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Returns values up to 2*usize by value #77434

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Oct 4, 2020
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Returns values up to 2*usize by value #77434

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5 changes: 3 additions & 2 deletions compiler/rustc_middle/src/ty/layout.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2787,8 +2787,9 @@ where
_ => return,
}

let max_by_val_size =
if is_ret { call::max_ret_by_val(cx) } else { Pointer.size(cx) };
// Return structures up to 2 pointers in size by value, matching `ScalarPair`. LLVM
// will usually return these in 2 registers, which is more efficient than by-ref.
let max_by_val_size = if is_ret { Pointer.size(cx) * 2 } else { Pointer.size(cx) };
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I wonder if it makes sense to limit this to returns, but also whether 3 might work better than 2 as the multiplier.
But I guess there's a risk it might get downgraded from registers... I really wish LLVM modeled ABIs more precisely so we can control that.

let size = arg.layout.size;

if arg.layout.is_unsized() || size > max_by_val_size {
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12 changes: 0 additions & 12 deletions compiler/rustc_target/src/abi/call/mod.rs
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Expand Up @@ -610,15 +610,3 @@ 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),
Comment on lines -618 to -620
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I was hoping removing this string comparison would help perf but I guess not?


_ => spec.data_layout().pointer_size,
}
}