use super::crt_objects::LinkSelfContainedDefault; use super::{cvs, LinkerFlavor, LldFlavor, PanicStrategy, RelocModel, TargetOptions, TlsModel}; pub fn options() -> TargetOptions { macro_rules! args { ($prefix:literal) => { &[ // By default LLD only gives us one page of stack (64k) which is a // little small. Default to a larger stack closer to other PC platforms // (1MB) and users can always inject their own link-args to override this. concat!($prefix, "-z"), concat!($prefix, "stack-size=1048576"), // By default LLD's memory layout is: // // 1. First, a blank page // 2. Next, all static data // 3. Finally, the main stack (which grows down) // // This has the unfortunate consequence that on stack overflows you // corrupt static data and can cause some exceedingly weird bugs. To // help detect this a little sooner we instead request that the stack is // placed before static data. // // This means that we'll generate slightly larger binaries as references // to static data will take more bytes in the ULEB128 encoding, but // stack overflow will be guaranteed to trap as it underflows instead of // corrupting static data. concat!($prefix, "--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. concat!($prefix, "--allow-undefined"), // Rust code should never have warnings, and warnings are often // indicative of bugs, let's prevent them. concat!($prefix, "--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. concat!($prefix, "--no-demangle"), ] }; } let mut pre_link_args = TargetOptions::link_args(LinkerFlavor::Lld(LldFlavor::Wasm), args!("")); super::add_link_args(&mut pre_link_args, LinkerFlavor::Gcc, args!("-Wl,")); TargetOptions { is_like_wasm: true, families: cvs!["wasm"], // 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, // relatively self-explanatory! exe_suffix: ".wasm".into(), dll_prefix: "".into(), dll_suffix: ".wasm".into(), eh_frame_header: 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".into()), lld_flavor: LldFlavor::Wasm, linker_is_gnu: false, pre_link_args, // FIXME: Figure out cases in which WASM needs to link with a native toolchain. link_self_contained: LinkSelfContainedDefault::True, // This has no effect in LLVM 8 or prior, but in LLVM 9 and later when // PIC code is implemented this has quite a drastic 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_thread_local`) 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_thread_local: true, tls_model: TlsModel::LocalExec, // gdb scripts don't work on wasm blobs emit_debug_gdb_scripts: false, // There's more discussion of this at // https://bugs.llvm.org/show_bug.cgi?id=52442 but the general result is // that this isn't useful for wasm and has tricky issues with // representation, so this is disabled. generate_arange_section: false, ..Default::default() } }