diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index f8b06bf2e976..9e9fab1e6a50 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -926,7 +926,7 @@ pub fn default_configuration(sess: &Session) -> ast::CrateConfig {
     let os = &sess.target.target.target_os;
     let env = &sess.target.target.target_env;
     let vendor = &sess.target.target.target_vendor;
-    let max_atomic_width = sess.target.target.options.max_atomic_width;
+    let max_atomic_width = sess.target.target.max_atomic_width();
 
     let fam = if let Some(ref fam) = sess.target.target.options.target_family {
         intern(fam)
diff --git a/src/librustc_back/target/aarch64_apple_ios.rs b/src/librustc_back/target/aarch64_apple_ios.rs
index 660ed0ac7b84..0d85d1ad90ad 100644
--- a/src/librustc_back/target/aarch64_apple_ios.rs
+++ b/src/librustc_back/target/aarch64_apple_ios.rs
@@ -25,7 +25,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             features: "+neon,+fp-armv8,+cyclone".to_string(),
             eliminate_frame_pointer: false,
-            max_atomic_width: 128,
+            max_atomic_width: Some(128),
             .. base
         },
     })
diff --git a/src/librustc_back/target/aarch64_linux_android.rs b/src/librustc_back/target/aarch64_linux_android.rs
index 7f54dab5b538..8440ed1c4d19 100644
--- a/src/librustc_back/target/aarch64_linux_android.rs
+++ b/src/librustc_back/target/aarch64_linux_android.rs
@@ -12,7 +12,7 @@ use target::{Target, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
-    base.max_atomic_width = 128;
+    base.max_atomic_width = Some(128);
     // As documented in http://developer.android.com/ndk/guides/cpu-features.html
     // the neon (ASIMD) and FP must exist on all android aarch64 targets.
     base.features = "+neon,+fp-armv8".to_string();
diff --git a/src/librustc_back/target/aarch64_unknown_linux_gnu.rs b/src/librustc_back/target/aarch64_unknown_linux_gnu.rs
index cca965f9d4ff..bb5376ff3ef0 100644
--- a/src/librustc_back/target/aarch64_unknown_linux_gnu.rs
+++ b/src/librustc_back/target/aarch64_unknown_linux_gnu.rs
@@ -12,7 +12,7 @@ use target::{Target, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
-    base.max_atomic_width = 128;
+    base.max_atomic_width = Some(128);
     Ok(Target {
         llvm_target: "aarch64-unknown-linux-gnu".to_string(),
         target_endian: "little".to_string(),
diff --git a/src/librustc_back/target/arm_linux_androideabi.rs b/src/librustc_back/target/arm_linux_androideabi.rs
index f3a18b13c678..e1906610441f 100644
--- a/src/librustc_back/target/arm_linux_androideabi.rs
+++ b/src/librustc_back/target/arm_linux_androideabi.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
     base.features = "+v7,+vfp3,+d16".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "arm-linux-androideabi".to_string(),
diff --git a/src/librustc_back/target/arm_unknown_linux_gnueabi.rs b/src/librustc_back/target/arm_unknown_linux_gnueabi.rs
index e666a8460e5e..7c6da341da63 100644
--- a/src/librustc_back/target/arm_unknown_linux_gnueabi.rs
+++ b/src/librustc_back/target/arm_unknown_linux_gnueabi.rs
@@ -12,7 +12,7 @@ use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         llvm_target: "arm-unknown-linux-gnueabi".to_string(),
         target_endian: "little".to_string(),
diff --git a/src/librustc_back/target/arm_unknown_linux_gnueabihf.rs b/src/librustc_back/target/arm_unknown_linux_gnueabihf.rs
index d65c89abc206..45f8f722059e 100644
--- a/src/librustc_back/target/arm_unknown_linux_gnueabihf.rs
+++ b/src/librustc_back/target/arm_unknown_linux_gnueabihf.rs
@@ -12,7 +12,7 @@ use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         llvm_target: "arm-unknown-linux-gnueabihf".to_string(),
         target_endian: "little".to_string(),
diff --git a/src/librustc_back/target/arm_unknown_linux_musleabi.rs b/src/librustc_back/target/arm_unknown_linux_musleabi.rs
index 028c91eadaeb..547afb3e8e58 100644
--- a/src/librustc_back/target/arm_unknown_linux_musleabi.rs
+++ b/src/librustc_back/target/arm_unknown_linux_musleabi.rs
@@ -16,7 +16,7 @@ pub fn target() -> TargetResult {
     // Most of these settings are copied from the arm_unknown_linux_gnueabi
     // target.
     base.features = "+v6".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         // It's important we use "gnueabi" and not "musleabi" here. LLVM uses it
         // to determine the calling convention and float ABI, and it doesn't
diff --git a/src/librustc_back/target/arm_unknown_linux_musleabihf.rs b/src/librustc_back/target/arm_unknown_linux_musleabihf.rs
index c7dda186f425..7e9996dea983 100644
--- a/src/librustc_back/target/arm_unknown_linux_musleabihf.rs
+++ b/src/librustc_back/target/arm_unknown_linux_musleabihf.rs
@@ -16,7 +16,7 @@ pub fn target() -> TargetResult {
     // Most of these settings are copied from the arm_unknown_linux_gnueabihf
     // target.
     base.features = "+v6,+vfp2".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         // It's important we use "gnueabihf" and not "musleabihf" here. LLVM
         // uses it to determine the calling convention and float ABI, and it
diff --git a/src/librustc_back/target/armv7_apple_ios.rs b/src/librustc_back/target/armv7_apple_ios.rs
index 71533a09b167..2bef25327a43 100644
--- a/src/librustc_back/target/armv7_apple_ios.rs
+++ b/src/librustc_back/target/armv7_apple_ios.rs
@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         target_vendor: "apple".to_string(),
         options: TargetOptions {
             features: "+v7,+vfp3,+neon".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })
diff --git a/src/librustc_back/target/armv7_linux_androideabi.rs b/src/librustc_back/target/armv7_linux_androideabi.rs
index 1c59262e0419..21ad18223dbc 100644
--- a/src/librustc_back/target/armv7_linux_androideabi.rs
+++ b/src/librustc_back/target/armv7_linux_androideabi.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
     base.features = "+v7,+thumb2,+vfp3,+d16".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "armv7-none-linux-android".to_string(),
diff --git a/src/librustc_back/target/armv7_unknown_linux_gnueabihf.rs b/src/librustc_back/target/armv7_unknown_linux_gnueabihf.rs
index 7e0306a03e2f..1abf8b0b69a2 100644
--- a/src/librustc_back/target/armv7_unknown_linux_gnueabihf.rs
+++ b/src/librustc_back/target/armv7_unknown_linux_gnueabihf.rs
@@ -26,7 +26,7 @@ pub fn target() -> TargetResult {
             // Info about features at https://wiki.debian.org/ArmHardFloatPort
             features: "+v7,+vfp3,+d16,+thumb2".to_string(),
             cpu: "generic".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })
diff --git a/src/librustc_back/target/armv7_unknown_linux_musleabihf.rs b/src/librustc_back/target/armv7_unknown_linux_musleabihf.rs
index e40704e5d499..efed7963c95e 100644
--- a/src/librustc_back/target/armv7_unknown_linux_musleabihf.rs
+++ b/src/librustc_back/target/armv7_unknown_linux_musleabihf.rs
@@ -17,7 +17,7 @@ pub fn target() -> TargetResult {
     // target.
     base.features = "+v7,+vfp3,+neon".to_string();
     base.cpu = "cortex-a8".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         // It's important we use "gnueabihf" and not "musleabihf" here. LLVM
         // uses it to determine the calling convention and float ABI, and LLVM
diff --git a/src/librustc_back/target/armv7s_apple_ios.rs b/src/librustc_back/target/armv7s_apple_ios.rs
index f24b9969910e..0b3ebf1294bd 100644
--- a/src/librustc_back/target/armv7s_apple_ios.rs
+++ b/src/librustc_back/target/armv7s_apple_ios.rs
@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         target_vendor: "apple".to_string(),
         options: TargetOptions {
             features: "+v7,+vfp4,+neon".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })
diff --git a/src/librustc_back/target/asmjs_unknown_emscripten.rs b/src/librustc_back/target/asmjs_unknown_emscripten.rs
index 667f7cf2c214..d86a9b093272 100644
--- a/src/librustc_back/target/asmjs_unknown_emscripten.rs
+++ b/src/librustc_back/target/asmjs_unknown_emscripten.rs
@@ -21,7 +21,7 @@ pub fn target() -> Result<Target, String> {
         linker_is_gnu: true,
         allow_asm: false,
         obj_is_bitcode: true,
-        max_atomic_width: 32,
+        max_atomic_width: Some(32),
         post_link_args: vec!["-s".to_string(), "ERROR_ON_UNDEFINED_SYMBOLS=1".to_string()],
         .. Default::default()
     };
diff --git a/src/librustc_back/target/i386_apple_ios.rs b/src/librustc_back/target/i386_apple_ios.rs
index 94146fe9d988..319ada4f8e17 100644
--- a/src/librustc_back/target/i386_apple_ios.rs
+++ b/src/librustc_back/target/i386_apple_ios.rs
@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         target_env: "".to_string(),
         target_vendor: "apple".to_string(),
         options: TargetOptions {
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })
diff --git a/src/librustc_back/target/i686_apple_darwin.rs b/src/librustc_back/target/i686_apple_darwin.rs
index 4876a3489d47..d3b09d9a0f11 100644
--- a/src/librustc_back/target/i686_apple_darwin.rs
+++ b/src/librustc_back/target/i686_apple_darwin.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::apple_base::opts();
     base.cpu = "yonah".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/i686_linux_android.rs b/src/librustc_back/target/i686_linux_android.rs
index 1de629238a13..a2c007d49696 100644
--- a/src/librustc_back/target/i686_linux_android.rs
+++ b/src/librustc_back/target/i686_linux_android.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
 
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     // http://developer.android.com/ndk/guides/abis.html#x86
     base.cpu = "pentiumpro".to_string();
diff --git a/src/librustc_back/target/i686_pc_windows_gnu.rs b/src/librustc_back/target/i686_pc_windows_gnu.rs
index 2c19b8109c36..0c2c5433e6c4 100644
--- a/src/librustc_back/target/i686_pc_windows_gnu.rs
+++ b/src/librustc_back/target/i686_pc_windows_gnu.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::windows_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     // Mark all dynamic libraries and executables as compatible with the larger 4GiB address
     // space available to x86 Windows binaries on x86_64.
diff --git a/src/librustc_back/target/i686_pc_windows_msvc.rs b/src/librustc_back/target/i686_pc_windows_msvc.rs
index cb02fcc308c2..2290d2057f13 100644
--- a/src/librustc_back/target/i686_pc_windows_msvc.rs
+++ b/src/librustc_back/target/i686_pc_windows_msvc.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::windows_msvc_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     // Mark all dynamic libraries and executables as compatible with the larger 4GiB address
     // space available to x86 Windows binaries on x86_64.
diff --git a/src/librustc_back/target/i686_unknown_dragonfly.rs b/src/librustc_back/target/i686_unknown_dragonfly.rs
index f96ec004b481..d8f8431e66e7 100644
--- a/src/librustc_back/target/i686_unknown_dragonfly.rs
+++ b/src/librustc_back/target/i686_unknown_dragonfly.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::dragonfly_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/i686_unknown_freebsd.rs b/src/librustc_back/target/i686_unknown_freebsd.rs
index 3489ecfe614d..ddbc74f25c9c 100644
--- a/src/librustc_back/target/i686_unknown_freebsd.rs
+++ b/src/librustc_back/target/i686_unknown_freebsd.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::freebsd_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/i686_unknown_haiku.rs b/src/librustc_back/target/i686_unknown_haiku.rs
index 862016704f47..9078206c9e06 100644
--- a/src/librustc_back/target/i686_unknown_haiku.rs
+++ b/src/librustc_back/target/i686_unknown_haiku.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::haiku_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/i686_unknown_linux_gnu.rs b/src/librustc_back/target/i686_unknown_linux_gnu.rs
index f2e865c015e3..bf9c28b0c10e 100644
--- a/src/librustc_back/target/i686_unknown_linux_gnu.rs
+++ b/src/librustc_back/target/i686_unknown_linux_gnu.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/i686_unknown_linux_musl.rs b/src/librustc_back/target/i686_unknown_linux_musl.rs
index a0a8de46e2f5..3d563fa6e5d9 100644
--- a/src/librustc_back/target/i686_unknown_linux_musl.rs
+++ b/src/librustc_back/target/i686_unknown_linux_musl.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::linux_musl_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
     base.pre_link_args.push("-Wl,-melf_i386".to_string());
 
diff --git a/src/librustc_back/target/le32_unknown_nacl.rs b/src/librustc_back/target/le32_unknown_nacl.rs
index 9ba6591f587c..a98a33feb5e9 100644
--- a/src/librustc_back/target/le32_unknown_nacl.rs
+++ b/src/librustc_back/target/le32_unknown_nacl.rs
@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         exe_suffix: ".pexe".to_string(),
         linker_is_gnu: true,
         allow_asm: false,
-        max_atomic_width: 32,
+        max_atomic_width: Some(32),
         .. Default::default()
     };
     Ok(Target {
diff --git a/src/librustc_back/target/mips64_unknown_linux_gnuabi64.rs b/src/librustc_back/target/mips64_unknown_linux_gnuabi64.rs
index 7e45b3206536..9f0b6fcc4364 100644
--- a/src/librustc_back/target/mips64_unknown_linux_gnuabi64.rs
+++ b/src/librustc_back/target/mips64_unknown_linux_gnuabi64.rs
@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
             // NOTE(mips64r2) matches C toolchain
             cpu: "mips64r2".to_string(),
             features: "+mips64r2".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             ..super::linux_base::opts()
         },
     })
diff --git a/src/librustc_back/target/mips64el_unknown_linux_gnuabi64.rs b/src/librustc_back/target/mips64el_unknown_linux_gnuabi64.rs
index 338a5da1e1d1..9c4531c5b212 100644
--- a/src/librustc_back/target/mips64el_unknown_linux_gnuabi64.rs
+++ b/src/librustc_back/target/mips64el_unknown_linux_gnuabi64.rs
@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
             // NOTE(mips64r2) matches C toolchain
             cpu: "mips64r2".to_string(),
             features: "+mips64r2".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             ..super::linux_base::opts()
         },
     })
diff --git a/src/librustc_back/target/mips_unknown_linux_gnu.rs b/src/librustc_back/target/mips_unknown_linux_gnu.rs
index ab967f6b40fb..e744dce2bb4a 100644
--- a/src/librustc_back/target/mips_unknown_linux_gnu.rs
+++ b/src/librustc_back/target/mips_unknown_linux_gnu.rs
@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32r2".to_string(),
             features: "+mips32r2".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })
diff --git a/src/librustc_back/target/mips_unknown_linux_musl.rs b/src/librustc_back/target/mips_unknown_linux_musl.rs
index 4a69bce53bc9..4254c1b83e31 100644
--- a/src/librustc_back/target/mips_unknown_linux_musl.rs
+++ b/src/librustc_back/target/mips_unknown_linux_musl.rs
@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32r2".to_string(),
             features: "+mips32r2,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         }
     })
diff --git a/src/librustc_back/target/mips_unknown_linux_uclibc.rs b/src/librustc_back/target/mips_unknown_linux_uclibc.rs
index 529bd310391c..e6b2672a8e39 100644
--- a/src/librustc_back/target/mips_unknown_linux_uclibc.rs
+++ b/src/librustc_back/target/mips_unknown_linux_uclibc.rs
@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32r2".to_string(),
             features: "+mips32r2,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })
diff --git a/src/librustc_back/target/mipsel_unknown_linux_gnu.rs b/src/librustc_back/target/mipsel_unknown_linux_gnu.rs
index b66fb62cd591..4949055861e5 100644
--- a/src/librustc_back/target/mipsel_unknown_linux_gnu.rs
+++ b/src/librustc_back/target/mipsel_unknown_linux_gnu.rs
@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32".to_string(),
             features: "+mips32".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })
diff --git a/src/librustc_back/target/mipsel_unknown_linux_musl.rs b/src/librustc_back/target/mipsel_unknown_linux_musl.rs
index a0524e5e7638..f282ac7e88b8 100644
--- a/src/librustc_back/target/mipsel_unknown_linux_musl.rs
+++ b/src/librustc_back/target/mipsel_unknown_linux_musl.rs
@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32".to_string(),
             features: "+mips32,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         }
     })
diff --git a/src/librustc_back/target/mipsel_unknown_linux_uclibc.rs b/src/librustc_back/target/mipsel_unknown_linux_uclibc.rs
index 1040a0fbe172..0f9b562068f5 100644
--- a/src/librustc_back/target/mipsel_unknown_linux_uclibc.rs
+++ b/src/librustc_back/target/mipsel_unknown_linux_uclibc.rs
@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32".to_string(),
             features: "+mips32,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })
diff --git a/src/librustc_back/target/mod.rs b/src/librustc_back/target/mod.rs
index 610594927787..1843fc581f1c 100644
--- a/src/librustc_back/target/mod.rs
+++ b/src/librustc_back/target/mod.rs
@@ -353,9 +353,8 @@ pub struct TargetOptions {
     // will 'just work'.
     pub obj_is_bitcode: bool,
 
-    /// Maximum integer size in bits that this target can perform atomic
-    /// operations on.
-    pub max_atomic_width: u64,
+    /// Don't use this field; instead use the `.max_atomic_width()` method.
+    pub max_atomic_width: Option<u64>,
 
     /// Panic strategy: "unwind" or "abort"
     pub panic_strategy: PanicStrategy,
@@ -407,10 +406,7 @@ impl Default for TargetOptions {
             allow_asm: true,
             has_elf_tls: false,
             obj_is_bitcode: false,
-            // NOTE 0 is *not* the real default value of max_atomic_width. The default value is
-            // actually the pointer_width of the target. This default is injected in the
-            // Target::from_json function.
-            max_atomic_width: 0,
+            max_atomic_width: None,
             panic_strategy: PanicStrategy::Unwind,
         }
     }
@@ -431,6 +427,12 @@ impl Target {
         }
     }
 
+    /// Maximum integer size in bits that this target can perform atomic
+    /// operations on.
+    pub fn max_atomic_width(&self) -> u64 {
+        self.options.max_atomic_width.unwrap_or(self.target_pointer_width.parse().unwrap())
+    }
+
     /// Load a target descriptor from a JSON object.
     pub fn from_json(obj: Json) -> TargetResult {
         // While ugly, this code must remain this way to retain
@@ -469,9 +471,6 @@ impl Target {
             options: Default::default(),
         };
 
-        // Default max-atomic-width to target-pointer-width
-        base.options.max_atomic_width = base.target_pointer_width.parse().unwrap();
-
         macro_rules! key {
             ($key_name:ident) => ( {
                 let name = (stringify!($key_name)).replace("_", "-");
@@ -484,11 +483,11 @@ impl Target {
                     .map(|o| o.as_boolean()
                          .map(|s| base.options.$key_name = s));
             } );
-            ($key_name:ident, u64) => ( {
+            ($key_name:ident, Option<u64>) => ( {
                 let name = (stringify!($key_name)).replace("_", "-");
                 obj.find(&name[..])
                     .map(|o| o.as_u64()
-                         .map(|s| base.options.$key_name = s));
+                         .map(|s| base.options.$key_name = Some(s)));
             } );
             ($key_name:ident, PanicStrategy) => ( {
                 let name = (stringify!($key_name)).replace("_", "-");
@@ -562,7 +561,7 @@ impl Target {
         key!(exe_allocation_crate);
         key!(has_elf_tls, bool);
         key!(obj_is_bitcode, bool);
-        key!(max_atomic_width, u64);
+        key!(max_atomic_width, Option<u64>);
         try!(key!(panic_strategy, PanicStrategy));
 
         Ok(base)
@@ -708,10 +707,6 @@ impl ToJson for Target {
         target_option_val!(max_atomic_width);
         target_option_val!(panic_strategy);
 
-        if self.options.max_atomic_width.to_string() != self.target_pointer_width {
-            d.insert("max-atomic-width".to_string(), self.options.max_atomic_width.to_json());
-        }
-
         Json::Object(d)
     }
 }
diff --git a/src/librustc_back/target/powerpc64_unknown_linux_gnu.rs b/src/librustc_back/target/powerpc64_unknown_linux_gnu.rs
index 1c04e763417c..333bfad4a268 100644
--- a/src/librustc_back/target/powerpc64_unknown_linux_gnu.rs
+++ b/src/librustc_back/target/powerpc64_unknown_linux_gnu.rs
@@ -14,7 +14,7 @@ pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
     base.cpu = "ppc64".to_string();
     base.pre_link_args.push("-m64".to_string());
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "powerpc64-unknown-linux-gnu".to_string(),
diff --git a/src/librustc_back/target/powerpc64le_unknown_linux_gnu.rs b/src/librustc_back/target/powerpc64le_unknown_linux_gnu.rs
index 906e28d2f20c..e75da133314a 100644
--- a/src/librustc_back/target/powerpc64le_unknown_linux_gnu.rs
+++ b/src/librustc_back/target/powerpc64le_unknown_linux_gnu.rs
@@ -14,7 +14,7 @@ pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
     base.cpu = "ppc64le".to_string();
     base.pre_link_args.push("-m64".to_string());
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "powerpc64le-unknown-linux-gnu".to_string(),
diff --git a/src/librustc_back/target/powerpc_unknown_linux_gnu.rs b/src/librustc_back/target/powerpc_unknown_linux_gnu.rs
index aebf9cd68717..45d28dd031f7 100644
--- a/src/librustc_back/target/powerpc_unknown_linux_gnu.rs
+++ b/src/librustc_back/target/powerpc_unknown_linux_gnu.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
     base.pre_link_args.push("-m32".to_string());
-    base.max_atomic_width = 32;
+    base.max_atomic_width = Some(32);
 
     Ok(Target {
         llvm_target: "powerpc-unknown-linux-gnu".to_string(),
diff --git a/src/librustc_back/target/s390x_unknown_linux_gnu.rs b/src/librustc_back/target/s390x_unknown_linux_gnu.rs
index 79f2d290e376..6e2dd6cd67c9 100644
--- a/src/librustc_back/target/s390x_unknown_linux_gnu.rs
+++ b/src/librustc_back/target/s390x_unknown_linux_gnu.rs
@@ -18,7 +18,7 @@ pub fn target() -> TargetResult {
     // cabi_s390x.rs are for now hard-coded to assume the no-vector ABI.
     // Pass the -vector feature string to LLVM to respect this assumption.
     base.features = "-vector".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "s390x-unknown-linux-gnu".to_string(),
diff --git a/src/librustc_back/target/thumbv6m_none_eabi.rs b/src/librustc_back/target/thumbv6m_none_eabi.rs
index d00744353a64..6c22f9853845 100644
--- a/src/librustc_back/target/thumbv6m_none_eabi.rs
+++ b/src/librustc_back/target/thumbv6m_none_eabi.rs
@@ -29,7 +29,7 @@ pub fn target() -> TargetResult {
             features: "+strict-align".to_string(),
             // There are no atomic instructions available in the instruction set of the ARMv6-M
             // architecture
-            max_atomic_width: 0,
+            max_atomic_width: Some(0),
             .. super::thumb_base::opts()
         }
     })
diff --git a/src/librustc_back/target/thumbv7em_none_eabi.rs b/src/librustc_back/target/thumbv7em_none_eabi.rs
index 6f5216e0f776..ddad4e3624f3 100644
--- a/src/librustc_back/target/thumbv7em_none_eabi.rs
+++ b/src/librustc_back/target/thumbv7em_none_eabi.rs
@@ -33,7 +33,7 @@ pub fn target() -> TargetResult {
         target_vendor: "".to_string(),
 
         options: TargetOptions {
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             .. super::thumb_base::opts()
         },
     })
diff --git a/src/librustc_back/target/thumbv7em_none_eabihf.rs b/src/librustc_back/target/thumbv7em_none_eabihf.rs
index b695674aada8..a9fac48e8e5a 100644
--- a/src/librustc_back/target/thumbv7em_none_eabihf.rs
+++ b/src/librustc_back/target/thumbv7em_none_eabihf.rs
@@ -42,7 +42,7 @@ pub fn target() -> TargetResult {
             // Reference:
             // ARMv7-M Architecture Reference Manual - A2.5 The optional floating-point extension
             features: "+vfp4,+d16,+fp-only-sp".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             .. super::thumb_base::opts()
         }
     })
diff --git a/src/librustc_back/target/thumbv7m_none_eabi.rs b/src/librustc_back/target/thumbv7m_none_eabi.rs
index 1ff95e1080b4..ed61dd0459b4 100644
--- a/src/librustc_back/target/thumbv7m_none_eabi.rs
+++ b/src/librustc_back/target/thumbv7m_none_eabi.rs
@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         target_vendor: "".to_string(),
 
         options: TargetOptions {
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             .. super::thumb_base::opts()
         },
     })
diff --git a/src/librustc_back/target/wasm32_unknown_emscripten.rs b/src/librustc_back/target/wasm32_unknown_emscripten.rs
index 2923f2eb92e4..77ab4fcae700 100644
--- a/src/librustc_back/target/wasm32_unknown_emscripten.rs
+++ b/src/librustc_back/target/wasm32_unknown_emscripten.rs
@@ -23,7 +23,7 @@ pub fn target() -> Result<Target, String> {
         linker_is_gnu: true,
         allow_asm: false,
         obj_is_bitcode: true,
-        max_atomic_width: 32,
+        max_atomic_width: Some(32),
         post_link_args: vec!["-s".to_string(), "BINARYEN=1".to_string(),
                              "-s".to_string(), "ERROR_ON_UNDEFINED_SYMBOLS=1".to_string()],
         .. Default::default()
diff --git a/src/librustc_back/target/x86_64_apple_darwin.rs b/src/librustc_back/target/x86_64_apple_darwin.rs
index 65e4b1400fcf..b3c1561dbcc0 100644
--- a/src/librustc_back/target/x86_64_apple_darwin.rs
+++ b/src/librustc_back/target/x86_64_apple_darwin.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::apple_base::opts();
     base.cpu = "core2".to_string();
-    base.max_atomic_width = 128; // core2 support cmpxchg16b
+    base.max_atomic_width = Some(128); // core2 support cmpxchg16b
     base.eliminate_frame_pointer = false;
     base.pre_link_args.push("-m64".to_string());
 
diff --git a/src/librustc_back/target/x86_64_apple_ios.rs b/src/librustc_back/target/x86_64_apple_ios.rs
index 3b8b636b6dc6..7a58bb34ce7f 100644
--- a/src/librustc_back/target/x86_64_apple_ios.rs
+++ b/src/librustc_back/target/x86_64_apple_ios.rs
@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         target_env: "".to_string(),
         target_vendor: "apple".to_string(),
         options: TargetOptions {
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })
diff --git a/src/librustc_back/target/x86_64_pc_windows_gnu.rs b/src/librustc_back/target/x86_64_pc_windows_gnu.rs
index 086e0e6bf4fe..321585cd65eb 100644
--- a/src/librustc_back/target/x86_64_pc_windows_gnu.rs
+++ b/src/librustc_back/target/x86_64_pc_windows_gnu.rs
@@ -14,7 +14,7 @@ pub fn target() -> TargetResult {
     let mut base = super::windows_base::opts();
     base.cpu = "x86-64".to_string();
     base.pre_link_args.push("-m64".to_string());
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "x86_64-pc-windows-gnu".to_string(),
diff --git a/src/librustc_back/target/x86_64_pc_windows_msvc.rs b/src/librustc_back/target/x86_64_pc_windows_msvc.rs
index 064f06e9b31d..ea8909d213e8 100644
--- a/src/librustc_back/target/x86_64_pc_windows_msvc.rs
+++ b/src/librustc_back/target/x86_64_pc_windows_msvc.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::windows_msvc_base::opts();
     base.cpu = "x86-64".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "x86_64-pc-windows-msvc".to_string(),
diff --git a/src/librustc_back/target/x86_64_rumprun_netbsd.rs b/src/librustc_back/target/x86_64_rumprun_netbsd.rs
index fd6578c2a2a0..331372143969 100644
--- a/src/librustc_back/target/x86_64_rumprun_netbsd.rs
+++ b/src/librustc_back/target/x86_64_rumprun_netbsd.rs
@@ -16,7 +16,7 @@ pub fn target() -> TargetResult {
     base.pre_link_args.push("-m64".to_string());
     base.linker = "x86_64-rumprun-netbsd-gcc".to_string();
     base.ar = "x86_64-rumprun-netbsd-ar".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     base.dynamic_linking = false;
     base.has_rpath = false;
diff --git a/src/librustc_back/target/x86_64_sun_solaris.rs b/src/librustc_back/target/x86_64_sun_solaris.rs
index 2a1feb937f74..8e4fd94e7bce 100644
--- a/src/librustc_back/target/x86_64_sun_solaris.rs
+++ b/src/librustc_back/target/x86_64_sun_solaris.rs
@@ -14,7 +14,7 @@ pub fn target() -> TargetResult {
     let mut base = super::solaris_base::opts();
     base.pre_link_args.push("-m64".to_string());
     base.cpu = "x86-64".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "x86_64-pc-solaris".to_string(),
diff --git a/src/librustc_back/target/x86_64_unknown_bitrig.rs b/src/librustc_back/target/x86_64_unknown_bitrig.rs
index 382096558981..eda16c29466b 100644
--- a/src/librustc_back/target/x86_64_unknown_bitrig.rs
+++ b/src/librustc_back/target/x86_64_unknown_bitrig.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::bitrig_base::opts();
     base.cpu = "x86-64".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m64".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/x86_64_unknown_dragonfly.rs b/src/librustc_back/target/x86_64_unknown_dragonfly.rs
index 7e40d49b870a..194efb8fc232 100644
--- a/src/librustc_back/target/x86_64_unknown_dragonfly.rs
+++ b/src/librustc_back/target/x86_64_unknown_dragonfly.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::dragonfly_base::opts();
     base.cpu = "x86-64".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m64".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/x86_64_unknown_freebsd.rs b/src/librustc_back/target/x86_64_unknown_freebsd.rs
index f38cdd4bec55..b127bee163b8 100644
--- a/src/librustc_back/target/x86_64_unknown_freebsd.rs
+++ b/src/librustc_back/target/x86_64_unknown_freebsd.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::freebsd_base::opts();
     base.cpu = "x86-64".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m64".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/x86_64_unknown_haiku.rs b/src/librustc_back/target/x86_64_unknown_haiku.rs
index 171e88cee50c..7cf0599037c1 100644
--- a/src/librustc_back/target/x86_64_unknown_haiku.rs
+++ b/src/librustc_back/target/x86_64_unknown_haiku.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::haiku_base::opts();
     base.cpu = "x86-64".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m64".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/x86_64_unknown_linux_gnu.rs b/src/librustc_back/target/x86_64_unknown_linux_gnu.rs
index ef81d397a8f1..f95bcb556e57 100644
--- a/src/librustc_back/target/x86_64_unknown_linux_gnu.rs
+++ b/src/librustc_back/target/x86_64_unknown_linux_gnu.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
     base.cpu = "x86-64".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m64".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/x86_64_unknown_linux_musl.rs b/src/librustc_back/target/x86_64_unknown_linux_musl.rs
index 4bad7754b390..c3bf9dcca6ee 100644
--- a/src/librustc_back/target/x86_64_unknown_linux_musl.rs
+++ b/src/librustc_back/target/x86_64_unknown_linux_musl.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::linux_musl_base::opts();
     base.cpu = "x86-64".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m64".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/x86_64_unknown_netbsd.rs b/src/librustc_back/target/x86_64_unknown_netbsd.rs
index 2d0b1e2a9330..87a7c184644d 100644
--- a/src/librustc_back/target/x86_64_unknown_netbsd.rs
+++ b/src/librustc_back/target/x86_64_unknown_netbsd.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::netbsd_base::opts();
     base.cpu = "x86-64".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m64".to_string());
 
     Ok(Target {
diff --git a/src/librustc_back/target/x86_64_unknown_openbsd.rs b/src/librustc_back/target/x86_64_unknown_openbsd.rs
index 339dbd591a02..e9d645b0d38f 100644
--- a/src/librustc_back/target/x86_64_unknown_openbsd.rs
+++ b/src/librustc_back/target/x86_64_unknown_openbsd.rs
@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::openbsd_base::opts();
     base.cpu = "x86-64".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m64".to_string());
 
     Ok(Target {