compiler_builtins: base conditional compilation on the specification

of the target rather than on its triple (i.e. its name).

As explained in rust-lang#35474, the current conditional compilation logic, which
is based on the target triple (e.g. x86_64-unknown-linux-gnu), is not
robust in the presence of custom targets as those can have arbitrary
triples (e.g. cortex-m3).

To fix that, this commit changes the conditional compilation logic to
use the specification of the target (e.g. target_arch, target_os, etc.).
For that, compiler_builtins build script now depends on the rustc-cfg
crate, whose role is to shell out to `rustc --print cfg` and return its
output parsed as a `struct`.

With the goal of completely removing any direct dependency of the
conditional compilation logic on the target triple, this commit also
exposes the llvm-target field of the target specification via `rustc
--print cfg`.

closes rust-lang#35474

Author: Jorge Aparicio

src/libcompiler_builtins/Cargo.toml

@@ -13,3 +13,4 @@ core = { path = "../libcore" }
 
 [build-dependencies]
 gcc = "0.3.27"
+rustc-cfg = { git = "https://github.com/japaric/rustc-cfg", branch = "llvm-target" }

src/libcompiler_builtins/build.rs

@@ -34,11 +34,14 @@
 //! far far less than working with compiler-rt's build system over time.
 
 extern crate gcc;
+extern crate rustc_cfg;
 
 use std::collections::BTreeMap;
 use std::env;
 use std::path::Path;
 
+use rustc_cfg::Cfg;
+
 struct Sources {
     // SYMBOL -> PATH TO SOURCE
     map: BTreeMap<&'static str, &'static str>,
@@ -73,9 +76,23 @@ impl Sources {
 
 fn main() {
     let target = env::var("TARGET").unwrap();
+    let Cfg {
+        ref llvm_target,
+        ref target_arch,
+        ref target_os,
+        ref target_env,
+        ref target_vendor,
+        ..
+    } = Cfg::new(&target).unwrap();
+    // TODO(stage0) use `unwrap` instead of `unwrap_or`
+    // NOTE in the latest stable/beta release, `rustc --print cfg` doesn't include `llvm_target` in
+    // its output. In those cases simply fallback to the target triple, which is usually similar to
+    // llvm-target, as a workaround.
+    let llvm_target = llvm_target.as_ref().unwrap_or(&target).split('-').collect::<Vec<_>>();
+    let target_vendor = target_vendor.as_ref().unwrap();
     let cfg = &mut gcc::Config::new();
 
-    if target.contains("msvc") {
+    if target_env == "msvc" {
         // Don't pull in extra libraries on MSVC
         cfg.flag("/Zl");
 
@@ -183,7 +200,7 @@ fn main() {
                      "umoddi3.c",
                      "umodsi3.c"]);
 
-    if !target.contains("ios") {
+    if target_os != "ios" {
         sources.extend(&["absvti2.c",
                          "addtf3.c",
                          "addvti3.c",
@@ -226,7 +243,7 @@ fn main() {
                          "umodti3.c"]);
     }
 
-    if target.contains("apple") {
+    if target_vendor == "apple" {
         sources.extend(&["atomic_flag_clear.c",
                          "atomic_flag_clear_explicit.c",
                          "atomic_flag_test_and_set.c",
@@ -235,20 +252,20 @@ fn main() {
                          "atomic_thread_fence.c"]);
     }
 
-    if !target.contains("windows") {
+    if target_os != "windows" {
         sources.extend(&["emutls.c"]);
     }
 
-    if target.contains("msvc") {
-        if target.contains("x86_64") {
+    if target_env == "msvc" {
+        if llvm_target[0] == "x86_64" {
             sources.extend(&["x86_64/floatdidf.c", "x86_64/floatdisf.c", "x86_64/floatdixf.c"]);
         }
     } else {
-        if !target.contains("freebsd") {
+        if target_os != "freebsd" {
             sources.extend(&["gcc_personality_v0.c"]);
         }
 
-        if target.contains("x86_64") {
+        if target_arch == "x86_64" {
             sources.extend(&["x86_64/chkstk.S",
                              "x86_64/chkstk2.S",
                              "x86_64/floatdidf.c",
@@ -259,7 +276,7 @@ fn main() {
                              "x86_64/floatundixf.S"]);
         }
 
-        if target.contains("i386") || target.contains("i586") || target.contains("i686") {
+        if llvm_target[0] == "i386" || llvm_target[0] == "i586" || llvm_target[0] == "i686" {
             sources.extend(&["i386/ashldi3.S",
                              "i386/ashrdi3.S",
                              "i386/chkstk.S",
@@ -279,7 +296,7 @@ fn main() {
         }
     }
 
-    if target.contains("arm") && !target.contains("ios") {
+    if target_arch == "arm" && target_os != "ios" {
         sources.extend(&["arm/aeabi_cdcmp.S",
                          "arm/aeabi_cdcmpeq_check_nan.c",
                          "arm/aeabi_cfcmp.S",
@@ -315,7 +332,7 @@ fn main() {
                          "arm/umodsi3.S"]);
     }
 
-    if target.contains("armv7") {
+    if llvm_target[0] == "armv7" {
         sources.extend(&["arm/sync_fetch_and_add_4.S",
                          "arm/sync_fetch_and_add_8.S",
                          "arm/sync_fetch_and_and_4.S",
@@ -338,7 +355,7 @@ fn main() {
                          "arm/sync_fetch_and_xor_8.S"]);
     }
 
-    if target.contains("eabihf") {
+    if llvm_target.last().unwrap().ends_with("eabihf") {
         sources.extend(&["arm/adddf3vfp.S",
                          "arm/addsf3vfp.S",
                          "arm/divdf3vfp.S",
@@ -377,7 +394,7 @@ fn main() {
                          "arm/unordsf2vfp.S"]);
     }
 
-    if target.contains("aarch64") {
+    if target_arch == "aarch64" {
         sources.extend(&["comparetf2.c",
                          "extenddftf2.c",
                          "extendsftf2.c",

src/librustc/session/config.rs

@@ -930,6 +930,7 @@ 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 llvm_target = &sess.target.target.llvm_target;
     let max_atomic_width = sess.target.target.options.max_atomic_width;
 
     let fam = if let Some(ref fam) = sess.target.target.options.target_family {
@@ -949,6 +950,7 @@ pub fn default_configuration(sess: &Session) -> ast::CrateConfig {
         mk(InternedString::new("target_pointer_width"), intern(wordsz)),
         mk(InternedString::new("target_env"), intern(env)),
         mk(InternedString::new("target_vendor"), intern(vendor)),
+        mk(InternedString::new("llvm_target"), intern(llvm_target)),
     ];
     match &fam[..] {
         "windows" | "unix" => ret.push(attr::mk_word_item(fam)),