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Perform LTO optimisations with wasm-ld + -Clinker-plugin-lto #118378

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Nov 29, 2023
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Perform LTO optimisations with wasm-ld + -Clinker-plugin-lto #118378

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28 changes: 27 additions & 1 deletion compiler/rustc_codegen_ssa/src/back/linker.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1302,6 +1302,8 @@ impl<'a> Linker for WasmLd<'a> {
}

fn optimize(&mut self) {
// The -O flag is, as of late 2023, only used for merging of strings and debuginfo, and
// only differentiates -O0 and -O1. It does not apply to LTO.
self.cmd.arg(match self.sess.opts.optimize {
OptLevel::No => "-O0",
OptLevel::Less => "-O1",
Expand Down Expand Up @@ -1354,7 +1356,31 @@ impl<'a> Linker for WasmLd<'a> {
fn subsystem(&mut self, _subsystem: &str) {}

fn linker_plugin_lto(&mut self) {
// Do nothing for now
match self.sess.opts.cg.linker_plugin_lto {
LinkerPluginLto::Disabled => {
// Nothing to do
}
LinkerPluginLto::LinkerPluginAuto => {
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Suggested change
LinkerPluginLto::LinkerPluginAuto => {
LinkerPluginLto::LinkerPluginAuto | LinkerPluginLto::LinkerPlugin(_) => {

push_linker_plugin_lto_args could then be inlined and removed.

Upd: the match on self.sess.opts.optimize could be kept separate function, though, it is shared between fn optimize and fn linker_plugin_lto.

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@cormacrelf cormacrelf Nov 28, 2023 •
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I intentionally left them separate.

  • That the match turns out the same is more of a coincidence than anything else. I anticipate the --lto-O* args may change to support Os and Oz in future. (Especially with those being the most popular opt levels for webassembly.) If that happens, adding Os/Oz to a shared match statement would break the normal fn optimize that controls merging sections only, unrelated to LLVM.
  • push_linker_plugin_lto_args could be inlined, but I was going to do another PR that puts warnings in one of the match arms but not the other.

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Ok.

self.push_linker_plugin_lto_args();
}
LinkerPluginLto::LinkerPlugin(_) => {
self.push_linker_plugin_lto_args();
}
}
}
}

impl<'a> WasmLd<'a> {
fn push_linker_plugin_lto_args(&mut self) {
let opt_level = match self.sess.opts.optimize {
config::OptLevel::No => "O0",
config::OptLevel::Less => "O1",
config::OptLevel::Default => "O2",
config::OptLevel::Aggressive => "O3",
// wasm-ld only handles integer LTO opt levels. Use O2
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Suggested change
// wasm-ld only handles integer LTO opt levels. Use O2
// `wasm-ld` only handles integer LTO opt levels. Use O2.

config::OptLevel::Size | config::OptLevel::SizeMin => "O2",
};
self.cmd.arg(&format!("--lto-{opt_level}"));
}
}

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