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Make Rust ABI return types up to two pointers in registers #124373

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Make Rust ABI return types up to two pointers in registers #124373

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c8507ac
make Rust ABI return types up to two pointers in registers
GoldsteinE Apr 25, 2024
e10dae3
weird
GoldsteinE Apr 25, 2024
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24 changes: 18 additions & 6 deletions compiler/rustc_ty_utils/src/abi.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,7 @@ use rustc_session::config::OptLevel;
use rustc_span::def_id::DefId;
use rustc_target::abi::call::{
ArgAbi, ArgAttribute, ArgAttributes, ArgExtension, Conv, FnAbi, PassMode, Reg, RegKind,
RiscvInterruptKind,
RiscvInterruptKind, Uniform,
};
use rustc_target::abi::*;
use rustc_target::spec::abi::Abi as SpecAbi;
Expand Down Expand Up @@ -779,11 +779,23 @@ fn fn_abi_adjust_for_abi<'tcx>(
assert!(is_indirect_not_on_stack, "{:?}", arg);

let size = arg.layout.size;
if !arg.layout.is_unsized() && size <= Pointer(AddressSpace::DATA).size(cx) {
// We want to pass small aggregates as immediates, but using
// an LLVM aggregate type for this leads to bad optimizations,
// so we pick an appropriately sized integer type instead.
arg.cast_to(Reg { kind: RegKind::Integer, size });
if !arg.layout.is_unsized() {
let data_pointer_size = Pointer(AddressSpace::DATA).size(cx);
if size <= data_pointer_size {
// We want to pass small aggregates as immediates, but using
// an LLVM aggregate type for this leads to bad optimizations,
// so we pick an appropriately sized integer type instead.
arg.cast_to(Reg { kind: RegKind::Integer, size });
} else if size == data_pointer_size * 2 && size.bytes() % 2 == 0 {
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There's no comment or anything explaining this subtlety -- please add it.

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Yeah, my original idea on how this should look got shot down by CI, so I'll look into this some more and add comments to the final version.

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Right, so it would be useful to understand why the original assumption you made is not compatible with codegen and explain it here. It also may suggest a deeper bug in your implementation, though it's not certain unless you look into it.

// Aggregates like `[usize; 2]` or (on 64-bit arch) `[u128; 1]`
// can be passed as a scalar pair.
let part_size = Size::from_bytes(size.bytes() / 2);
arg.cast_to(Uniform {
unit: Reg { kind: RegKind::Integer, size: part_size },
total: size,
is_consecutive: false,
});
}
}

// If we deduced that this parameter was read-only, add that to the attribute list now.
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