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.

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

@@ -881,7 +881,38 @@ 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.
+        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");
+        }
         WasmLd { cmd, sess, info }
     }
 }
@@ -984,6 +1015,13 @@ 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;