1 files changed, 144 insertions, 0 deletions
diff --git a/compiler/rustc_target/src/spec/wasm32_base.rs b/compiler/rustc_target/src/spec/wasm32_base.rs
new file mode 100644
index 00000000000..62fc8f06183
--- /dev/null
+++ b/compiler/rustc_target/src/spec/wasm32_base.rs
@@ -0,0 +1,144 @@
+use super::crt_objects::CrtObjectsFallback;
+use super::{LinkerFlavor, LldFlavor, PanicStrategy, RelocModel, TargetOptions, TlsModel};
+use std::collections::BTreeMap;
+
+pub fn options() -> TargetOptions {
+    let mut lld_args = Vec::new();
+    let mut clang_args = Vec::new();
+    let mut arg = |arg: &str| {
+        lld_args.push(arg.to_string());
+        clang_args.push(format!("-Wl,{}", arg));
+    };
+
+    // By default LLD only gives us one page of stack (64k) which is a
+    // little small. Default to a larger stack closer to other PC platforms
+    // (1MB) and users can always inject their own link-args to override this.
+    arg("-z");
+    arg("stack-size=1048576");
+
+    // By default LLD's memory layout is:
+    //
+    // 1. First, a blank page
+    // 2. Next, all static data
+    // 3. Finally, the main stack (which grows down)
+    //
+    // This has the unfortunate consequence that on stack overflows you
+    // corrupt static data and can cause some exceedingly weird bugs. To
+    // help detect this a little sooner we instead request that the stack is
+    // placed before static data.
+    //
+    // This means that we'll generate slightly larger binaries as references
+    // to static data will take more bytes in the ULEB128 encoding, but
+    // stack overflow will be guaranteed to trap as it underflows instead of
+    // corrupting static data.
+    arg("--stack-first");
+
+    // FIXME we probably shouldn't pass this but instead pass an explicit list
+    // of symbols we'll allow to be undefined. We don't currently have a
+    // mechanism of knowing, however, which symbols are intended to be imported
+    // from the environment and which are intended to be imported from other
+    // objects linked elsewhere. This is a coarse approximation but is sure to
+    // hide some bugs and frustrate someone at some point, so we should ideally
+    // work towards a world where we can explicitly list symbols that are
+    // supposed to be imported and have all other symbols generate errors if
+    // they remain undefined.
+    arg("--allow-undefined");
+
+    // Rust code should never have warnings, and warnings are often
+    // indicative of bugs, let's prevent them.
+    arg("--fatal-warnings");
+
+    // LLD only implements C++-like demangling, which doesn't match our own
+    // mangling scheme. Tell LLD to not demangle anything and leave it up to
+    // us to demangle these symbols later. Currently rustc does not perform
+    // further demangling, but tools like twiggy and wasm-bindgen are intended
+    // to do so.
+    arg("--no-demangle");
+
+    // The symbol visibility story is a bit in flux right now with LLD.
+    // It's... not entirely clear to me what's going on, but this looks to
+    // make everything work when `export_symbols` isn't otherwise called for
+    // things like executables.
+    //
+    // This is really only here to get things working. If it can be removed and
+    // basic tests still work, then sounds like it should be removed!
+    arg("--export-dynamic");
+
+    let mut pre_link_args = BTreeMap::new();
+    pre_link_args.insert(LinkerFlavor::Lld(LldFlavor::Wasm), lld_args);
+    pre_link_args.insert(LinkerFlavor::Gcc, clang_args);
+
+    TargetOptions {
+        // we allow dynamic linking, but only cdylibs. Basically we allow a
+        // final library artifact that exports some symbols (a wasm module) but
+        // we don't allow intermediate `dylib` crate types
+        dynamic_linking: true,
+        only_cdylib: true,
+
+        // This means we'll just embed a `#[start]` function in the wasm module
+        executables: true,
+
+        // relatively self-explanatory!
+        exe_suffix: ".wasm".to_string(),
+        dll_prefix: String::new(),
+        dll_suffix: ".wasm".to_string(),
+        linker_is_gnu: false,
+
+        max_atomic_width: Some(64),
+
+        // Unwinding doesn't work right now, so the whole target unconditionally
+        // defaults to panic=abort. Note that this is guaranteed to change in
+        // the future once unwinding is implemented. Don't rely on this as we're
+        // basically guaranteed to change it once WebAssembly supports
+        // exceptions.
+        panic_strategy: PanicStrategy::Abort,
+
+        // Wasm doesn't have atomics yet, so tell LLVM that we're in a single
+        // threaded model which will legalize atomics to normal operations.
+        singlethread: true,
+
+        // no dynamic linking, no need for default visibility!
+        default_hidden_visibility: true,
+
+        // Symbol visibility takes care of this for the WebAssembly.
+        // Additionally the only known linker, LLD, doesn't support the script
+        // arguments just yet
+        limit_rdylib_exports: false,
+
+        // we use the LLD shipped with the Rust toolchain by default
+        linker: Some("rust-lld".to_owned()),
+        lld_flavor: LldFlavor::Wasm,
+
+        // No need for indirection here, simd types can always be passed by
+        // value as the whole module either has simd or not, which is different
+        // from x86 (for example) where programs can have functions that don't
+        // enable simd features.
+        simd_types_indirect: false,
+
+        pre_link_args,
+
+        crt_objects_fallback: Some(CrtObjectsFallback::Wasm),
+
+        // This has no effect in LLVM 8 or prior, but in LLVM 9 and later when
+        // PIC code is implemented this has quite a drastric effect if it stays
+        // at the default, `pic`. In an effort to keep wasm binaries as minimal
+        // as possible we're defaulting to `static` for now, but the hope is
+        // that eventually we can ship a `pic`-compatible standard library which
+        // works with `static` as well (or works with some method of generating
+        // non-relative calls and such later on).
+        relocation_model: RelocModel::Static,
+
+        // 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: TlsModel::LocalExec,
+
+        // gdb scripts don't work on wasm blobs
+        emit_debug_gdb_scripts: false,
+
+        ..Default::default()
+    }
+}