Custom LLVM calling convention

compiler
1

Rust uses aggregate function arguments and return types (structs and enums) a LOT. The C calling convention is suboptimal in this case, passing the aggregates on the stack even when they could be in registers if the calling convention were different.

Swift (which I believe has the same problem) uses a custom calling convention. It is suboptimal for Rust, though, because it involves a special register used for error handling which Rust doesn’t need. Have there been any thoughts on a custom calling convention specifically for Rust?

1 Like
2

Rust does in fact use its own calling convention – but it’s handled sort of “before” the LLVM level. That is, if the Rust type is Foo, we will often tell LLVM to expect a Foo*, essentially.

3

That is still less efficient than my proposal, in which Foo would be returned in registers unless it is big.

4

We can pass arguments and results in registers without adding a custom calling convention to LLVM. In fact, rustc already generates reasonable results if you request the C calling convention:

pub struct X(*const i32, *const i32);
pub extern "C" fn f(x: X) -> X { x }

Generates:

	movq	%rdi, %rax
	movq	%rsi, %rdx
	retq

Of course, it’s kind of pathetic that Rust’s default calling convention is worse than the C calling convention… but that can be fixed without a custom LLVM calling convention.

2 Likes
5

Since Rust uses LLVM fast-call, shouldn’t that be reported to LLVM’s developers?

6

I meant that rustc is making bad decisions about what structures to pass/return by pointer, not that LLVM is doing anything wrong.

7

What about a struct with four int64s? That can be returned in registers on x86-64 (at least if the ABI provides enough caller-saved registers), but I doubt that fastcc will do that.

As for whether a custom calling convention is worthwhile: fastcc is still optimized for C-like code, which usually passes aggregates by reference. Rust often passes aggregates by value, so it would be nice to optimize this case. I suspect this is one of the reasons that Swift uses a custom ABI. Note that GHC and an LLVM backend for Erlang HiPE also have custom calling conventions.

8

Is anything being done or are there plans to fix this? It seems ludicrous that if I want to strongly type an int by wrapping it in a struct all simple functions on it now have to read and write memory. EDIT: No, actually all is fine, I was looking at wrong piece of generated code.

9

It doesn’t always force it into memory.

pub struct Foo(u32);

pub fn extract(num: Foo) -> u32 {
  num.0
}

Gives me:

_ZN8rust_out7extract17h95ec4993a842ab8fE:
	movl	%edi, %eax
	retq
10

Ok sorry, ignore my comment, I was looking at the wrong piece of generated code where I was passing it by reference, doh!

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