Auto merge of #30417 - alexcrichton:better-detect-elf-tls, r=brson

Currently a compiler can be built with the `--disable-elf-tls` option for compatibility with OSX 10.6 which doesn't have ELF TLS. This is unfortunate, however, as a whole new compiler must be generated which can take some time. These commits add a new (feature gated) `cfg(target_thread_local)` annotation set by the compiler which indicates whether `#[thread_local]` is available for use. The compiler now interprets `MACOSX_DEPLOYMENT_TARGET` (a standard environment variable) to set this flag on OSX. With this we may want to start compiling our OSX nightlies with `MACOSX_DEPLOYMENT_TARGET` set to 10.6 which would allow the compiler out-of-the-box to generate 10.6-compatible binaries.

For now the compiler still by default targets OSX 10.7 by allowing ELF TLS by default (e.g. if `MACOSX_DEPLOYMENT_TARGET` isn't set).

src/librustc/session/config.rs

@@ -660,6 +660,9 @@ pub fn default_configuration(sess: &Session) -> ast::CrateConfig {
         "windows" | "unix" => ret.push(attr::mk_word_item(fam)),
         _ => (),
     }
+    if sess.target.target.options.has_elf_tls {
+        ret.push(attr::mk_word_item(InternedString::new("target_thread_local")));
+    }
     if sess.opts.debug_assertions {
         ret.push(attr::mk_word_item(InternedString::new("debug_assertions")));
     }

src/librustc_back/target/aarch64_unknown_linux_gnu.rs

@@ -11,7 +11,8 @@
 use target::Target;
 
 pub fn target() -> Target {
-    let base = super::linux_base::opts();
+    let mut base = super::linux_base::opts();
+    base.has_elf_tls = false;
     Target {
         llvm_target: "aarch64-unknown-linux-gnu".to_string(),
         target_endian: "little".to_string(),

src/librustc_back/target/apple_base.rs

@@ -8,10 +8,30 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::env;
+
 use target::TargetOptions;
-use std::default::Default;
 
 pub fn opts() -> TargetOptions {
+    // ELF TLS is only available in OSX 10.7+. If you try to compile for 10.6
+    // either the linker will complain if it is used or the binary will end up
+    // segfaulting at runtime when run on 10.6. Rust by default supports OSX
+    // 10.7+, but there is a standard environment variable,
+    // MACOSX_DEPLOYMENT_TARGET, which is used to signal targeting older
+    // versions of OSX. For example compiling on 10.10 with
+    // MACOSX_DEPLOYMENT_TARGET set to 10.6 will cause the linker to generate
+    // warnings about the usage of ELF TLS.
+    //
+    // Here we detect what version is being requested, defaulting to 10.7. ELF
+    // TLS is flagged as enabled if it looks to be supported.
+    let deployment_target = env::var("MACOSX_DEPLOYMENT_TARGET").ok();
+    let version = deployment_target.as_ref().and_then(|s| {
+        let mut i = s.splitn(2, ".");
+        i.next().and_then(|a| i.next().map(|b| (a, b)))
+    }).and_then(|(a, b)| {
+        a.parse::<u32>().and_then(|a| b.parse::<u32>().map(|b| (a, b))).ok()
+    }).unwrap_or((10, 7));
+
     TargetOptions {
         // OSX has -dead_strip, which doesn't rely on ffunction_sections
         function_sections: false,
@@ -25,6 +45,7 @@ pub fn opts() -> TargetOptions {
         archive_format: "bsd".to_string(),
         pre_link_args: Vec::new(),
         exe_allocation_crate: super::maybe_jemalloc(),
+        has_elf_tls: version >= (10, 7),
         .. Default::default()
     }
 }

src/librustc_back/target/apple_ios_base.rs

@@ -88,6 +88,7 @@ pub fn opts(arch: Arch) -> TargetOptions {
         dynamic_linking: false,
         executables: true,
         pre_link_args: pre_link_args(arch),
+        has_elf_tls: false,
         .. super::apple_base::opts()
     }
 }

src/librustc_back/target/linux_base.rs

@@ -30,6 +30,7 @@ pub fn opts() -> TargetOptions {
         position_independent_executables: true,
         archive_format: "gnu".to_string(),
         exe_allocation_crate: super::maybe_jemalloc(),
+        has_elf_tls: true,
         .. Default::default()
     }
 }

src/librustc_back/target/mod.rs

@@ -195,6 +195,10 @@ pub struct TargetOptions {
     /// Default crate for allocation symbols to link against
     pub lib_allocation_crate: String,
     pub exe_allocation_crate: String,
+
+    /// Flag indicating whether ELF TLS (e.g. #[thread_local]) is available for
+    /// this target.
+    pub has_elf_tls: bool,
 }
 
 impl Default for TargetOptions {
@@ -240,6 +244,7 @@ impl Default for TargetOptions {
             lib_allocation_crate: "alloc_system".to_string(),
             exe_allocation_crate: "alloc_system".to_string(),
             allow_asm: true,
+            has_elf_tls: false,
         }
     }
 }

src/librustc_driver/driver.rs

@@ -606,7 +606,7 @@ pub fn phase_2_configure_and_expand(sess: &Session,
         feature_gated_cfgs.sort();
         feature_gated_cfgs.dedup();
         for cfg in &feature_gated_cfgs {
-            cfg.check_and_emit(sess.diagnostic(), &features);
+            cfg.check_and_emit(sess.diagnostic(), &features, sess.codemap());
         }
     });
 

src/libstd/lib.rs

@@ -218,6 +218,7 @@
 #![feature(borrow_state)]
 #![feature(box_syntax)]
 #![feature(cfg_target_vendor)]
+#![feature(cfg_target_thread_local)]
 #![feature(char_internals)]
 #![feature(clone_from_slice)]
 #![feature(collections)]

src/libstd/thread/local.rs

@@ -15,10 +15,6 @@
 use cell::UnsafeCell;
 use mem;
 
-// Sure wish we had macro hygiene, no?
-#[doc(hidden)]
-pub use self::imp::Key as __KeyInner;
-
 /// A thread local storage key which owns its contents.
 ///
 /// This key uses the fastest possible implementation available to it for the
@@ -61,78 +57,42 @@ pub use self::imp::Key as __KeyInner;
 /// });
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
-pub struct LocalKey<T:'static> {
-    // The key itself may be tagged with #[thread_local], and this `Key` is
-    // stored as a `static`, and it's not valid for a static to reference the
-    // address of another thread_local static. For this reason we kinda wonkily
-    // work around this by generating a shim function which will give us the
-    // address of the inner TLS key at runtime.
+pub struct LocalKey<T: 'static> {
+    // This outer `LocalKey<T>` type is what's going to be stored in statics,
+    // but actual data inside will sometimes be tagged with #[thread_local].
+    // It's not valid for a true static to reference a #[thread_local] static,
+    // so we get around that by exposing an accessor through a layer of function
+    // indirection (this thunk).
+    //
+    // Note that the thunk is itself unsafe because the returned lifetime of the
+    // slot where data lives, `'static`, is not actually valid. The lifetime
+    // here is actually `'thread`!
     //
-    // This is trivially devirtualizable by LLVM because we never store anything
-    // to this field and rustc can declare the `static` as constant as well.
-    inner: fn() -> &'static __KeyInner<T>,
+    // Although this is an extra layer of indirection, it should in theory be
+    // trivially devirtualizable by LLVM because the value of `inner` never
+    // changes and the constant should be readonly within a crate. This mainly
+    // only runs into problems when TLS statics are exported across crates.
+    inner: unsafe fn() -> Option<&'static UnsafeCell<Option<T>>>,
 
     // initialization routine to invoke to create a value
     init: fn() -> T,
 }
 
-// Macro pain #4586:
-//
-// When cross compiling, rustc will load plugins and macros from the *host*
-// platform before search for macros from the target platform. This is primarily
-// done to detect, for example, plugins. Ideally the macro below would be
-// defined once per module below, but unfortunately this means we have the
-// following situation:
-//
-// 1. We compile libstd for x86_64-unknown-linux-gnu, this thread_local!() macro
-//    will inject #[thread_local] statics.
-// 2. We then try to compile a program for arm-linux-androideabi
-// 3. The compiler has a host of linux and a target of android, so it loads
-//    macros from the *linux* libstd.
-// 4. The macro generates a #[thread_local] field, but the android libstd does
-//    not use #[thread_local]
-// 5. Compile error about structs with wrong fields.
-//
-// To get around this, we're forced to inject the #[cfg] logic into the macro
-// itself. Woohoo.
-
 /// Declare a new thread local storage key of type `std::thread::LocalKey`.
 ///
 /// See [LocalKey documentation](thread/struct.LocalKey.html) for more
 /// information.
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[allow_internal_unstable]
-#[cfg(not(no_elf_tls))]
-macro_rules! thread_local {
-    (static $name:ident: $t:ty = $init:expr) => (
-        static $name: $crate::thread::LocalKey<$t> =
-            __thread_local_inner!($t, $init,
-                #[cfg_attr(all(any(target_os = "macos", target_os = "linux"),
-                               not(target_arch = "aarch64")),
-                           thread_local)]);
-    );
-    (pub static $name:ident: $t:ty = $init:expr) => (
-        pub static $name: $crate::thread::LocalKey<$t> =
-            __thread_local_inner!($t, $init,
-                #[cfg_attr(all(any(target_os = "macos", target_os = "linux"),
-                               not(target_arch = "aarch64")),
-                           thread_local)]);
-    );
-}
-
-#[macro_export]
-#[stable(feature = "rust1", since = "1.0.0")]
-#[allow_internal_unstable]
-#[cfg(no_elf_tls)]
 macro_rules! thread_local {
     (static $name:ident: $t:ty = $init:expr) => (
         static $name: $crate::thread::LocalKey<$t> =
-            __thread_local_inner!($t, $init, #[]);
+            __thread_local_inner!($t, $init);
     );
     (pub static $name:ident: $t:ty = $init:expr) => (
         pub static $name: $crate::thread::LocalKey<$t> =
-            __thread_local_inner!($t, $init, #[]);
+            __thread_local_inner!($t, $init);
     );
 }
 
@@ -143,12 +103,25 @@ macro_rules! thread_local {
 #[macro_export]
 #[allow_internal_unstable]
 macro_rules! __thread_local_inner {
-    ($t:ty, $init:expr, #[$($attr:meta),*]) => {{
-        $(#[$attr])*
-        static __KEY: $crate::thread::__LocalKeyInner<$t> =
-            $crate::thread::__LocalKeyInner::new();
+    ($t:ty, $init:expr) => {{
         fn __init() -> $t { $init }
-        fn __getit() -> &'static $crate::thread::__LocalKeyInner<$t> { &__KEY }
+
+        unsafe fn __getit() -> $crate::option::Option<
+            &'static $crate::cell::UnsafeCell<
+                $crate::option::Option<$t>>>
+        {
+            #[thread_local]
+            #[cfg(target_thread_local)]
+            static __KEY: $crate::thread::__ElfLocalKeyInner<$t> =
+                $crate::thread::__ElfLocalKeyInner::new();
+
+            #[cfg(not(target_thread_local))]
+            static __KEY: $crate::thread::__OsLocalKeyInner<$t> =
+                $crate::thread::__OsLocalKeyInner::new();
+
+            __KEY.get()
+        }
+
         $crate::thread::LocalKey::new(__getit, __init)
     }}
 }
@@ -190,11 +163,11 @@ impl<T: 'static> LocalKey<T> {
     #[unstable(feature = "thread_local_internals",
                reason = "recently added to create a key",
                issue = "0")]
-    pub const fn new(inner: fn() -> &'static __KeyInner<T>,
+    pub const fn new(inner: unsafe fn() -> Option<&'static UnsafeCell<Option<T>>>,
                      init: fn() -> T) -> LocalKey<T> {
         LocalKey {
             inner: inner,
-            init: init
+            init: init,
         }
     }
 
@@ -211,10 +184,10 @@ impl<T: 'static> LocalKey<T> {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn with<F, R>(&'static self, f: F) -> R
                       where F: FnOnce(&T) -> R {
-        let slot = (self.inner)();
         unsafe {
-            let slot = slot.get().expect("cannot access a TLS value during or \
-                                          after it is destroyed");
+            let slot = (self.inner)();
+            let slot = slot.expect("cannot access a TLS value during or \
+                                    after it is destroyed");
             f(match *slot.get() {
                 Some(ref inner) => inner,
                 None => self.init(slot),
@@ -270,7 +243,7 @@ impl<T: 'static> LocalKey<T> {
                issue = "27716")]
     pub fn state(&'static self) -> LocalKeyState {
         unsafe {
-            match (self.inner)().get() {
+            match (self.inner)() {
                 Some(cell) => {
                     match *cell.get() {
                         Some(..) => LocalKeyState::Valid,
@@ -283,11 +256,9 @@ impl<T: 'static> LocalKey<T> {
     }
 }
 
-#[cfg(all(any(target_os = "macos", target_os = "linux"),
-          not(target_arch = "aarch64"),
-          not(no_elf_tls)))]
+#[cfg(target_thread_local)]
 #[doc(hidden)]
-mod imp {
+pub mod elf {
     use cell::{Cell, UnsafeCell};
     use intrinsics;
     use ptr;
@@ -431,11 +402,8 @@ mod imp {
     }
 }
 
-#[cfg(any(not(any(target_os = "macos", target_os = "linux")),
-          target_arch = "aarch64",
-          no_elf_tls))]
 #[doc(hidden)]
-mod imp {
+pub mod os {
     use prelude::v1::*;
 
     use cell::{Cell, UnsafeCell};

src/libstd/thread/mod.rs

@@ -191,7 +191,10 @@ pub use self::local::{LocalKey, LocalKeyState};
 pub use self::scoped_tls::ScopedKey;
 
 #[unstable(feature = "libstd_thread_internals", issue = "0")]
-#[doc(hidden)] pub use self::local::__KeyInner as __LocalKeyInner;
+#[cfg(target_thread_local)]
+#[doc(hidden)] pub use self::local::elf::Key as __ElfLocalKeyInner;
+#[unstable(feature = "libstd_thread_internals", issue = "0")]
+#[doc(hidden)] pub use self::local::os::Key as __OsLocalKeyInner;
 #[unstable(feature = "libstd_thread_internals", issue = "0")]
 #[doc(hidden)] pub use self::scoped_tls::__KeyInner as __ScopedKeyInner;
 

src/libsyntax/feature_gate.rs

@@ -236,6 +236,9 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Option<u32>, Status
 
     // allow using type ascription in expressions
     ("type_ascription", "1.6.0", Some(23416), Active),
+
+    // Allows cfg(target_thread_local)
+    ("cfg_target_thread_local", "1.7.0", Some(29594), Active),
 ];
 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 
@@ -414,6 +417,8 @@ const GATED_CFGS: &'static [(&'static str, &'static str, fn(&Features) -> bool)]
     // (name in cfg, feature, function to check if the feature is enabled)
     ("target_feature", "cfg_target_feature", cfg_fn!(|x| x.cfg_target_feature)),
     ("target_vendor", "cfg_target_vendor", cfg_fn!(|x| x.cfg_target_vendor)),
+    ("target_thread_local", "cfg_target_thread_local",
+     cfg_fn!(|x| x.cfg_target_thread_local)),
 ];
 
 #[derive(Debug, Eq, PartialEq)]
@@ -449,10 +454,13 @@ impl PartialOrd for GatedCfgAttr {
 }
 
 impl GatedCfgAttr {
-    pub fn check_and_emit(&self, diagnostic: &Handler, features: &Features) {
+    pub fn check_and_emit(&self,
+                          diagnostic: &Handler,
+                          features: &Features,
+                          codemap: &CodeMap) {
         match *self {
             GatedCfgAttr::GatedCfg(ref cfg) => {
-                cfg.check_and_emit(diagnostic, features);
+                cfg.check_and_emit(diagnostic, features, codemap);
             }
             GatedCfgAttr::GatedAttr(span) => {
                 if !features.stmt_expr_attributes {
@@ -479,9 +487,12 @@ impl GatedCfg {
                       }
                   })
     }
-    fn check_and_emit(&self, diagnostic: &Handler, features: &Features) {
+    fn check_and_emit(&self,
+                      diagnostic: &Handler,
+                      features: &Features,
+                      codemap: &CodeMap) {
         let (cfg, feature, has_feature) = GATED_CFGS[self.index];
-        if !has_feature(features) {
+        if !has_feature(features) && !codemap.span_allows_unstable(self.span) {
             let explain = format!("`cfg({})` is experimental and subject to change", cfg);
             emit_feature_err(diagnostic, feature, self.span, GateIssue::Language, &explain);
         }
@@ -541,6 +552,7 @@ pub struct Features {
     pub type_macros: bool,
     pub cfg_target_feature: bool,
     pub cfg_target_vendor: bool,
+    pub cfg_target_thread_local: bool,
     pub augmented_assignments: bool,
     pub braced_empty_structs: bool,
     pub staged_api: bool,
@@ -575,6 +587,7 @@ impl Features {
             type_macros: false,
             cfg_target_feature: false,
             cfg_target_vendor: false,
+            cfg_target_thread_local: false,
             augmented_assignments: false,
             braced_empty_structs: false,
             staged_api: false,
@@ -1157,6 +1170,7 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &Handler,
         type_macros: cx.has_feature("type_macros"),
         cfg_target_feature: cx.has_feature("cfg_target_feature"),
         cfg_target_vendor: cx.has_feature("cfg_target_vendor"),
+        cfg_target_thread_local: cx.has_feature("cfg_target_thread_local"),
         augmented_assignments: cx.has_feature("augmented_assignments"),
         braced_empty_structs: cx.has_feature("braced_empty_structs"),
         staged_api: cx.has_feature("staged_api"),