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Auto merge of #128004 - folkertdev:naked-fn-asm, r=Amanieu

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codegen `#[naked]` functions using global asm

tracking issue: https://github.com/rust-lang/rust/issues/90957

Fixes #124375

This implements the approach suggested in the tracking issue: use the existing global assembly infrastructure to emit the body of `#[naked]` functions. The main advantage is that we now have full control over what gets generated, and are no longer dependent on LLVM not sneakily messing with our output (inlining, adding extra instructions, etc).

I discussed this approach with `@Amanieu` and while I think the general direction is correct, there is probably a bunch of stuff that needs to change or move around here. I'll leave some inline comments on things that I'm not sure about.

Combined with https://github.com/rust-lang/rust/pull/127853, if both accepted, I think that resolves all steps from the tracking issue.

r? `@Amanieu`
This commit is contained in:
bors 2024-12-11 21:51:07 +00:00
commit 1daec069fb
18 changed files with 635 additions and 106 deletions
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8
compiler/rustc_codegen_llvm/src/asm.rs
View file

@ -442,6 +442,14 @@ impl<'tcx> AsmCodegenMethods<'tcx> for CodegenCx<'_, 'tcx> {
);
}
}
fn mangled_name(&self, instance: Instance<'tcx>) -> String {
let llval = self.get_fn(instance);
llvm::build_string(|s| unsafe {
llvm::LLVMRustGetMangledName(llval, s);
})
.expect("symbol is not valid UTF-8")
}
}
pub(crate) fn inline_asm_call<'ll>(

14
compiler/rustc_codegen_llvm/src/attributes.rs
View file

@ -395,17 +395,9 @@ pub(crate) fn llfn_attrs_from_instance<'ll, 'tcx>(
to_add.push(MemoryEffects::None.create_attr(cx.llcx));
}
if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
to_add.push(AttributeKind::Naked.create_attr(cx.llcx));
// HACK(jubilee): "indirect branch tracking" works by attaching prologues to functions.
// And it is a module-level attribute, so the alternative is pulling naked functions into
// new LLVM modules. Otherwise LLVM's "naked" functions come with endbr prefixes per
// https://github.com/rust-lang/rust/issues/98768
to_add.push(AttributeKind::NoCfCheck.create_attr(cx.llcx));
if llvm_util::get_version() < (19, 0, 0) {
// Prior to LLVM 19, branch-target-enforcement was disabled by setting the attribute to
// the string "false". Now it is disabled by absence of the attribute.
to_add.push(llvm::CreateAttrStringValue(cx.llcx, "branch-target-enforcement", "false"));
}
// do nothing; a naked function is converted into an extern function
// and a global assembly block. LLVM's support for naked functions is
// not used.
} else {
// Do not set sanitizer attributes for naked functions.
to_add.extend(sanitize_attrs(cx, codegen_fn_attrs.no_sanitize));