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centralize -Zmin-function-alignment logic #142854

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Merged
merged 1 commit into from
Jun 23, 2025
Merged

centralize -Zmin-function-alignment logic #142854

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6 changes: 1 addition & 5 deletions compiler/rustc_codegen_llvm/src/attributes.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -491,11 +491,7 @@ pub(crate) fn llfn_attrs_from_instance<'ll, 'tcx>(
let allocated_pointer = AttributeKind::AllocatedPointer.create_attr(cx.llcx);
attributes::apply_to_llfn(llfn, AttributePlace::Argument(0), &[allocated_pointer]);
}
// function alignment can be set globally with the `-Zmin-function-alignment=<n>` flag;
// the alignment from a `#[repr(align(<n>))]` is used if it specifies a higher alignment.
if let Some(align) =
Ord::max(cx.tcx.sess.opts.unstable_opts.min_function_alignment, codegen_fn_attrs.alignment)
{
if let Some(align) = codegen_fn_attrs.alignment {
llvm::set_alignment(llfn, align);
}
if let Some(backchain) = backchain_attr(cx) {
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6 changes: 6 additions & 0 deletions compiler/rustc_codegen_ssa/src/codegen_attrs.rs
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Expand Up @@ -126,6 +126,12 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
}
}

// Apply the minimum function alignment here, so that individual backends don't have to.
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It's actually more important than that, if you consider that -C flags can differ between translation units.

This ensures functions defined in a crate compiled with -Cmin-function-alignment get that alignment independent of the crate they are codegen'd in.

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If a function from crate A is generated in another crate B compiled with -Zmin-function-alignment=B, I would expect the alignment to be at least B.

codegen_fn_attrs.alignment = Ord::max(
codegen_fn_attrs.alignment,
tcx.sess.opts.unstable_opts.min_function_alignment,
);

let Some(Ident { name, .. }) = attr.ident() else {
continue;
};
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8 changes: 2 additions & 6 deletions compiler/rustc_codegen_ssa/src/mir/naked_asm.rs
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Expand Up @@ -131,12 +131,8 @@ fn prefix_and_suffix<'tcx>(
let attrs = tcx.codegen_fn_attrs(instance.def_id());
let link_section = attrs.link_section.map(|symbol| symbol.as_str().to_string());

// function alignment can be set globally with the `-Zmin-function-alignment=<n>` flag;
// the alignment from a `#[repr(align(<n>))]` is used if it specifies a higher alignment.
// if no alignment is specified, an alignment of 4 bytes is used.
let min_function_alignment = tcx.sess.opts.unstable_opts.min_function_alignment;
let align_bytes =
Ord::max(min_function_alignment, attrs.alignment).map(|a| a.bytes()).unwrap_or(4);
// If no alignment is specified, an alignment of 4 bytes is used.
let align_bytes = attrs.alignment.map(|a| a.bytes()).unwrap_or(4);

// In particular, `.arm` can also be written `.code 32` and `.thumb` as `.code 16`.
let (arch_prefix, arch_suffix) = if is_arm {
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7 changes: 1 addition & 6 deletions compiler/rustc_const_eval/src/interpret/memory.rs
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Expand Up @@ -877,12 +877,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
if let Some(fn_val) = self.get_fn_alloc(id) {
let align = match fn_val {
FnVal::Instance(instance) => {
// Function alignment can be set globally with the `-Zmin-function-alignment=<n>` flag;
// the alignment from a `#[repr(align(<n>))]` is used if it specifies a higher alignment.
let fn_align = self.tcx.codegen_fn_attrs(instance.def_id()).alignment;
let global_align = self.tcx.sess.opts.unstable_opts.min_function_alignment;
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Is there any way we can have a lint against accessing tcx.sess.opts in the interpreter? Or is there some way the query machinery could help us ensure this? This is a soundness bug waiting to happen. :/ And it can't be just the interpreter, right? Other queries also must be cross-crate-stable in the same way?


Ord::max(global_align, fn_align).unwrap_or(Align::ONE)
self.tcx.codegen_fn_attrs(instance.def_id()).alignment.unwrap_or(Align::ONE)
}
// Machine-specific extra functions currently do not support alignment restrictions.
FnVal::Other(_) => Align::ONE,
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