rustc_target/spec/base/
wasm.rs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
use crate::spec::{
    Cc, LinkSelfContainedDefault, LinkerFlavor, PanicStrategy, RelocModel, TargetOptions, TlsModel,
    add_link_args, cvs,
};

pub(crate) 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,

        // 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()
    }
}