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Auto merge of #79547 - erikdesjardins:byval, r=nagisa

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Pass arguments up to 2*usize by value

In https://github.com/rust-lang/rust/pull/77434#discussion_r498719533, `@eddyb` said:

> I wonder if it makes sense to limit this to returns [...]

Let's do a perf run and find out.

It seems the `extern "C"` ABI will pass arguments up to 2*usize in registers: https://godbolt.org/z/n8E6zc. (modified from https://github.com/rust-lang/rust/issues/26494#issuecomment-619506345)

r? `@nagisa`
This commit is contained in:
bors 2020-12-02 15:17:32 +00:00
commit a094ff9590
3 changed files with 17 additions and 12 deletions
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12
compiler/rustc_middle/src/ty/layout.rs
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@ -2848,7 +2848,7 @@ where
|| abi == SpecAbi::RustIntrinsic
|| abi == SpecAbi::PlatformIntrinsic
{
let fixup = |arg: &mut ArgAbi<'tcx, Ty<'tcx>>, is_ret: bool| {
let fixup = |arg: &mut ArgAbi<'tcx, Ty<'tcx>>| {
if arg.is_ignore() {
return;
}
@ -2886,9 +2886,9 @@ where
_ => return,
}
// 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) };
// Pass and return structures up to 2 pointers in size by value, matching `ScalarPair`.
// LLVM will usually pass these in 2 registers, which is more efficient than by-ref.
let max_by_val_size = Pointer.size(cx) * 2;
let size = arg.layout.size;
if arg.layout.is_unsized() || size > max_by_val_size {
@ -2900,9 +2900,9 @@ where
arg.cast_to(Reg { kind: RegKind::Integer, size });
}
};
fixup(&mut self.ret, true);
fixup(&mut self.ret);
for arg in &mut self.args {
fixup(arg, false);
fixup(arg);
}
return;
}