Index: clang/lib/Basic/Targets/X86.h
===================================================================
--- clang/lib/Basic/Targets/X86.h
+++ clang/lib/Basic/Targets/X86.h
@@ -346,9 +346,8 @@
          (1 << TargetInfo::LongDouble));
 
     // x86-32 has atomics up to 8 bytes
-    // FIXME: Check that we actually have cmpxchg8b before setting
-    // MaxAtomicInlineWidth. (cmpxchg8b is an i586 instruction.)
-    MaxAtomicPromoteWidth = MaxAtomicInlineWidth = 64;
+    MaxAtomicPromoteWidth = 64;
+    MaxAtomicInlineWidth = 32;
   }
 
   BuiltinVaListKind getBuiltinVaListKind() const override {
@@ -384,6 +383,11 @@
     return X86TargetInfo::validateOperandSize(Constraint, Size);
   }
 
+  void setMaxAtomicWidth() override {
+    if (CPU >= CK_i586)
+      MaxAtomicInlineWidth = 64;
+  }
+
   ArrayRef<Builtin::Info> getTargetBuiltins() const override;
 };
 
Index: clang/test/CodeGen/atomic-ops.c
===================================================================
--- clang/test/CodeGen/atomic-ops.c
+++ clang/test/CodeGen/atomic-ops.c
@@ -1,10 +1,10 @@
-// RUN: %clang_cc1 %s -emit-llvm -o - -ffreestanding -ffake-address-space-map -triple=i686-apple-darwin9 | FileCheck %s
+// RUN: %clang_cc1 %s -emit-llvm -o - -ffreestanding -ffake-address-space-map -triple=i686-apple-darwin9 -target-cpu pentium4 | FileCheck %s
 // REQUIRES: x86-registered-target
 
 // Also test serialization of atomic operations here, to avoid duplicating the
 // test.
-// RUN: %clang_cc1 %s -emit-pch -o %t -ffreestanding -ffake-address-space-map -triple=i686-apple-darwin9
-// RUN: %clang_cc1 %s -include-pch %t -ffreestanding -ffake-address-space-map -triple=i686-apple-darwin9 -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 %s -emit-pch -o %t -ffreestanding -ffake-address-space-map -triple=i686-apple-darwin9 -target-cpu pentium4
+// RUN: %clang_cc1 %s -include-pch %t -ffreestanding -ffake-address-space-map -triple=i686-apple-darwin9 -target-cpu pentium4 -emit-llvm -o - | FileCheck %s
 #ifndef ALREADY_INCLUDED
 #define ALREADY_INCLUDED
 
Index: clang/test/Preprocessor/init.c
===================================================================
--- clang/test/Preprocessor/init.c
+++ clang/test/Preprocessor/init.c
@@ -10083,7 +10083,7 @@
 // AVR:#define __WINT_TYPE__ int
 
 
-// RUN: %clang_cc1 -E -dM -ffreestanding \
+// RUN: %clang_cc1 -E -dM -ffreestanding -target-cpu pentium4 \
 // RUN:    -triple i686-windows-msvc -fms-compatibility -x c++ < /dev/null \
 // RUN:  | FileCheck -match-full-lines -check-prefix MSVC-X32 %s
 
Index: clang/test/Sema/atomic-ops.c
===================================================================
--- clang/test/Sema/atomic-ops.c
+++ clang/test/Sema/atomic-ops.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 %s -verify -ffreestanding -fsyntax-only -triple=i686-linux-gnu -std=c11
+// RUN: %clang_cc1 %s -verify -ffreestanding -fsyntax-only -triple=i686-linux-gnu -std=c11 -target-cpu pentium4
 
 // Basic parsing/Sema tests for __c11_atomic_*