fix extern "aapcs" fn

[japaric]

to actually use the AAPCS calling convention

closes#37810

This is technically a [breaking-change] because it changes the ABI of
extern "aapcs" functions that (a) involve f32/f64 arguments/return
values and (b) are compiled for arm-eabihf targets from
"aapcs-vfp" (wrong) to "aapcs" (correct).

Appendix:

What these ABIs mean?

• In the "aapcs-vfp" ABI or "hard float" calling convention: Floating
  point values are passed/returned through FPU registers (s0, s1, d0, etc.)

• Whereas, in the "aapcs" ABI or "soft float" calling convention:
  Floating point values are passed/returned through general purpose
  registers (r0, r1, etc.)

Mixing these ABIs can cause problems if the caller assumes that the
routine is using one of these ABIs but it's actually using the other
one.

---

r? @alexcrichton We are going this extern "aapcs" fn thing to implement some intrinsics (floatundidf) for the eabihf targets in order to comply with LLVM's calling convention of intrinsics.

Oh, and the value of the enum came from here.

cc @TimNN@parched

[rust-highfive]

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[parched]

this doesn't match ffi.rs it's missing Arm

[japaric]

It works:

#[no_mangle]pubextern"aapcs"fnfoo(x:f64) -> f64{
x
}#[no_mangle]pubextern"C"fnbar(x:f64) -> f64{
x
}

00000000 <foo>:0: e24dd008 subsp,sp, #84: ec410b10 vmov d0, r0, r18: ed8d0b00 vstr d0,[sp] c: eaffffff b 10 <foo+0x10>10: ed9d0b00 vldr d0,[sp]14: ec510b10 vmov r0, r1, d018: e28dd008 addsp,sp, #8 1c: e12fff1e bx lrDisassembly of section .text.bar:00000000 <bar>:0: e24dd008 subsp,sp, #84: ed8d0b00 vstr d0,[sp]8: eaffffff b c <bar+0xc> c: ed9d0b00 vldr d0,[sp]10: e28dd008 addsp,sp, #814: e12fff1e bx lr

[japaric]

(Compiled for arm-unknown-linux-gnueabihf)

[TimNN]

Maybe we should add a note about this (aapcs is now always soft float) to the aapcs entry in reference.md?

I'm not sure about the exact policy here, however technically this is a breaking change so should probably tagged as such ([breaking-change]).

[alexcrichton]

Strictly speaking yeah it's a breaking change but in practice I imagine this will fix more code than it breaks (if it breaks anything...).

@japaric want to edit the commit message and I'll r+?

also cc @rust-lang/compiler, just to be aware

[japaric]

want to edit the commit message and I'll r+?

Done.

[alexcrichton]

@bors: r+

[bors]

📌 Commit 456ceba has been approved by alexcrichton

[bors]

⌛ Testing commit 456ceba with merge fb025b4...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: alexcrichton
Pushing fb025b4 to master...