Commits on Jul 25, 2019

1. 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.

   

   alexcrichton committed Jul 25, 2019
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2. std: Use native #[thread_local] TLS on wasm …

   This commit moves `thread_local!` on WebAssembly targets to using the
   `#[thread_local]` attribute in LLVM. This was recently implemented
   upstream and is [in the process of being documented][dox]. This change
   only takes affect if modules are compiled with `+atomics` which is
   currently unstable and a pretty esoteric method of compiling wasm
   artifacts.
   
   This "new power" of the wasm toolchain means that the old
   `wasm-bindgen-threads` feature of the standard library can be removed
   since it should now be possible to create a fully functioning threaded
   wasm module without intrusively dealing with libstd symbols or
   intrinsics. Yay!
   
   [dox]: WebAssembly/tool-conventions#116

   

   alexcrichton committed Jul 25, 2019
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