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scottmcmscottmcm
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Shrink from_raw_parts's MIR so that Vec::deref MIR-inlines again
1 parent 556216a commit 3e4a577

5 files changed

Lines changed: 190 additions & 35 deletions

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‎library/core/src/ptr/metadata.rs‎

Lines changed: 56 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -92,10 +92,14 @@ pub trait Thin = Pointee<Metadata = ()>;
9292
#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
9393
#[inline]
9494
pub const fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {
95-
// SAFETY: Accessing the value from the `PtrRepr` union is safe since *const T
96-
// and PtrComponents<T> have the same memory layouts. Only std can make this
97-
// guarantee.
98-
unsafe { PtrRepr { const_ptr: ptr }.components.metadata }
95+
metadata_flexible(ptr)
96+
}
97+
98+
#[inline]
99+
const fn metadata_flexible<P: RawPointer>(ptr: P) -> <P::Pointee as Pointee>::Metadata {
100+
// SAFETY: Transmuting like this is safe since `P` and `PtrComponents<P, _>`
101+
// have the same memory layouts. Only std can make this guarantee.
102+
unsafe { crate::intrinsics::transmute_unchecked::<P, PtrComponents<P::Pointee>>(ptr).metadata }
99103
}
100104

101105
/// Forms a (possibly-wide) raw pointer from a data pointer and metadata.
@@ -112,10 +116,7 @@ pub const fn from_raw_parts<T: ?Sized>(
112116
data_pointer: *const (),
113117
metadata: <T as Pointee>::Metadata,
114118
) -> *const T {
115-
// SAFETY: Accessing the value from the `PtrRepr` union is safe since *const T
116-
// and PtrComponents<T> have the same memory layouts. Only std can make this
117-
// guarantee.
118-
unsafe { PtrRepr { components: PtrComponents { data_pointer, metadata } }.const_ptr }
119+
from_raw_parts_flexible(data_pointer, metadata)
119120
}
120121

121122
/// Performs the same functionality as [`from_raw_parts`], except that a
@@ -129,30 +130,64 @@ pub const fn from_raw_parts_mut<T: ?Sized>(
129130
data_pointer: *mut (),
130131
metadata: <T as Pointee>::Metadata,
131132
) -> *mut T {
132-
// SAFETY: Accessing the value from the `PtrRepr` union is safe since *const T
133-
// and PtrComponents<T> have the same memory layouts. Only std can make this
134-
// guarantee.
135-
unsafe { PtrRepr { components: PtrComponents { data_pointer, metadata } }.mut_ptr }
133+
from_raw_parts_flexible(data_pointer, metadata)
136134
}
137135

138-
#[repr(C)]
139-
union PtrRepr<T: ?Sized> {
140-
const_ptr: *const T,
141-
mut_ptr: *mut T,
142-
components: PtrComponents<T>,
136+
/// Just like [`from_raw_parts`] and [`from_raw_parts_mut`], but more flexible
137+
/// in terms of which types it can take, allowing smaller MIR.
138+
// See <https://github.com/rust-lang/rust/issues/123174>
139+
#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
140+
#[inline]
141+
pub(crate) const fn from_raw_parts_flexible<P: RawPointer>(
142+
data_pointer: impl RawPointer<Pointee: Thin>,
143+
metadata: <P::Pointee as Pointee>::Metadata,
144+
) -> P {
145+
// SAFETY: Transmuting like this is safe since `P` and `PtrComponents<P, _>`
146+
// have the same memory layouts. Only std can make this guarantee.
147+
unsafe {
148+
crate::intrinsics::transmute_unchecked::<PtrComponents<P::Pointee, _>, P>(PtrComponents {
149+
data_pointer,
150+
metadata,
151+
})
152+
}
153+
}
154+
155+
use private_bounds::RawPointer;
156+
mod private_bounds {
157+
/// Internal trait to avoid bad instantiations of [`PtrComponents`]
158+
///
159+
/// # Safety
160+
///
161+
/// Must have the same layout as `*const Self::Pointee` and be able to hold provenance.
162+
pub unsafe trait RawPointer {
163+
type Pointee: ?Sized + super::Pointee;
164+
}
165+
}
166+
167+
// SAFETY: `*const T` is obviously a raw pointer
168+
unsafe impl<T: ?Sized> RawPointer for *const T {
169+
type Pointee = T;
170+
}
171+
// SAFETY: `*mut T` is obviously a raw pointer
172+
unsafe impl<T: ?Sized> RawPointer for *mut T {
173+
type Pointee = T;
174+
}
175+
// SAFETY: `NonNull<T>` is a transparent newtype around a `*const T`.
176+
unsafe impl<T: ?Sized> RawPointer for super::NonNull<T> {
177+
type Pointee = T;
143178
}
144179

145180
#[repr(C)]
146-
struct PtrComponents<T: ?Sized> {
147-
data_pointer: *const (),
181+
struct PtrComponents<T: ?Sized, P = *const ()> {
182+
data_pointer: P,
148183
metadata: <T as Pointee>::Metadata,
149184
}
150185

151186
// Manual impl needed to avoid `T: Copy` bound.
152-
impl<T: ?Sized> Copy for PtrComponents<T> {}
187+
impl<T: ?Sized, P: Copy> Copy for PtrComponents<T, P> {}
153188

154189
// Manual impl needed to avoid `T: Clone` bound.
155-
impl<T: ?Sized> Clone for PtrComponents<T> {
190+
impl<T: ?Sized, P: Copy> Clone for PtrComponents<T, P> {
156191
fn clone(&self) -> Self {
157192
*self
158193
}

‎library/core/src/ptr/mod.rs‎

Lines changed: 3 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -411,6 +411,7 @@ pub use crate::intrinsics::copy;
411411
pub use crate::intrinsics::write_bytes;
412412

413413
mod metadata;
414+
pub(crate) use metadata::from_raw_parts_flexible;
414415
#[unstable(feature = "ptr_metadata", issue = "81513")]
415416
pub use metadata::{from_raw_parts, from_raw_parts_mut, metadata, DynMetadata, Pointee, Thin};
416417

@@ -812,7 +813,7 @@ pub const fn from_mut<T: ?Sized>(r: &mut T) -> *mut T {
812813
#[rustc_allow_const_fn_unstable(ptr_metadata)]
813814
#[rustc_diagnostic_item = "ptr_slice_from_raw_parts"]
814815
pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
815-
from_raw_parts(data.cast(), len)
816+
from_raw_parts_flexible(data, len)
816817
}
817818

818819
/// Forms a raw mutable slice from a pointer and a length.
@@ -858,7 +859,7 @@ pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
858859
#[rustc_const_unstable(feature = "const_slice_from_raw_parts_mut", issue = "67456")]
859860
#[rustc_diagnostic_item = "ptr_slice_from_raw_parts_mut"]
860861
pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
861-
from_raw_parts_mut(data.cast(), len)
862+
from_raw_parts_flexible(data, len)
862863
}
863864

864865
/// Swaps the values at two mutable locations of the same type, without

‎library/core/src/ptr/non_null.rs‎

Lines changed: 1 addition & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -265,10 +265,7 @@ impl<T: ?Sized> NonNull<T> {
265265
data_pointer: NonNull<()>,
266266
metadata: <T as super::Pointee>::Metadata,
267267
) -> NonNull<T> {
268-
// SAFETY: The result of `ptr::from::raw_parts_mut` is non-null because `data_pointer` is.
269-
unsafe {
270-
NonNull::new_unchecked(super::from_raw_parts_mut(data_pointer.as_ptr(), metadata))
271-
}
268+
super::from_raw_parts_flexible(data_pointer, metadata)
272269
}
273270

274271
/// Decompose a (possibly wide) pointer into its data pointer and metadata components.

‎tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-abort.mir‎

Lines changed: 65 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -3,12 +3,73 @@
33
fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
44
debug v => _1;
55
let mut _0: &[u8];
6-
7-
bb0: {
8-
_0 = <Vec<u8> as Deref>::deref(move _1) -> [return: bb1, unwind unreachable];
6+
scope 1 (inlined <Vec<u8> as Deref>::deref) {
7+
debug self => _1;
8+
let mut _4: *const u8;
9+
let mut _5: usize;
10+
scope 2 {
11+
scope 3 (inlined Vec::<u8>::as_ptr) {
12+
debug self => _1;
13+
let mut _2: &alloc::raw_vec::RawVec<u8>;
14+
scope 4 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
15+
debug self => _2;
16+
let mut _3: std::ptr::NonNull<u8>;
17+
scope 5 (inlined Unique::<u8>::as_ptr) {
18+
debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _3;
19+
debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
20+
scope 6 (inlined NonNull::<u8>::as_ptr) {
21+
debug self => _3;
22+
}
23+
}
24+
}
25+
}
26+
scope 7 (inlined std::slice::from_raw_parts::<'_, u8>) {
27+
debug data => _4;
28+
debug len => _5;
29+
let _7: *const [u8];
30+
scope 8 {
31+
scope 9 (inlined core::ub_checks::check_language_ub) {
32+
scope 10 (inlined core::ub_checks::check_language_ub::runtime) {
33+
}
34+
}
35+
scope 11 (inlined std::mem::size_of::<u8>) {
36+
}
37+
scope 12 (inlined align_of::<u8>) {
38+
}
39+
scope 13 (inlined slice_from_raw_parts::<u8>) {
40+
debug data => _4;
41+
debug len => _5;
42+
scope 14 (inlined std::ptr::metadata::from_raw_parts_flexible::<*const [u8], *const u8>) {
43+
debug data_pointer => _4;
44+
debug metadata => _5;
45+
let mut _6: std::ptr::metadata::PtrComponents<[u8], *const u8>;
46+
scope 15 {
47+
}
48+
}
49+
}
50+
}
51+
}
52+
}
953
}
1054

11-
bb1: {
55+
bb0: {
56+
StorageLive(_4);
57+
StorageLive(_2);
58+
_2 = &((*_1).0: alloc::raw_vec::RawVec<u8>);
59+
StorageLive(_3);
60+
_3 = ((((*_1).0: alloc::raw_vec::RawVec<u8>).0: std::ptr::Unique<u8>).0: std::ptr::NonNull<u8>);
61+
_4 = (_3.0: *const u8);
62+
StorageDead(_3);
63+
StorageDead(_2);
64+
StorageLive(_5);
65+
_5 = ((*_1).1: usize);
66+
StorageLive(_6);
67+
_6 = std::ptr::metadata::PtrComponents::<[u8], *const u8> { data_pointer: _4, metadata: _5 };
68+
_7 = move _6 as *const [u8] (Transmute);
69+
StorageDead(_6);
70+
StorageDead(_5);
71+
StorageDead(_4);
72+
_0 = &(*_7);
1273
return;
1374
}
1475
}

‎tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-unwind.mir‎

Lines changed: 65 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -3,12 +3,73 @@
33
fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
44
debug v => _1;
55
let mut _0: &[u8];
6-
7-
bb0: {
8-
_0 = <Vec<u8> as Deref>::deref(move _1) -> [return: bb1, unwind continue];
6+
scope 1 (inlined <Vec<u8> as Deref>::deref) {
7+
debug self => _1;
8+
let mut _4: *const u8;
9+
let mut _5: usize;
10+
scope 2 {
11+
scope 3 (inlined Vec::<u8>::as_ptr) {
12+
debug self => _1;
13+
let mut _2: &alloc::raw_vec::RawVec<u8>;
14+
scope 4 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
15+
debug self => _2;
16+
let mut _3: std::ptr::NonNull<u8>;
17+
scope 5 (inlined Unique::<u8>::as_ptr) {
18+
debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _3;
19+
debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
20+
scope 6 (inlined NonNull::<u8>::as_ptr) {
21+
debug self => _3;
22+
}
23+
}
24+
}
25+
}
26+
scope 7 (inlined std::slice::from_raw_parts::<'_, u8>) {
27+
debug data => _4;
28+
debug len => _5;
29+
let _7: *const [u8];
30+
scope 8 {
31+
scope 9 (inlined core::ub_checks::check_language_ub) {
32+
scope 10 (inlined core::ub_checks::check_language_ub::runtime) {
33+
}
34+
}
35+
scope 11 (inlined std::mem::size_of::<u8>) {
36+
}
37+
scope 12 (inlined align_of::<u8>) {
38+
}
39+
scope 13 (inlined slice_from_raw_parts::<u8>) {
40+
debug data => _4;
41+
debug len => _5;
42+
scope 14 (inlined std::ptr::metadata::from_raw_parts_flexible::<*const [u8], *const u8>) {
43+
debug data_pointer => _4;
44+
debug metadata => _5;
45+
let mut _6: std::ptr::metadata::PtrComponents<[u8], *const u8>;
46+
scope 15 {
47+
}
48+
}
49+
}
50+
}
51+
}
52+
}
953
}
1054

11-
bb1: {
55+
bb0: {
56+
StorageLive(_4);
57+
StorageLive(_2);
58+
_2 = &((*_1).0: alloc::raw_vec::RawVec<u8>);
59+
StorageLive(_3);
60+
_3 = ((((*_1).0: alloc::raw_vec::RawVec<u8>).0: std::ptr::Unique<u8>).0: std::ptr::NonNull<u8>);
61+
_4 = (_3.0: *const u8);
62+
StorageDead(_3);
63+
StorageDead(_2);
64+
StorageLive(_5);
65+
_5 = ((*_1).1: usize);
66+
StorageLive(_6);
67+
_6 = std::ptr::metadata::PtrComponents::<[u8], *const u8> { data_pointer: _4, metadata: _5 };
68+
_7 = move _6 as *const [u8] (Transmute);
69+
StorageDead(_6);
70+
StorageDead(_5);
71+
StorageDead(_4);
72+
_0 = &(*_7);
1273
return;
1374
}
1475
}

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