Clarify behavior of x87 FP registers in inline assembly

[Amanieu]

This came up in a discussion on Zulip.

cc @joshtriplett

[joshtriplett]

Can you clarify and spell out how the writer of an asm! block can clobber all x87 registers? Do they need to write out a clobber for each register individually? You wrote st(*) here; someone might expect that syntax to work. Does it, or could it?

[Amanieu]

All 8 registers need to be explicitly clobbered, there is no wildcard syntax. The reason for this is that in the future we may decide to support x87 registers as proper operands. Also there is no precedent for a wildcard register name that covers multiple real registers.

[joshtriplett]

Also there is no precedent for a wildcard register name that covers multiple real registers.

In theory we could do that with something like clobber_abi("x87"), but perhaps that's not worth catering for.

[joshtriplett]

@bors r+ rollup

[loveJesus]

Happy Easter and hello thanks for your work on rust. I am trying to implement the following in the new asm! syntax but i'm not really sure particularly about how to go about having the fstp instruction store into a rust variable. Thank you!

#[inline(always)]
pub fn cos_chirho(a: f32) -> f32 { 
let mut res: f32 = unsafe { mem::uninitialized() };

unsafe { asm!(
    r##"
        flds $1;
        fcos;
        fstps $0;
    "##
    : "=*m"(&mut res as *mut f32)
    : "*m"(&a as *const f32)
) };

res
}

[Amanieu]

Just pass the memory address in a reg and then use it as normal:

asm!(
	"flds [{}]",
	"fcos",
	"fstps [{}]",
	in(reg) &a
	in(reg) &mut res,
);

[loveJesus]

God is good! thank you.
Changed the flds to fld dword ptr and likewise for fstp. It works!

I did not know however to use an in(reg) instead of out(reg) for the &mut parameter. It was giving me (exit code: 0xc0000005, STATUS_ACCESS_VIOLATION) when it would compile with out(reg) *&mut. It makes sense that an in can work since we modify the contents pointed to by the register and not the register itself. Sorry to have bothered you with this. Do you think me attempting to make a pull request with this example would be helpful?

[Amanieu]

Rust-by-example might be a better place for some example code.