bjoernager/rust
bjoernager/rust
1
Fork 0
Code Activity

Auto merge of #76986 - jonas-schievink:ret-in-reg, r=nagisa

Browse source 
Return values up to 128 bits in registers

This fixes https://github.com/rust-lang/rust/issues/26494#issuecomment-619506345 by making Rust's default ABI pass return values up to 128 bits in size in registers, just like the System V ABI.

The result is that these methods from the comment linked above now generate the same code, making the Rust ABI as efficient as the `"C"` ABI:

```rust
pub struct Stats { x: u32, y: u32, z: u32, }

pub extern "C" fn sum_c(a: &Stats, b: &Stats) -> Stats {
    return Stats {x: a.x + b.x, y: a.y + b.y, z: a.z + b.z };
}

pub fn sum_rust(a: &Stats, b: &Stats) -> Stats {
    return Stats {x: a.x + b.x, y: a.y + b.y, z: a.z + b.z };
}
```

```asm
sum_rust:
	movl	(%rsi), %eax
	addl	(%rdi), %eax
	movl	4(%rsi), %ecx
	addl	4(%rdi), %ecx
	movl	8(%rsi), %edx
	addl	8(%rdi), %edx
	shlq	$32, %rcx
	orq	%rcx, %rax
	retq
```
This commit is contained in:
bors 2020-09-27 02:35:11 +00:00
commit 62fe055aba
3 changed files with 52 additions and 4 deletions
Download patch file Download diff file Expand all files Collapse all files

12
compiler/rustc_target/src/abi/call/mod.rs
View file

@ -610,3 +610,15 @@ impl<'a, Ty> FnAbi<'a, Ty> {
Ok(())
}
}
/// Returns the maximum size of return values to be passed by value in the Rust ABI.
///
/// Return values beyond this size will use an implicit out-pointer instead.
pub fn max_ret_by_val<C: HasTargetSpec + HasDataLayout>(spec: &C) -> Size {
match spec.target_spec().arch.as_str() {
// System-V will pass return values up to 128 bits in RAX/RDX.
"x86_64" => Size::from_bits(128),
_ => spec.data_layout().pointer_size,
}
}