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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");
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 {
is_like_wasm: true,
families: vec!["wasm".to_string()],
// 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(),
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".to_owned()),
lld_flavor: LldFlavor::Wasm,
linker_is_gnu: 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()
}
}