Rollup merge of rust-lang#62809 - alexcrichton:wasm-llvm-9, r=nikic

rustc: Update wasm32 support for LLVM 9

This commit brings in a number of minor updates for rustc's support for
the wasm target which has changed in the LLVM 9 update. Notable updates
include:

* The compiler now no longer manually inserts the `producers` section,
  instead relying on LLVM to do so. LLVM uses the `llvm.ident` metadata
  for the `processed-by` directive (which is now emitted on the wasm
  target in this PR) and it uses debuginfo to figure out what `language`
  to put in the `producers` section.

* Threaded WebAssembly code now requires different flags to be passed
  with LLD. In LLD we now pass:

  * `--shared-memory` - required since objects are compiled with
    atomics. This also means that the generated memory will be marked as
    `shared`.
  * `--max-memory=1GB` - required with the `--shared-memory` argument
    since shared memories in WebAssembly must have a maximum size. The
    1GB number is intended to be a conservative estimate for rustc, but
    it should be overridable with `-C link-arg` if necessary.
  * `--passive-segments` - this has become the default for multithreaded
    memory, but when compiling a threaded module all data segments need
    to be marked as passive to ensure they don't re-initialize memory
    for each thread. This will also cause LLD to emit a synthetic
    function to initialize memory which users will have to arrange to
    call.
  * The `__heap_base` and `__data_end` globals are explicitly exported
    since they're now hidden by default due to the `--export` flags we
    pass to LLD.

Author: Centril

src/librustc_codegen_llvm/debuginfo/metadata.rs

@@ -913,9 +913,12 @@ pub fn compile_unit_metadata(
     }
 
     debug!("compile_unit_metadata: {:?}", name_in_debuginfo);
+    let rustc_producer = format!(
+        "rustc version {}",
+        option_env!("CFG_VERSION").expect("CFG_VERSION"),
+    );
     // FIXME(#41252) Remove "clang LLVM" if we can get GDB and LLVM to play nice.
-    let producer = format!("clang LLVM (rustc version {})",
-                           (option_env!("CFG_VERSION")).expect("CFG_VERSION"));
+    let producer = format!("clang LLVM ({})", rustc_producer);
 
     let name_in_debuginfo = name_in_debuginfo.to_string_lossy();
     let name_in_debuginfo = SmallCStr::new(&name_in_debuginfo);
@@ -980,6 +983,21 @@ pub fn compile_unit_metadata(
                                               gcov_metadata);
         }
 
+        // Insert `llvm.ident` metadata on the wasm32 targets since that will
+        // get hooked up to the "producer" sections `processed-by` information.
+        if tcx.sess.opts.target_triple.triple().starts_with("wasm32") {
+            let name_metadata = llvm::LLVMMDStringInContext(
+                debug_context.llcontext,
+                rustc_producer.as_ptr() as *const _,
+                rustc_producer.as_bytes().len() as c_uint,
+            );
+            llvm::LLVMAddNamedMetadataOperand(
+                debug_context.llmod,
+                const_cstr!("llvm.ident").as_ptr(),
+                llvm::LLVMMDNodeInContext(debug_context.llcontext, &name_metadata, 1),
+            );
+        }
+
         return unit_metadata;
     };
 

src/librustc_codegen_ssa/back/link.rs

@@ -678,14 +678,6 @@ fn link_natively<'a, B: ArchiveBuilder<'a>>(sess: &'a Session,
             sess.fatal(&format!("failed to run dsymutil: {}", e))
         }
     }
-
-    if sess.opts.target_triple.triple() == "wasm32-unknown-unknown" {
-        super::wasm::add_producer_section(
-            &out_filename,
-            &sess.edition().to_string(),
-            option_env!("CFG_VERSION").unwrap_or("unknown"),
-        );
-    }
 }
 
 /// Returns a boolean indicating whether the specified crate should be ignored

src/librustc_codegen_ssa/back/linker.rs

@@ -901,7 +901,45 @@ pub struct WasmLd<'a> {
 }
 
 impl<'a> WasmLd<'a> {
-    fn new(cmd: Command, sess: &'a Session, info: &'a LinkerInfo) -> WasmLd<'a> {
+    fn new(mut cmd: Command, sess: &'a Session, info: &'a LinkerInfo) -> WasmLd<'a> {
+        // If the atomics feature is enabled for wasm then we need a whole bunch
+        // of flags:
+        //
+        // * `--shared-memory` - the link won't even succeed without this, flags
+        //   the one linear memory as `shared`
+        //
+        // * `--max-memory=1G` - when specifying a shared memory this must also
+        //   be specified. We conservatively choose 1GB but users should be able
+        //   to override this with `-C link-arg`.
+        //
+        // * `--import-memory` - it doesn't make much sense for memory to be
+        //   exported in a threaded module because typically you're
+        //   sharing memory and instantiating the module multiple times. As a
+        //   result if it were exported then we'd just have no sharing.
+        //
+        // * `--passive-segments` - all memory segments should be passive to
+        //   prevent each module instantiation from reinitializing memory.
+        //
+        // * `--export=__wasm_init_memory` - when using `--passive-segments` the
+        //   linker will synthesize this function, and so we need to make sure
+        //   that our usage of `--export` below won't accidentally cause this
+        //   function to get deleted.
+        //
+        // * `--export=*tls*` - when `#[thread_local]` symbols are used these
+        //   symbols are how the TLS segments are initialized and configured.
+        let atomics = sess.opts.cg.target_feature.contains("+atomics") ||
+            sess.target.target.options.features.contains("+atomics");
+        if atomics {
+            cmd.arg("--shared-memory");
+            cmd.arg("--max-memory=1073741824");
+            cmd.arg("--import-memory");
+            cmd.arg("--passive-segments");
+            cmd.arg("--export=__wasm_init_memory");
+            cmd.arg("--export=__wasm_init_tls");
+            cmd.arg("--export=__tls_size");
+            cmd.arg("--export=__tls_align");
+            cmd.arg("--export=__tls_base");
+        }
         WasmLd { cmd, sess, info }
     }
 }
@@ -1004,6 +1042,13 @@ impl<'a> Linker for WasmLd<'a> {
         for sym in self.info.exports[&crate_type].iter() {
             self.cmd.arg("--export").arg(&sym);
         }
+
+        // LLD will hide these otherwise-internal symbols since our `--export`
+        // list above is a whitelist of what to export. Various bits and pieces
+        // of tooling use this, so be sure these symbols make their way out of
+        // the linker as well.
+        self.cmd.arg("--export=__heap_base");
+        self.cmd.arg("--export=__data_end");
     }
 
     fn subsystem(&mut self, _subsystem: &str) {

src/librustc_codegen_ssa/back/mod.rs

@@ -6,4 +6,3 @@ pub mod command;
 pub mod symbol_export;
 pub mod archive;
 pub mod rpath;
-pub mod wasm;

src/librustc_codegen_ssa/back/wasm.rs

@@ -132,6 +132,14 @@ pub fn options() -> TargetOptions {
         // non-relative calls and such later on).
         relocation_model: "static".to_string(),
 
+        // When the atomics feature is activated then these two keys matter,
+        // otherwise they're basically ignored by the standard library. In this
+        // mode, however, the `#[thread_local]` attribute works (i.e.
+        // `has_elf_tls`) and we need to get it to work by specifying
+        // `local-exec` as that's all that's implemented in LLVM today for wasm.
+        has_elf_tls: true,
+        tls_model: "local-exec".to_string(),
+
         .. Default::default()
     }
 }

src/librustc_target/spec/wasm32_base.rs

@@ -75,11 +75,6 @@ panic_immediate_abort = ["core/panic_immediate_abort"]
 # requires rebuilding the standard library to use it.
 wasm_syscall = []
 
-# An off-by-default features to enable libstd to assume that wasm-bindgen is in
-# the environment for hooking up some thread-related information like the
-# current thread id and accessing/getting the current thread's TCB
-wasm-bindgen-threads = []
-
 # Enable std_detect default features for stdarch/crates/std_detect:
 # https://github.com/rust-lang/stdarch/blob/master/crates/std_detect/Cargo.toml
 std_detect_file_io = []

src/libstd/Cargo.toml

@@ -47,6 +47,8 @@ pub mod stdio;
 pub mod thread;
 #[path = "../wasm/thread_local.rs"]
 pub mod thread_local;
+#[path = "../wasm/fast_thread_local.rs"]
+pub mod fast_thread_local;
 pub mod time;
 pub mod ext;
 

src/libstd/sys/wasi/mod.rs

@@ -0,0 +1,9 @@
+#![unstable(feature = "thread_local_internals", issue = "0")]
+
+pub unsafe fn register_dtor(_t: *mut u8, _dtor: unsafe extern fn(*mut u8)) {
+    // FIXME: right now there is no concept of "thread exit", but this is likely
+    // going to show up at some point in the form of an exported symbol that the
+    // wasm runtime is oging to be expected to call. For now we basically just
+    // ignore the arguments, but if such a function starts to exist it will
+    // likely look like the OSX implementation in `unix/fast_thread_local.rs`
+}

src/libstd/sys/wasm/fast_thread_local.rs

@@ -37,6 +37,8 @@ pub mod stack_overflow;
 pub mod thread;
 pub mod time;
 pub mod stdio;
+pub mod thread_local;
+pub mod fast_thread_local;
 
 pub use crate::sys_common::os_str_bytes as os_str;
 
@@ -48,13 +50,10 @@ cfg_if::cfg_if! {
         pub mod mutex;
         #[path = "rwlock_atomics.rs"]
         pub mod rwlock;
-        #[path = "thread_local_atomics.rs"]
-        pub mod thread_local;
     } else {
         pub mod condvar;
         pub mod mutex;
         pub mod rwlock;
-        pub mod thread_local;
     }
 }