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USIZE_MARKER: make sure the function is monomorphized only once #123780

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USIZE_MARKER: make sure the function is monomorphized only once #123780

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15 changes: 11 additions & 4 deletions library/core/src/fmt/rt.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -209,8 +209,15 @@ extern "C" {
// first argument. The read_volatile here ensures that we can safely ready out a
// usize from the passed reference and that this address does not point at a
// non-usize taking function.
static USIZE_MARKER: fn(&usize, &mut Formatter<'_>) -> Result = |ptr, _| {
// SAFETY: ptr is a reference
let _v: usize = unsafe { crate::ptr::read_volatile(ptr) };
loop {}
static USIZE_MARKER: fn(&usize, &mut Formatter<'_>) -> Result = {
// Make sure this function is only monomorphized once, and not considered cross-crate
// inlineable.
#[inline(never)]
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#[inline(never)] is a suggestion to the optimizer, not a hard requirement. As such this change still doesn't guarantee that the function is only monomorphized once.

https://doc.rust-lang.org/reference/attributes/codegen.html#the-inline-attribute

  • #[inline(never)] suggests that an inline expansion should never be performed.

Note: #[inline] in every form is a hint, with no requirements on the language to place a copy of the attributed function in the caller.

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Yeah, I don't think there is a way to actually do what USIZE_MARKER wants to do. But I don't think t-libs will let me remove this thing entirely, so using #[inline(never)] seems like an improvement...

fn usize_marker(ptr: &usize, _: &mut Formatter<'_>) -> Result {
// SAFETY: ptr is a reference
let _v: usize = unsafe { crate::ptr::read_volatile(ptr) };
loop {}
}

usize_marker
};
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