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Auto merge of rust-lang#117116 - calebzulawski:repr-simd-packed, r=wo…
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…rkingjubilee

Implement repr(packed) for repr(simd)

This allows creating vectors with non-power-of-2 lengths that do not have padding.  See rust-lang/portable-simd#319
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bors committed Dec 11, 2023
2 parents c13187c + aa00bae commit 8b1ba11
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Showing 3 changed files with 83 additions and 3 deletions.
9 changes: 8 additions & 1 deletion compiler/rustc_codegen_llvm/src/intrinsic.rs
Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,7 @@ use crate::value::Value;
use rustc_codegen_ssa::base::{compare_simd_types, wants_msvc_seh, wants_wasm_eh};
use rustc_codegen_ssa::common::{IntPredicate, TypeKind};
use rustc_codegen_ssa::errors::{ExpectedPointerMutability, InvalidMonomorphization};
use rustc_codegen_ssa::mir::operand::OperandRef;
use rustc_codegen_ssa::mir::operand::{OperandRef, OperandValue};
use rustc_codegen_ssa::mir::place::PlaceRef;
use rustc_codegen_ssa::traits::*;
use rustc_hir as hir;
Expand Down Expand Up @@ -946,6 +946,13 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
tcx.normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), callee_ty.fn_sig(tcx));
let arg_tys = sig.inputs();

// Vectors must be immediates (non-power-of-2 #[repr(packed)] are not)
for (ty, arg) in arg_tys.iter().zip(args) {
if ty.is_simd() && !matches!(arg.val, OperandValue::Immediate(_)) {
return_error!(InvalidMonomorphization::SimdArgument { span, name, ty: *ty });
}
}

if name == sym::simd_select_bitmask {
let (len, _) = require_simd!(arg_tys[1], SimdArgument);

Expand Down
18 changes: 16 additions & 2 deletions compiler/rustc_ty_utils/src/layout.rs
Original file line number Diff line number Diff line change
Expand Up @@ -433,7 +433,21 @@ fn layout_of_uncached<'tcx>(
.size
.checked_mul(e_len, dl)
.ok_or_else(|| error(cx, LayoutError::SizeOverflow(ty)))?;
let align = dl.vector_align(size);

let (abi, align) = if def.repr().packed() && !e_len.is_power_of_two() {
// Non-power-of-two vectors have padding up to the next power-of-two.
// If we're a packed repr, remove the padding while keeping the alignment as close
// to a vector as possible.
(
Abi::Aggregate { sized: true },
AbiAndPrefAlign {
abi: Align::max_for_offset(size),
pref: dl.vector_align(size).pref,
},
)
} else {
(Abi::Vector { element: e_abi, count: e_len }, dl.vector_align(size))
};
let size = size.align_to(align.abi);

// Compute the placement of the vector fields:
Expand All @@ -446,7 +460,7 @@ fn layout_of_uncached<'tcx>(
tcx.mk_layout(LayoutS {
variants: Variants::Single { index: FIRST_VARIANT },
fields,
abi: Abi::Vector { element: e_abi, count: e_len },
abi,
largest_niche: e_ly.largest_niche,
size,
align,
Expand Down
59 changes: 59 additions & 0 deletions tests/ui/simd/repr_packed.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,59 @@
// run-pass

#![feature(repr_simd, platform_intrinsics)]
#![allow(non_camel_case_types)]

#[repr(simd, packed)]
struct Simd<T, const N: usize>([T; N]);

#[repr(simd)]
struct FullSimd<T, const N: usize>([T; N]);

fn check_size_align<T, const N: usize>() {
use std::mem;
assert_eq!(mem::size_of::<Simd<T, N>>(), mem::size_of::<[T; N]>());
assert_eq!(mem::size_of::<Simd<T, N>>() % mem::align_of::<Simd<T, N>>(), 0);
}

fn check_ty<T>() {
check_size_align::<T, 1>();
check_size_align::<T, 2>();
check_size_align::<T, 3>();
check_size_align::<T, 4>();
check_size_align::<T, 8>();
check_size_align::<T, 9>();
check_size_align::<T, 15>();
}

extern "platform-intrinsic" {
fn simd_add<T>(a: T, b: T) -> T;
}

fn main() {
check_ty::<u8>();
check_ty::<i16>();
check_ty::<u32>();
check_ty::<i64>();
check_ty::<usize>();
check_ty::<f32>();
check_ty::<f64>();

unsafe {
// powers-of-two have no padding and work as usual
let x: Simd<f64, 4> =
simd_add(Simd::<f64, 4>([0., 1., 2., 3.]), Simd::<f64, 4>([2., 2., 2., 2.]));
assert_eq!(std::mem::transmute::<_, [f64; 4]>(x), [2., 3., 4., 5.]);

// non-powers-of-two have padding and need to be expanded to full vectors
fn load<T, const N: usize>(v: Simd<T, N>) -> FullSimd<T, N> {
unsafe {
let mut tmp = core::mem::MaybeUninit::<FullSimd<T, N>>::uninit();
std::ptr::copy_nonoverlapping(&v as *const _, tmp.as_mut_ptr().cast(), 1);
tmp.assume_init()
}
}
let x: FullSimd<f64, 3> =
simd_add(load(Simd::<f64, 3>([0., 1., 2.])), load(Simd::<f64, 3>([2., 2., 2.])));
assert_eq!(x.0, [2., 3., 4.]);
}
}

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