compiler/rustc_target/src/spec/wasm_base.rs

   1 use super::crt_objects::CrtObjectsFallback;
   2 use super::{LinkerFlavor, LldFlavor, PanicStrategy, RelocModel, TargetOptions, TlsModel};
   3 use std::collections::BTreeMap;
   4
   5 pub fn options() -> TargetOptions {
   6     let mut lld_args = Vec::new();
   7     let mut clang_args = Vec::new();
   8     let mut arg = |arg: &str| {
   9         lld_args.push(arg.to_string());
  10         clang_args.push(format!("-Wl,{}", arg));
  11     };
  12
  13     // By default LLD only gives us one page of stack (64k) which is a
  14     // little small. Default to a larger stack closer to other PC platforms
  15     // (1MB) and users can always inject their own link-args to override this.
  16     arg("-z");
  17     arg("stack-size=1048576");
  18
  19     // By default LLD's memory layout is:
  20     //
  21     // 1. First, a blank page
  22     // 2. Next, all static data
  23     // 3. Finally, the main stack (which grows down)
  24     //
  25     // This has the unfortunate consequence that on stack overflows you
  26     // corrupt static data and can cause some exceedingly weird bugs. To
  27     // help detect this a little sooner we instead request that the stack is
  28     // placed before static data.
  29     //
  30     // This means that we'll generate slightly larger binaries as references
  31     // to static data will take more bytes in the ULEB128 encoding, but
  32     // stack overflow will be guaranteed to trap as it underflows instead of
  33     // corrupting static data.
  34     arg("--stack-first");
  35
  36     // FIXME we probably shouldn't pass this but instead pass an explicit list
  37     // of symbols we'll allow to be undefined. We don't currently have a
  38     // mechanism of knowing, however, which symbols are intended to be imported
  39     // from the environment and which are intended to be imported from other
  40     // objects linked elsewhere. This is a coarse approximation but is sure to
  41     // hide some bugs and frustrate someone at some point, so we should ideally
  42     // work towards a world where we can explicitly list symbols that are
  43     // supposed to be imported and have all other symbols generate errors if
  44     // they remain undefined.
  45     arg("--allow-undefined");
  46
  47     // Rust code should never have warnings, and warnings are often
  48     // indicative of bugs, let's prevent them.
  49     arg("--fatal-warnings");
  50
  51     // LLD only implements C++-like demangling, which doesn't match our own
  52     // mangling scheme. Tell LLD to not demangle anything and leave it up to
  53     // us to demangle these symbols later. Currently rustc does not perform
  54     // further demangling, but tools like twiggy and wasm-bindgen are intended
  55     // to do so.
  56     arg("--no-demangle");
  57
  58     let mut pre_link_args = BTreeMap::new();
  59     pre_link_args.insert(LinkerFlavor::Lld(LldFlavor::Wasm), lld_args);
  60     pre_link_args.insert(LinkerFlavor::Gcc, clang_args);
  61
  62     TargetOptions {
  63         is_like_wasm: true,
  64         families: vec!["wasm".to_string()],
  65
  66         // we allow dynamic linking, but only cdylibs. Basically we allow a
  67         // final library artifact that exports some symbols (a wasm module) but
  68         // we don't allow intermediate `dylib` crate types
  69         dynamic_linking: true,
  70         only_cdylib: true,
  71
  72         // This means we'll just embed a `#[start]` function in the wasm module
  73         executables: true,
  74
  75         // relatively self-explanatory!
  76         exe_suffix: ".wasm".to_string(),
  77         dll_prefix: String::new(),
  78         dll_suffix: ".wasm".to_string(),
  79         eh_frame_header: false,
  80
  81         max_atomic_width: Some(64),
  82
  83         // Unwinding doesn't work right now, so the whole target unconditionally
  84         // defaults to panic=abort. Note that this is guaranteed to change in
  85         // the future once unwinding is implemented. Don't rely on this as we're
  86         // basically guaranteed to change it once WebAssembly supports
  87         // exceptions.
  88         panic_strategy: PanicStrategy::Abort,
  89
  90         // Wasm doesn't have atomics yet, so tell LLVM that we're in a single
  91         // threaded model which will legalize atomics to normal operations.
  92         singlethread: true,
  93
  94         // no dynamic linking, no need for default visibility!
  95         default_hidden_visibility: true,
  96
  97         // Symbol visibility takes care of this for the WebAssembly.
  98         // Additionally the only known linker, LLD, doesn't support the script
  99         // arguments just yet
 100         limit_rdylib_exports: false,
 101
 102         // we use the LLD shipped with the Rust toolchain by default
 103         linker: Some("rust-lld".to_owned()),
 104         lld_flavor: LldFlavor::Wasm,
 105         linker_is_gnu: false,
 106
 107         pre_link_args,
 108
 109         crt_objects_fallback: Some(CrtObjectsFallback::Wasm),
 110
 111         // This has no effect in LLVM 8 or prior, but in LLVM 9 and later when
 112         // PIC code is implemented this has quite a drastric effect if it stays
 113         // at the default, `pic`. In an effort to keep wasm binaries as minimal
 114         // as possible we're defaulting to `static` for now, but the hope is
 115         // that eventually we can ship a `pic`-compatible standard library which
 116         // works with `static` as well (or works with some method of generating
 117         // non-relative calls and such later on).
 118         relocation_model: RelocModel::Static,
 119
 120         // When the atomics feature is activated then these two keys matter,
 121         // otherwise they're basically ignored by the standard library. In this
 122         // mode, however, the `#[thread_local]` attribute works (i.e.
 123         // `has_elf_tls`) and we need to get it to work by specifying
 124         // `local-exec` as that's all that's implemented in LLVM today for wasm.
 125         has_elf_tls: true,
 126         tls_model: TlsModel::LocalExec,
 127
 128         // gdb scripts don't work on wasm blobs
 129         emit_debug_gdb_scripts: false,
 130
 131         ..Default::default()
 132     }
 133 }