compiler/rustc_target/src/spec/base/wasm.rs - toolchain/rustc - Git at Google

 use crate::spec::{
     add_link_args, cvs, Cc, LinkSelfContainedDefault, LinkerFlavor, 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!($prefix, "--allow-undefined"),
                 // LLD only implements C++-like demangling, which doesn't match our own
                 // mangling scheme. Tell LLD to not demangle anything and leave it up to
                 // us to demangle these symbols later. Currently rustc does not perform
                 // further demangling, but tools like twiggy and wasm-bindgen are intended
                 // to do so.
                 concat!($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"],

         // 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()),
         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,

         // 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()
     }
 }
	use crate::spec::{
	add_link_args, cvs, Cc, LinkSelfContainedDefault, LinkerFlavor, 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!($prefix, "--allow-undefined"),
	// LLD only implements C++-like demangling, which doesn't match our own
	// mangling scheme. Tell LLD to not demangle anything and leave it up to
	// us to demangle these symbols later. Currently rustc does not perform
	// further demangling, but tools like twiggy and wasm-bindgen are intended
	// to do so.
	concat!($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"],

	// 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()),
	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,

	// 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()
	}
	}