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Rollup merge of #144706 - zachs18:fix-144661, r=RalfJung

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Do not give function allocations alignment in consteval and Miri.

We do not yet have a (clear and T-lang approved) design for how `#[align(N)]` on functions should affect function pointers' addresses on various platforms, so for now do not give function pointers alignment in consteval and Miri.

----

Old summary:

Not a full solution to <https://github.com/rust-lang/rust/issues/144661>, but fixes the immediate issue by making function allocations all have alignment 1 in consteval, ignoring `#[rustc_align(N)]`, so the compiler doesn't know if any offset other than 0 is non-null.

A more "principlied" solution would probably be to make function pointers to `#[instruction_set(arm::t32)]` functions be at offset 1 of an align-`max(2, align attribute)` allocation instead of at offset 0 of their allocation during consteval, and on wasm to either disallow `#[align(N)]` where N > 1, or to pad the function table such that the function pointer of a `#[align(N)]` function is a multiple of `N` at runtime.
This commit is contained in:
Stuart Cook 2025-08-04 11:24:39 +10:00 • committed by GitHub
commit 0225f8b09c
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2 changed files with 6 additions and 27 deletions
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8
compiler/rustc_const_eval/src/interpret/memory.rs
View file

@ -937,8 +937,12 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
// (both global from `alloc_map` and local from `extra_fn_ptr_map`)
if let Some(fn_val) = self.get_fn_alloc(id) {
let align = match fn_val {
FnVal::Instance(instance) => {
self.tcx.codegen_instance_attrs(instance.def).alignment.unwrap_or(Align::ONE)
FnVal::Instance(_instance) => {
// FIXME: Until we have a clear design for the effects of align(N) functions
// on the address of function pointers, we don't consider the align(N)
// attribute on functions in the interpreter.
// See <https://github.com/rust-lang/rust/issues/144661> for more context.
Align::ONE
}
// Machine-specific extra functions currently do not support alignment restrictions.
FnVal::Other(_) => Align::ONE,

25
src/tools/miri/tests/pass/fn_align.rs
View file

@ -1,25 +0,0 @@
//@compile-flags: -Zmin-function-alignment=8
// FIXME(rust-lang/rust#82232, rust-lang/rust#143834): temporarily renamed to mitigate `#[align]`
// nameres ambiguity
#![feature(rustc_attrs)]
#![feature(fn_align)]
// When a function uses `align(N)`, the function address should be a multiple of `N`.
#[rustc_align(256)]
fn foo() {}
#[rustc_align(16)]
fn bar() {}
#[rustc_align(4)]
fn baz() {}
fn main() {
assert!((foo as usize).is_multiple_of(256));
assert!((bar as usize).is_multiple_of(16));
// The maximum of `align(N)` and `-Zmin-function-alignment=N` is used.
assert!((baz as usize).is_multiple_of(8));
}