Index: docs/ReleaseNotes.rst
===================================================================
--- docs/ReleaseNotes.rst
+++ docs/ReleaseNotes.rst
@@ -142,6 +142,14 @@
 ABI Changes in Clang
 --------------------
 
+- The System V i386 psABI requires __m64 to be passed in MMX registers.
+  Clang historically had a bug where it failed to apply this rule, and
+  some platforms (e.g. Darwin, PS4, and FreeBSD) have opted to maintain
+  compatibility with the old Clang behavior, so we only apply it on
+  platforms that have specifically requested it (currently just Linux and
+  NetBSD). You can switch back to old API behavior with flag:
+  -fclang-abi-compat=8.0.
+
 - ...
 
 OpenMP Support in Clang
Index: include/clang/Basic/LangOptions.h
===================================================================
--- include/clang/Basic/LangOptions.h
+++ include/clang/Basic/LangOptions.h
@@ -138,6 +138,11 @@
     /// rather than returning the required alignment.
     Ver7,
 
+    /// Attempt to be ABI-compatible with code generated by Clang 8.0.x
+    /// (SVN r363116). This causes __m64 to be passed in MMX register 
+    /// instead of integer register.
+    Ver8,
+
     /// Conform to the underlying platform's C and C++ ABIs as closely
     /// as we can.
     Latest
Index: lib/CodeGen/TargetInfo.cpp
===================================================================
--- lib/CodeGen/TargetInfo.cpp
+++ lib/CodeGen/TargetInfo.cpp
@@ -1088,6 +1088,11 @@
   }
 
   bool isPassInMMXRegABI() const {
+    // Clang <= 8.0 did not do this for compatiblity with old behavior.
+    if (getContext().getLangOpts().getClangABICompat() <=
+        LangOptions::ClangABI::Ver8)
+      return false;
+
     // The System V i386 psABI requires __m64 to be passed in MMX registers.
     // Clang historically had a bug where it failed to apply this rule, and
     // some platforms (e.g. Darwin, PS4, and FreeBSD) have opted to maintain
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -3104,6 +3104,8 @@
         Opts.setClangABICompat(LangOptions::ClangABI::Ver6);
       else if (Major <= 7)
         Opts.setClangABICompat(LangOptions::ClangABI::Ver7);
+      else if (Major <= 8)
+        Opts.setClangABICompat(LangOptions::ClangABI::Ver8);
     } else if (Ver != "latest") {
       Diags.Report(diag::err_drv_invalid_value)
           << A->getAsString(Args) << A->getValue();
Index: test/CodeGen/x86_32-m64.c
===================================================================
--- test/CodeGen/x86_32-m64.c
+++ test/CodeGen/x86_32-m64.c
@@ -3,6 +3,7 @@
 // RUN: %clang_cc1 -w -O2 -fblocks -triple i386-apple-darwin9 -target-cpu yonah -emit-llvm -o - %s | FileCheck %s --check-prefixes=CHECK,DARWIN
 // RUN: %clang_cc1 -w -O2 -fblocks -triple i386-pc-elfiamcu -mfloat-abi soft -emit-llvm -o - %s | FileCheck %s --check-prefixes=CHECK,IAMCU
 // RUN: %clang_cc1 -w -O2 -fblocks -triple i386-pc-win32 -emit-llvm -o - %s | FileCheck %s --check-prefixes=CHECK,WIN32
+// RUN: %clang_cc1 -w -O2 -fblocks -triple i386-pc-linux-gnu -target-cpu pentium4 -emit-llvm -fclang-abi-compat=8.0 -o - %s | FileCheck %s --check-prefixes=CHECK,OLDABI
 
 #include <mmintrin.h>
 __m64 m64;
@@ -24,6 +25,8 @@
 // WIN32-LABEL: define dso_local <1 x i64> @caller(i64 %__m1.coerce, i64 %__m2.coerce)
 // WIN32: call void @callee(i64 %__m2.coerce, i64 %__m1.coerce)
 // WIN32: ret <1 x i64>
+// OLDABI: tail call void @callee(i64 %__m2.coerce, i64 %__m1.coerce)
+// OLDABI: ret <1 x i64>
   callee(__m2, __m1);
   return m64;
 }