Rollup merge of #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.

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;
     };