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

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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
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12
compiler/rustc_middle/src/ty/layout.rs
View file

@ -2735,6 +2735,7 @@ where
can_unwind: fn_can_unwind(cx.tcx().sess.panic_strategy(), codegen_fn_attr_flags, conv),
};
fn_abi.adjust_for_abi(cx, sig.abi);
debug!("FnAbi::new_internal = {:?}", fn_abi);
fn_abi
}
@ -2748,7 +2749,7 @@ where
|| abi == SpecAbi::RustIntrinsic
|| abi == SpecAbi::PlatformIntrinsic
{
let fixup = |arg: &mut ArgAbi<'tcx, Ty<'tcx>>| {
let fixup = |arg: &mut ArgAbi<'tcx, Ty<'tcx>>, is_ret: bool| {
if arg.is_ignore() {
return;
}
@ -2786,8 +2787,11 @@ where
_ => return,
}
let max_by_val_size =
if is_ret { call::max_ret_by_val(cx) } else { Pointer.size(cx) };
let size = arg.layout.size;
if arg.layout.is_unsized() || size > Pointer.size(cx) {
if arg.layout.is_unsized() || size > max_by_val_size {
arg.make_indirect();
} else {
// We want to pass small aggregates as immediates, but using
@ -2796,9 +2800,9 @@ where
arg.cast_to(Reg { kind: RegKind::Integer, size });
}
};
fixup(&mut self.ret);
fixup(&mut self.ret, true);
for arg in &mut self.args {
fixup(arg);
fixup(arg, false);
}
if let PassMode::Indirect(ref mut attrs, _) = self.ret.mode {
attrs.set(ArgAttribute::StructRet);