Can't use const fn directly as inline assembly constant input

[parched]

I hoped this would work

const fn get_3() -> u32
{
    3
}

pub unsafe fn use_get_3()
{
    asm!("" : : "i"(get_3()))
}

but it generates (without optimizations)

  %0 = call i32 @_ZN8rust_out5get_317hccb90954bf88bf3cE()
  call void asm "", "i,~{dirflag},~{fpsr},~{flags}"(i32 %0)

giving error: invalid operand for inline asm constraint 'i'.

The current solution seems to be

pub unsafe fn use_three()
{
    const THREE: u32 = get_3();
    asm!("" : : "i"(THREE))
}

which generates

  call void asm "", "i,~{dirflag},~{fpsr},~{flags}"(i32 3)

like I hoped the first case would.

Could we make the first case guaranteed to work?

[eddyb]

We could make this work the same we did for SIMD intrinsics (requiring certain arguments are constant, and promoting them to ensure codegen can always succeed):
(we just need a way to detect whether a certain inline asm input needs to be constant)

rust/src/librustc_mir/transform/promote_consts.rs

Lines 219 to 238 in 54b7d21

	if let TerminatorKind::Call { ref func, .. } = *kind {
	if let ty::FnDef(def_id, _) = func.ty(self.body, self.tcx).kind {
	let fn_sig = self.tcx.fn_sig(def_id);
	if let Abi::RustIntrinsic | Abi::PlatformIntrinsic = fn_sig.abi() {
	let name = self.tcx.item_name(def_id);
	// FIXME(eddyb) use `#[rustc_args_required_const(2)]` for shuffles.
	if name.as_str().starts_with("simd_shuffle") {
	self.candidates.push(Candidate::Argument { bb: location.block, index: 2 });
	return; // Don't double count `simd_shuffle` candidates
	}
	}
	if let Some(constant_args) = args_required_const(self.tcx, def_id) {
	for index in constant_args {
	self.candidates.push(Candidate::Argument { bb: location.block, index });
	}
	}
	}
	}

cc @Amanieu @oli-obk @ecstatic-morse

[hanna-kruppe]

Note that the current proposal for inline assembly includes const <expr> operands that explicitly are constant expressions, and the prototype implementation treats them more or less as you describe. I don't think it's worth doing the same for "legacy" inline asm.

[eddyb]

Oh, nice, for some reason I expected if that were the case we'd get a comment on these issues stating it, I should've checked first.

[Amanieu]

This issue does not apply to the new asm! (RFC 2850) which has proper support for const operands.

The legacy llvm_asm! is deprecated and is no longer maintained.