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Merged
rust-bors merged 3 commits into from
Jan 31, 2026
Merged

GVN: Elide more intermediate transmutes #151622

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3a33ab0
GVN: Elide more intermediate transmutes
scottmcm Jan 24, 2026
929e280
Update `ptr::Alignment` to go through transmuting
scottmcm Jan 26, 2026
9288c20
Adjust `Alignment` to emphasize that we don't look at its field
scottmcm Jan 26, 2026
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61 changes: 52 additions & 9 deletions compiler/rustc_mir_transform/src/gvn.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1591,10 +1591,12 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
(Transmute, PtrToPtr) if self.pointers_have_same_metadata(from, to) => {
Some(Transmute)
}
// If would be legal to always do this, but we don't want to hide information
// It would be legal to always do this, but we don't want to hide information
// from the backend that it'd otherwise be able to use for optimizations.
(Transmute, Transmute)
if !self.type_may_have_niche_of_interest_to_backend(from) =>
if !self.transmute_may_have_niche_of_interest_to_backend(
inner_from, from, to,
) =>
{
Some(Transmute)
}
Expand Down Expand Up @@ -1642,24 +1644,65 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
}
}

/// Returns `false` if we know for sure that this type has no interesting niche,
/// and thus we can skip transmuting through it without worrying.
/// Returns `false` if we're confident that the middle type doesn't have an
/// interesting niche so we can skip that step when transmuting.
///
/// The backend will emit `assume`s when transmuting between types with niches,
/// so we want to preserve `i32 -> char -> u32` so that that data is around,
/// but it's fine to skip whole-range-is-value steps like `A -> u32 -> B`.
fn type_may_have_niche_of_interest_to_backend(&self, ty: Ty<'tcx>) -> bool {
let Ok(layout) = self.ecx.layout_of(ty) else {
fn transmute_may_have_niche_of_interest_to_backend(
&self,
from_ty: Ty<'tcx>,
middle_ty: Ty<'tcx>,
to_ty: Ty<'tcx>,
) -> bool {
let Ok(middle_layout) = self.ecx.layout_of(middle_ty) else {
// If it's too generic or something, then assume it might be interesting later.
return true;
};

if layout.uninhabited {
if middle_layout.uninhabited {
return true;
}

match layout.backend_repr {
BackendRepr::Scalar(a) => !a.is_always_valid(&self.ecx),
match middle_layout.backend_repr {
BackendRepr::Scalar(mid) => {
if mid.is_always_valid(&self.ecx) {
// With no niche it's never interesting, so don't bother
// looking at the layout of the other two types.
false
} else if let Ok(from_layout) = self.ecx.layout_of(from_ty)
&& !from_layout.uninhabited
&& from_layout.size == middle_layout.size
&& let BackendRepr::Scalar(from_a) = from_layout.backend_repr
&& let mid_range = mid.valid_range(&self.ecx)
&& let from_range = from_a.valid_range(&self.ecx)
&& mid_range.contains_range(from_range, middle_layout.size)
{
// The `from_range` is a (non-strict) subset of `mid_range`
// such as if we're doing `bool` -> `ascii::Char` -> `_`,
// where `from_range: 0..=1` and `mid_range: 0..=127`,
// and thus the middle doesn't tell us anything we don't
// already know from the initial type.
false
} else if let Ok(to_layout) = self.ecx.layout_of(to_ty)
&& !to_layout.uninhabited
&& to_layout.size == middle_layout.size
&& let BackendRepr::Scalar(to_a) = to_layout.backend_repr
&& let mid_range = mid.valid_range(&self.ecx)
&& let to_range = to_a.valid_range(&self.ecx)
&& mid_range.contains_range(to_range, middle_layout.size)
{
// The `to_range` is a (non-strict) subset of `mid_range`
// such as if we're doing `_` -> `ascii::Char` -> `bool`,
// where `mid_range: 0..=127` and `to_range: 0..=1`,
// and thus the middle doesn't tell us anything we don't
// already know from the final type.
false
Comment on lines 1674 to 1701
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@cjgillot cjgillot Jan 25, 2026

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Do you mind commenting the logic? This looks good to me, but it took me a few frowns to understand, for instance why ranges are compared the way you wrote it

} else {
true
}
}
BackendRepr::ScalarPair(a, b) => {
!a.is_always_valid(&self.ecx) || !b.is_always_valid(&self.ecx)
}
Expand Down
14 changes: 11 additions & 3 deletions library/core/src/ptr/alignment.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,12 @@ use crate::{cmp, fmt, hash, mem, num};
#[unstable(feature = "ptr_alignment_type", issue = "102070")]
#[derive(Copy, Clone, PartialEq, Eq)]
#[repr(transparent)]
pub struct Alignment(AlignmentEnum);
pub struct Alignment {
// This field is never used directly (nor is the enum),
// as it's just there to convey the validity invariant.
// (Hopefully it'll eventually be a pattern type instead.)
_inner_repr_trick: AlignmentEnum,
}
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@cjgillot cjgillot Jan 25, 2026

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Can this change be done in its own commit? This would make easier to understand what changes from the mir opt diff.

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@scottmcm scottmcm Jan 26, 2026

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Absolutely. I split it to three:

  1. Just the compiler change to the mir-opt pass
  2. The change to Alignment::as_usize on its own
  3. The field change in Alignment

That way all the library changes are separated.


// Alignment is `repr(usize)`, but via extra steps.
const _: () = assert!(size_of::<Alignment>() == size_of::<usize>());
Expand All @@ -37,7 +42,7 @@ impl Alignment {
/// assert_eq!(Alignment::MIN.as_usize(), 1);
/// ```
#[unstable(feature = "ptr_alignment_type", issue = "102070")]
pub const MIN: Self = Self(AlignmentEnum::_Align1Shl0);
pub const MIN: Self = Self::new(1).unwrap();

/// Returns the alignment for a type.
///
Expand Down Expand Up @@ -166,7 +171,10 @@ impl Alignment {
#[unstable(feature = "ptr_alignment_type", issue = "102070")]
#[inline]
pub const fn as_usize(self) -> usize {
self.0 as usize
// Going through `as_nonzero` helps this be more clearly the inverse of
// `new_unchecked`, letting MIR optimizations fold it away.

self.as_nonzero().get()
}

/// Returns the alignment as a <code>[NonZero]<[usize]></code>.
Expand Down
8 changes: 2 additions & 6 deletions ...opt/dataflow-const-prop/default_boxed_slice.main.DataflowConstProp.32bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -16,12 +16,11 @@
scope 4 (inlined std::ptr::Unique::<[bool; 0]>::dangling) {
let mut _5: std::ptr::NonNull<[bool; 0]>;
scope 5 (inlined NonNull::<[bool; 0]>::dangling) {
let mut _6: std::num::NonZero<usize>;
scope 6 {
scope 8 (inlined std::ptr::Alignment::as_nonzero) {
}
scope 9 (inlined NonNull::<[bool; 0]>::without_provenance) {
let _7: *const [bool; 0];
let _6: *const [bool; 0];
scope 10 {
}
scope 11 (inlined NonZero::<usize>::get) {
Expand All @@ -45,11 +44,8 @@
StorageLive(_4);
StorageLive(_5);
StorageLive(_6);
_6 = const NonZero::<usize>(core::num::niche_types::NonZeroUsizeInner(1_usize));
StorageLive(_7);
_7 = const {0x1 as *const [bool; 0]};
_6 = const {0x1 as *const [bool; 0]};
_5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};
StorageDead(_7);
StorageDead(_6);
_4 = const std::ptr::Unique::<[bool; 0]> {{ pointer: NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }}, _marker: PhantomData::<[bool; 0]> }};
StorageDead(_5);
Expand Down
8 changes: 2 additions & 6 deletions ...pt/dataflow-const-prop/default_boxed_slice.main.DataflowConstProp.32bit.panic-unwind.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -16,12 +16,11 @@
scope 4 (inlined std::ptr::Unique::<[bool; 0]>::dangling) {
let mut _5: std::ptr::NonNull<[bool; 0]>;
scope 5 (inlined NonNull::<[bool; 0]>::dangling) {
let mut _6: std::num::NonZero<usize>;
scope 6 {
scope 8 (inlined std::ptr::Alignment::as_nonzero) {
}
scope 9 (inlined NonNull::<[bool; 0]>::without_provenance) {
let _7: *const [bool; 0];
let _6: *const [bool; 0];
scope 10 {
}
scope 11 (inlined NonZero::<usize>::get) {
Expand All @@ -45,11 +44,8 @@
StorageLive(_4);
StorageLive(_5);
StorageLive(_6);
_6 = const NonZero::<usize>(core::num::niche_types::NonZeroUsizeInner(1_usize));
StorageLive(_7);
_7 = const {0x1 as *const [bool; 0]};
_6 = const {0x1 as *const [bool; 0]};
_5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};
StorageDead(_7);
StorageDead(_6);
_4 = const std::ptr::Unique::<[bool; 0]> {{ pointer: NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }}, _marker: PhantomData::<[bool; 0]> }};
StorageDead(_5);
Expand Down
8 changes: 2 additions & 6 deletions ...opt/dataflow-const-prop/default_boxed_slice.main.DataflowConstProp.64bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -16,12 +16,11 @@
scope 4 (inlined std::ptr::Unique::<[bool; 0]>::dangling) {
let mut _5: std::ptr::NonNull<[bool; 0]>;
scope 5 (inlined NonNull::<[bool; 0]>::dangling) {
let mut _6: std::num::NonZero<usize>;
scope 6 {
scope 8 (inlined std::ptr::Alignment::as_nonzero) {
}
scope 9 (inlined NonNull::<[bool; 0]>::without_provenance) {
let _7: *const [bool; 0];
let _6: *const [bool; 0];
scope 10 {
}
scope 11 (inlined NonZero::<usize>::get) {
Expand All @@ -45,11 +44,8 @@
StorageLive(_4);
StorageLive(_5);
StorageLive(_6);
_6 = const NonZero::<usize>(core::num::niche_types::NonZeroUsizeInner(1_usize));
StorageLive(_7);
_7 = const {0x1 as *const [bool; 0]};
_6 = const {0x1 as *const [bool; 0]};
_5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};
StorageDead(_7);
StorageDead(_6);
_4 = const std::ptr::Unique::<[bool; 0]> {{ pointer: NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }}, _marker: PhantomData::<[bool; 0]> }};
StorageDead(_5);
Expand Down
8 changes: 2 additions & 6 deletions ...pt/dataflow-const-prop/default_boxed_slice.main.DataflowConstProp.64bit.panic-unwind.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -16,12 +16,11 @@
scope 4 (inlined std::ptr::Unique::<[bool; 0]>::dangling) {
let mut _5: std::ptr::NonNull<[bool; 0]>;
scope 5 (inlined NonNull::<[bool; 0]>::dangling) {
let mut _6: std::num::NonZero<usize>;
scope 6 {
scope 8 (inlined std::ptr::Alignment::as_nonzero) {
}
scope 9 (inlined NonNull::<[bool; 0]>::without_provenance) {
let _7: *const [bool; 0];
let _6: *const [bool; 0];
scope 10 {
}
scope 11 (inlined NonZero::<usize>::get) {
Expand All @@ -45,11 +44,8 @@
StorageLive(_4);
StorageLive(_5);
StorageLive(_6);
_6 = const NonZero::<usize>(core::num::niche_types::NonZeroUsizeInner(1_usize));
StorageLive(_7);
_7 = const {0x1 as *const [bool; 0]};
_6 = const {0x1 as *const [bool; 0]};
_5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};
StorageDead(_7);
StorageDead(_6);
_4 = const std::ptr::Unique::<[bool; 0]> {{ pointer: NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }}, _marker: PhantomData::<[bool; 0]> }};
StorageDead(_5);
Expand Down
13 changes: 4 additions & 9 deletions tests/mir-opt/dataflow-const-prop/default_boxed_slice.main.GVN.32bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -16,12 +16,11 @@
scope 4 (inlined std::ptr::Unique::<[bool; 0]>::dangling) {
let mut _5: std::ptr::NonNull<[bool; 0]>;
scope 5 (inlined NonNull::<[bool; 0]>::dangling) {
let mut _6: std::num::NonZero<usize>;
scope 6 {
scope 8 (inlined std::ptr::Alignment::as_nonzero) {
}
scope 9 (inlined NonNull::<[bool; 0]>::without_provenance) {
let _7: *const [bool; 0];
let _6: *const [bool; 0];
scope 10 {
}
scope 11 (inlined NonZero::<usize>::get) {
Expand All @@ -45,14 +44,10 @@
StorageLive(_4);
StorageLive(_5);
StorageLive(_6);
- _6 = const std::ptr::Alignment::of::<[bool; 0]>::{constant#0} as std::num::NonZero<usize> (Transmute);
+ _6 = const NonZero::<usize>(core::num::niche_types::NonZeroUsizeInner(1_usize));
StorageLive(_7);
- _7 = copy _6 as *const [bool; 0] (Transmute);
- _5 = NonNull::<[bool; 0]> { pointer: copy _7 };
+ _7 = const {0x1 as *const [bool; 0]};
- _6 = const std::ptr::Alignment::of::<[bool; 0]>::{constant#0} as *const [bool; 0] (Transmute);
- _5 = NonNull::<[bool; 0]> { pointer: copy _6 };
+ _6 = const {0x1 as *const [bool; 0]};
+ _5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};
StorageDead(_7);
StorageDead(_6);
- _4 = std::ptr::Unique::<[bool; 0]> { pointer: move _5, _marker: const PhantomData::<[bool; 0]> };
+ _4 = const std::ptr::Unique::<[bool; 0]> {{ pointer: NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }}, _marker: PhantomData::<[bool; 0]> }};
Expand Down
13 changes: 4 additions & 9 deletions tests/mir-opt/dataflow-const-prop/default_boxed_slice.main.GVN.32bit.panic-unwind.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -16,12 +16,11 @@
scope 4 (inlined std::ptr::Unique::<[bool; 0]>::dangling) {
let mut _5: std::ptr::NonNull<[bool; 0]>;
scope 5 (inlined NonNull::<[bool; 0]>::dangling) {
let mut _6: std::num::NonZero<usize>;
scope 6 {
scope 8 (inlined std::ptr::Alignment::as_nonzero) {
}
scope 9 (inlined NonNull::<[bool; 0]>::without_provenance) {
let _7: *const [bool; 0];
let _6: *const [bool; 0];
scope 10 {
}
scope 11 (inlined NonZero::<usize>::get) {
Expand All @@ -45,14 +44,10 @@
StorageLive(_4);
StorageLive(_5);
StorageLive(_6);
- _6 = const std::ptr::Alignment::of::<[bool; 0]>::{constant#0} as std::num::NonZero<usize> (Transmute);
+ _6 = const NonZero::<usize>(core::num::niche_types::NonZeroUsizeInner(1_usize));
StorageLive(_7);
- _7 = copy _6 as *const [bool; 0] (Transmute);
- _5 = NonNull::<[bool; 0]> { pointer: copy _7 };
+ _7 = const {0x1 as *const [bool; 0]};
- _6 = const std::ptr::Alignment::of::<[bool; 0]>::{constant#0} as *const [bool; 0] (Transmute);
- _5 = NonNull::<[bool; 0]> { pointer: copy _6 };
+ _6 = const {0x1 as *const [bool; 0]};
+ _5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};
StorageDead(_7);
StorageDead(_6);
- _4 = std::ptr::Unique::<[bool; 0]> { pointer: move _5, _marker: const PhantomData::<[bool; 0]> };
+ _4 = const std::ptr::Unique::<[bool; 0]> {{ pointer: NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }}, _marker: PhantomData::<[bool; 0]> }};
Expand Down
13 changes: 4 additions & 9 deletions tests/mir-opt/dataflow-const-prop/default_boxed_slice.main.GVN.64bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -16,12 +16,11 @@
scope 4 (inlined std::ptr::Unique::<[bool; 0]>::dangling) {
let mut _5: std::ptr::NonNull<[bool; 0]>;
scope 5 (inlined NonNull::<[bool; 0]>::dangling) {
let mut _6: std::num::NonZero<usize>;
scope 6 {
scope 8 (inlined std::ptr::Alignment::as_nonzero) {
}
scope 9 (inlined NonNull::<[bool; 0]>::without_provenance) {
let _7: *const [bool; 0];
let _6: *const [bool; 0];
scope 10 {
}
scope 11 (inlined NonZero::<usize>::get) {
Expand All @@ -45,14 +44,10 @@
StorageLive(_4);
StorageLive(_5);
StorageLive(_6);
- _6 = const std::ptr::Alignment::of::<[bool; 0]>::{constant#0} as std::num::NonZero<usize> (Transmute);
+ _6 = const NonZero::<usize>(core::num::niche_types::NonZeroUsizeInner(1_usize));
StorageLive(_7);
- _7 = copy _6 as *const [bool; 0] (Transmute);
- _5 = NonNull::<[bool; 0]> { pointer: copy _7 };
+ _7 = const {0x1 as *const [bool; 0]};
- _6 = const std::ptr::Alignment::of::<[bool; 0]>::{constant#0} as *const [bool; 0] (Transmute);
- _5 = NonNull::<[bool; 0]> { pointer: copy _6 };
+ _6 = const {0x1 as *const [bool; 0]};
+ _5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};
StorageDead(_7);
StorageDead(_6);
- _4 = std::ptr::Unique::<[bool; 0]> { pointer: move _5, _marker: const PhantomData::<[bool; 0]> };
+ _4 = const std::ptr::Unique::<[bool; 0]> {{ pointer: NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }}, _marker: PhantomData::<[bool; 0]> }};
Expand Down
13 changes: 4 additions & 9 deletions tests/mir-opt/dataflow-const-prop/default_boxed_slice.main.GVN.64bit.panic-unwind.diff
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -16,12 +16,11 @@
scope 4 (inlined std::ptr::Unique::<[bool; 0]>::dangling) {
let mut _5: std::ptr::NonNull<[bool; 0]>;
scope 5 (inlined NonNull::<[bool; 0]>::dangling) {
let mut _6: std::num::NonZero<usize>;
scope 6 {
scope 8 (inlined std::ptr::Alignment::as_nonzero) {
}
scope 9 (inlined NonNull::<[bool; 0]>::without_provenance) {
let _7: *const [bool; 0];
let _6: *const [bool; 0];
scope 10 {
}
scope 11 (inlined NonZero::<usize>::get) {
Expand All @@ -45,14 +44,10 @@
StorageLive(_4);
StorageLive(_5);
StorageLive(_6);
- _6 = const std::ptr::Alignment::of::<[bool; 0]>::{constant#0} as std::num::NonZero<usize> (Transmute);
+ _6 = const NonZero::<usize>(core::num::niche_types::NonZeroUsizeInner(1_usize));
StorageLive(_7);
- _7 = copy _6 as *const [bool; 0] (Transmute);
- _5 = NonNull::<[bool; 0]> { pointer: copy _7 };
+ _7 = const {0x1 as *const [bool; 0]};
- _6 = const std::ptr::Alignment::of::<[bool; 0]>::{constant#0} as *const [bool; 0] (Transmute);
- _5 = NonNull::<[bool; 0]> { pointer: copy _6 };
+ _6 = const {0x1 as *const [bool; 0]};
+ _5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};
StorageDead(_7);
StorageDead(_6);
- _4 = std::ptr::Unique::<[bool; 0]> { pointer: move _5, _marker: const PhantomData::<[bool; 0]> };
+ _4 = const std::ptr::Unique::<[bool; 0]> {{ pointer: NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }}, _marker: PhantomData::<[bool; 0]> }};
Expand Down
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