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//! This target is a confluence of Linux and Wasm models, inheriting most
//! aspects from their respective base targets
use crate::spec::{
Cc, LinkSelfContainedDefault, LinkerFlavor, PanicStrategy, RelocModel, TargetOptions, TlsModel,
add_link_args, crt_objects, cvs,
};
pub(crate) fn opts() -> 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!($prefix, "--allow-undefined"),
// LLD only implements C++-like demangling, which doesn't match our own
// mangling scheme. Tell LLD to not demangle anything and leave it up to
// us to demangle these symbols later. Currently rustc does not perform
// further demangling, but tools like twiggy and wasm-bindgen are intended
// to do so.
concat!($prefix, "--no-demangle"),
]
};
}
let mut pre_link_args = TargetOptions::link_args(LinkerFlavor::WasmLld(Cc::No), args!(""));
add_link_args(&mut pre_link_args, LinkerFlavor::WasmLld(Cc::Yes), args!("-Wl,"));
TargetOptions {
is_like_wasm: true,
families: cvs!["wasm", "unix"],
os: "linux".into(),
env: "musl".into(),
// 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,
// 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()),
linker_flavor: LinkerFlavor::WasmLld(Cc::No),
pre_link_args,
// FIXME: Figure out cases in which WASM needs to link with a native toolchain.
//
// rust-lang/rust#104137: cannot blindly remove this without putting in
// some other way to compensate for lack of `-nostartfiles` in linker
// invocation.
link_self_contained: LinkSelfContainedDefault::True,
pre_link_objects_self_contained: crt_objects::pre_wasi_self_contained(),
post_link_objects_self_contained: crt_objects::post_wasi_self_contained(),
// 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,
// Supporting Linux requires multithreading supported by Wasm's thread
// proposal
singlethread: false,
// 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,
// Right now this is a bit of a workaround but we're currently saying that
// the target by default has a static crt which we're taking as a signal
// for "use the bundled crt". If that's turned off then the system's crt
// will be used, but this means that default usage of this target doesn't
// need an external compiler but it's still interoperable with an external
// compiler if configured correctly.
crt_static_default: true,
crt_static_respected: true,
// Allow `+crt-static` to create a "cdylib" output which is just a wasm file
// without a main function.
crt_static_allows_dylibs: true,
// Wasm start ignores arguments -- relies on API call from interface.
main_needs_argc_argv: false,
// Wasm toolchains mangle the name of "main" to distinguish between different
// signatures.
entry_name: "__main_void".into(),
// Wasm Feature flags for supporting Linux
features: "+atomics,+bulk-memory,+mutable-globals,+sign-ext".into(),
..Default::default()
}
}
|
