add blog post "Stabilizing naked functions"

[folkertdev]

Providing an introduction to naked functions, some background on the 10-year journey to stabilization, and some future features related to inline assembly.

cc @tgross35 @joshtriplett

• I'm not sure what my mandate is exactly here, so I just filled in something
• I'm also not sure what is good in terms of scheduling (but probably not too long after the release?)

Rendered

[jieyouxu]

Hi, noticed this while reviewing the 1.88 relnotes (#1651 (comment)).

I feel like this should be a public Blog Post, because this is about the stabilized user-facing language feature, especially about "well, why do we need this language feature in the first place"?

Sure, it has some tidbits on the implementation details on how we got to stabilized naked functions, and some possible future directions, but I think that might also be interesting for the community at large :)

[folkertdev]

Sure, I've adjusted it, and made some further small tweaks/typo fixes.

[joshtriplett]

This looks good to me. 👍 🚢

[folkertdev]

Made some final tweaks

• updated the date to today
• made all headerstart with an uppercase letter
• clarify the name a bit
• link to the 1.88.0 release blog post

With that, I think this is ready, but it looks like it needs a re-approve?

[tgross35]

A handful of optional comments from one final read through, but overall this looks pretty great!

[tgross35]

This suggestion might be a bit much since this section is basically an addendum, but it's probably worth adding at least a couple of hints

/// Division and modulo of two numbers using Arm's nonstandard ABI.
///
/// ```c
/// typedef struct { int quot; int rem; } idiv_return;
///  __value_in_regs idiv_return __aeabi_idivmod(int num, int denom);
/// ```
// SAFETY: the assembly implements the expected ABI, and "custom" ensures this
// cannot be called directly.
#[unsafe(naked)]
pub unsafe extern "custom" fn __aeabi_idivmod() {
    core::arch::naked_asm!(
        "push {{r0, r1, r4, lr}}", // Back up clobbers
        "bl {trampoline}",         // Call an `extern "C"` function for a / b
        "pop {{r1, r2}}",
        "muls r2, r2, r0",         // Perform the modulo
        "subs r1, r1, r2",
        "pop {{r4, pc}}",          // Restore clobbers, implicit return by setting `pc`
        trampoline = sym crate::arm::__aeabi_idiv,
    );
}

[folkertdev]

If we're giving detail we may as well give all the detail. Also maybe backport this to compiler-builtins itself?

[tgross35]

I had the same thought, I'll probably update that whole module next time I have a reason to touch it

[tgross35]

Looks great to me as well!