3 -fdebug-default-version=4 -x assembler -c -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF3
+// RUN: %clang -### -target x86_64-linux-gnu -gdwarf-4 -fdebug-default-version=2 -x assembler -c -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF4
+// RUN: %clang -### -target x86_64-linux-gnu -gdwarf-5 -x assembler -c -fdebug-default-version=2 -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF5
+// RUN: %clang -### -target x86_64-linux-gnu -fdebug-default-version=5 -g -x assembler -c -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF5
+
+// The -isysroot is used as a hack to avoid LIT messing with the SDKROOT
+// environment variable which indirecty overrides the version in the target
+// triple used here.
+// RUN: %clang -### -target x86_64-apple-macosx10.11 -fdebug-default-version=5 -g -x assembler -c -o - %s -isysroot %t 2>&1 | FileCheck %s --check-prefix=DWARF5
+// RUN: %clang -### -target x86_64-apple-darwin14 -g -fdebug-default-version=4 -x assembler -c -o - %s -isysroot %t 2>&1 | FileCheck %s --check-prefix=DWARF4
+
+// RUN: %clang -### -target powerpc-unknown-openbsd -fdebug-default-version=4 -g -x assembler -c -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF4
+// RUN: %clang -### -target powerpc-unknown-freebsd -g -fdebug-default-version=4 -x assembler -c -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF4
+// RUN: %clang -### -target i386-pc-solaris -fdebug-default-version=4 -g -x assembler -c -o - %s 2>&1 | FileCheck %s --check-prefix=DWARF4
+
+
+int main (void) {
+  return 0;
+}
+
+// NOCODEVIEW-NOT: "-gcodeview"
+// CODEVIEW: "-gcodeview"
+
+// NODEBUGINFO-NOT: "-debug-info-kind="
+
+// DWARF2: "-dwarf-version=2"
+// DWARF3: "-dwarf-version=3"
+// DWARF4: "-dwarf-version=4"
+// DWARF5: "-dwarf-version=5"
+
+// NOCODEVIEW-NOT: "-gcodeview"
+// NODWARF-NOT: "-dwarf-version="
+// NODWARF-NOT: "-dwarf-version="