[mir-opt] Allow SRoA when an object is transmuted to a field type by scottmcm · Pull Request #158291 · rust-lang/rust

scottmcm · 2026-06-23T08:12:46Z

Inspired by #158202 (comment), this allows SRoA to expand a local consumed by a CastKind::Transmute if that transmute is to the type of a field, since if it's to the type of a field we can just have it take the local for that field like we would if it was a field projection.

Future PRs could do more complex stuff, like using layout to always allow SRoA of x in Transmute(x) when x is https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/layout/type.TyAndLayout.html#method.non_1zst_field, but this is enough for the specific case I needed.

r? mir-opt

rustbot · 2026-06-23T08:12:49Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

…d, r=aapoalas Implement `ptr::{read,write}_unaligned` via `repr(packed)` We already support doing unaligned reads & writes via `repr(packed)` fields, so this just uses that support from the backend rather than needing to thinks about `memcpy` and `intrinsics::forget` and such. Turns out in codegen this ends up essentially identical because the packed type read gets `BackendRepr::Memory` and thus the read/write of the packed type *is* an `align 1` memcpy, just like the library code before this PR. But doing this gives much simpler MIR and rust-lang#158291 will make it better than the previous version by not needing to emit the `memcpy` at all for things like `read_unaligned::<u32>`. First commit are some tests that pass on master; Second commit changes the implementation and shows the corresponding test changes. r? libs

rustbot · 2026-07-02T05:30:17Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

scottmcm · 2026-07-02T05:36:03Z

- StorageLive(_4);
- _4 = copy (*_3);
- _5 = copy _4 as T (Transmute);
- StorageDead(_4);
+ _4 = copy ((*_3).0: T);


The library change to use packed landed in #158427, so now this shows the critical change from this optimization...
View changes since the review

scottmcm · 2026-07-02T05:36:52Z

- // CHECK-NOT: !noundef
- // CHECK-NOT: !range
+ // CHECK-SAME: !range [[R16:![0-9]+]]
+ // CHECK-SAME: !noundef


...which then means these loads get metadata on them without codegen changes.
View changes since the review

scottmcm · 2026-07-03T11:18:04Z

The more I poked at this the less I'm confident that SRoA is the correct answer here, especially since I tried going a bit further and ended up just getting things like this

 bb0: {
StorageLive(_3);
_3 = PtrMetadata(copy _1);
StorageLive(_5);
StorageLive(_4);
_4 = &raw const (*_1);
- _5 = copy _4 as std::ptr::NonNull<[T]> (Transmute);+ _5 = copy _4 as (*const [T]) is !null (Transmute);
StorageDead(_4);
StorageLive(_7);
StorageLive(_6);
_6 = copy _5 as *mut [T] (Transmute);
_7 = copy _6 as *mut T (PtrToPtr);
StorageDead(_6);
- _8 = copy _7 as std::ptr::NonNull<T> (Transmute);+ _8 = copy _7 as (*const T) is !null (Transmute);
StorageDead(_7);
StorageDead(_5);
switchInt(const <T as std::mem::SizedTypeProperties>::IS_ZST) -> [0: bb1, otherwise: bb2];
}

which is kinda the opposite of the direction in #153085 which I think would be nicer to see when reading the MIR.

scottmcm · 2026-07-09T18:42:13Z

Closing this in favour of fixing the motivating example in layout instead, #159033

scottmcm · 2026-06-23T08:12:46Z

Inspired by #158202 (comment), this allows SRoA to expand a local consumed by a CastKind::Transmute if that transmute is to the type of a field, since if it's to the type of a field we can just have it take the local for that field like we would if it was a field projection.

Future PRs could do more complex stuff, like using layout to always allow SRoA of x in Transmute(x) when x is https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/layout/type.TyAndLayout.html#method.non_1zst_field, but this is enough for the specific case I needed.

r? mir-opt

rustbot · 2026-06-23T08:12:49Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

…d, r=aapoalas Implement `ptr::{read,write}_unaligned` via `repr(packed)` We already support doing unaligned reads & writes via `repr(packed)` fields, so this just uses that support from the backend rather than needing to thinks about `memcpy` and `intrinsics::forget` and such. Turns out in codegen this ends up essentially identical because the packed type read gets `BackendRepr::Memory` and thus the read/write of the packed type *is* an `align 1` memcpy, just like the library code before this PR. But doing this gives much simpler MIR and rust-lang#158291 will make it better than the previous version by not needing to emit the `memcpy` at all for things like `read_unaligned::<u32>`. First commit are some tests that pass on master; Second commit changes the implementation and shows the corresponding test changes. r? libs

rustbot · 2026-07-02T05:30:17Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

scottmcm · 2026-07-02T05:36:03Z

- StorageLive(_4);
- _4 = copy (*_3);
- _5 = copy _4 as T (Transmute);
- StorageDead(_4);
+ _4 = copy ((*_3).0: T);


The library change to use packed landed in #158427, so now this shows the critical change from this optimization...
View changes since the review

scottmcm · 2026-07-02T05:36:52Z

- // CHECK-NOT: !noundef
- // CHECK-NOT: !range
+ // CHECK-SAME: !range [[R16:![0-9]+]]
+ // CHECK-SAME: !noundef


...which then means these loads get metadata on them without codegen changes.
View changes since the review

scottmcm · 2026-07-03T11:18:04Z

The more I poked at this the less I'm confident that SRoA is the correct answer here, especially since I tried going a bit further and ended up just getting things like this

 bb0: {
StorageLive(_3);
_3 = PtrMetadata(copy _1);
StorageLive(_5);
StorageLive(_4);
_4 = &raw const (*_1);
- _5 = copy _4 as std::ptr::NonNull<[T]> (Transmute);+ _5 = copy _4 as (*const [T]) is !null (Transmute);
StorageDead(_4);
StorageLive(_7);
StorageLive(_6);
_6 = copy _5 as *mut [T] (Transmute);
_7 = copy _6 as *mut T (PtrToPtr);
StorageDead(_6);
- _8 = copy _7 as std::ptr::NonNull<T> (Transmute);+ _8 = copy _7 as (*const T) is !null (Transmute);
StorageDead(_7);
StorageDead(_5);
switchInt(const <T as std::mem::SizedTypeProperties>::IS_ZST) -> [0: bb1, otherwise: bb2];
}

which is kinda the opposite of the direction in #153085 which I think would be nicer to see when reading the MIR.

scottmcm · 2026-07-09T18:42:13Z

Closing this in favour of fixing the motivating example in layout instead, #159033

scottmcm · 2026-06-23T08:12:46Z

Inspired by #158202 (comment), this allows SRoA to expand a local consumed by a CastKind::Transmute if that transmute is to the type of a field, since if it's to the type of a field we can just have it take the local for that field like we would if it was a field projection.

Future PRs could do more complex stuff, like using layout to always allow SRoA of x in Transmute(x) when x is https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/layout/type.TyAndLayout.html#method.non_1zst_field, but this is enough for the specific case I needed.

r? mir-opt

rustbot · 2026-06-23T08:12:49Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

…d, r=aapoalas Implement `ptr::{read,write}_unaligned` via `repr(packed)` We already support doing unaligned reads & writes via `repr(packed)` fields, so this just uses that support from the backend rather than needing to thinks about `memcpy` and `intrinsics::forget` and such. Turns out in codegen this ends up essentially identical because the packed type read gets `BackendRepr::Memory` and thus the read/write of the packed type *is* an `align 1` memcpy, just like the library code before this PR. But doing this gives much simpler MIR and rust-lang#158291 will make it better than the previous version by not needing to emit the `memcpy` at all for things like `read_unaligned::<u32>`. First commit are some tests that pass on master; Second commit changes the implementation and shows the corresponding test changes. r? libs

rustbot · 2026-07-02T05:30:17Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

scottmcm · 2026-07-02T05:36:03Z

- StorageLive(_4);
- _4 = copy (*_3);
- _5 = copy _4 as T (Transmute);
- StorageDead(_4);
+ _4 = copy ((*_3).0: T);


The library change to use packed landed in #158427, so now this shows the critical change from this optimization...
View changes since the review

scottmcm · 2026-07-02T05:36:52Z

- // CHECK-NOT: !noundef
- // CHECK-NOT: !range
+ // CHECK-SAME: !range [[R16:![0-9]+]]
+ // CHECK-SAME: !noundef


...which then means these loads get metadata on them without codegen changes.
View changes since the review

scottmcm · 2026-07-03T11:18:04Z

The more I poked at this the less I'm confident that SRoA is the correct answer here, especially since I tried going a bit further and ended up just getting things like this

 bb0: {
StorageLive(_3);
_3 = PtrMetadata(copy _1);
StorageLive(_5);
StorageLive(_4);
_4 = &raw const (*_1);
- _5 = copy _4 as std::ptr::NonNull<[T]> (Transmute);+ _5 = copy _4 as (*const [T]) is !null (Transmute);
StorageDead(_4);
StorageLive(_7);
StorageLive(_6);
_6 = copy _5 as *mut [T] (Transmute);
_7 = copy _6 as *mut T (PtrToPtr);
StorageDead(_6);
- _8 = copy _7 as std::ptr::NonNull<T> (Transmute);+ _8 = copy _7 as (*const T) is !null (Transmute);
StorageDead(_7);
StorageDead(_5);
switchInt(const <T as std::mem::SizedTypeProperties>::IS_ZST) -> [0: bb1, otherwise: bb2];
}

which is kinda the opposite of the direction in #153085 which I think would be nicer to see when reading the MIR.

scottmcm · 2026-07-09T18:42:13Z

Closing this in favour of fixing the motivating example in layout instead, #159033

scottmcm · 2026-06-23T08:12:46Z

Inspired by #158202 (comment), this allows SRoA to expand a local consumed by a CastKind::Transmute if that transmute is to the type of a field, since if it's to the type of a field we can just have it take the local for that field like we would if it was a field projection.

Future PRs could do more complex stuff, like using layout to always allow SRoA of x in Transmute(x) when x is https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/layout/type.TyAndLayout.html#method.non_1zst_field, but this is enough for the specific case I needed.

r? mir-opt

rustbot · 2026-06-23T08:12:49Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

…d, r=aapoalas Implement `ptr::{read,write}_unaligned` via `repr(packed)` We already support doing unaligned reads & writes via `repr(packed)` fields, so this just uses that support from the backend rather than needing to thinks about `memcpy` and `intrinsics::forget` and such. Turns out in codegen this ends up essentially identical because the packed type read gets `BackendRepr::Memory` and thus the read/write of the packed type *is* an `align 1` memcpy, just like the library code before this PR. But doing this gives much simpler MIR and rust-lang#158291 will make it better than the previous version by not needing to emit the `memcpy` at all for things like `read_unaligned::<u32>`. First commit are some tests that pass on master; Second commit changes the implementation and shows the corresponding test changes. r? libs

rustbot · 2026-07-02T05:30:17Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

scottmcm · 2026-07-02T05:36:03Z

- StorageLive(_4);
- _4 = copy (*_3);
- _5 = copy _4 as T (Transmute);
- StorageDead(_4);
+ _4 = copy ((*_3).0: T);


The library change to use packed landed in #158427, so now this shows the critical change from this optimization...
View changes since the review

scottmcm · 2026-07-02T05:36:52Z

- // CHECK-NOT: !noundef
- // CHECK-NOT: !range
+ // CHECK-SAME: !range [[R16:![0-9]+]]
+ // CHECK-SAME: !noundef


...which then means these loads get metadata on them without codegen changes.
View changes since the review

scottmcm · 2026-07-03T11:18:04Z

The more I poked at this the less I'm confident that SRoA is the correct answer here, especially since I tried going a bit further and ended up just getting things like this

 bb0: {
StorageLive(_3);
_3 = PtrMetadata(copy _1);
StorageLive(_5);
StorageLive(_4);
_4 = &raw const (*_1);
- _5 = copy _4 as std::ptr::NonNull<[T]> (Transmute);+ _5 = copy _4 as (*const [T]) is !null (Transmute);
StorageDead(_4);
StorageLive(_7);
StorageLive(_6);
_6 = copy _5 as *mut [T] (Transmute);
_7 = copy _6 as *mut T (PtrToPtr);
StorageDead(_6);
- _8 = copy _7 as std::ptr::NonNull<T> (Transmute);+ _8 = copy _7 as (*const T) is !null (Transmute);
StorageDead(_7);
StorageDead(_5);
switchInt(const <T as std::mem::SizedTypeProperties>::IS_ZST) -> [0: bb1, otherwise: bb2];
}

which is kinda the opposite of the direction in #153085 which I think would be nicer to see when reading the MIR.

scottmcm · 2026-07-09T18:42:13Z

Closing this in favour of fixing the motivating example in layout instead, #159033

scottmcm · 2026-06-23T08:12:46Z

Inspired by #158202 (comment), this allows SRoA to expand a local consumed by a CastKind::Transmute if that transmute is to the type of a field, since if it's to the type of a field we can just have it take the local for that field like we would if it was a field projection.

Future PRs could do more complex stuff, like using layout to always allow SRoA of x in Transmute(x) when x is https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/layout/type.TyAndLayout.html#method.non_1zst_field, but this is enough for the specific case I needed.

r? mir-opt

rustbot · 2026-06-23T08:12:49Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

…d, r=aapoalas Implement `ptr::{read,write}_unaligned` via `repr(packed)` We already support doing unaligned reads & writes via `repr(packed)` fields, so this just uses that support from the backend rather than needing to thinks about `memcpy` and `intrinsics::forget` and such. Turns out in codegen this ends up essentially identical because the packed type read gets `BackendRepr::Memory` and thus the read/write of the packed type *is* an `align 1` memcpy, just like the library code before this PR. But doing this gives much simpler MIR and rust-lang#158291 will make it better than the previous version by not needing to emit the `memcpy` at all for things like `read_unaligned::<u32>`. First commit are some tests that pass on master; Second commit changes the implementation and shows the corresponding test changes. r? libs

rustbot · 2026-07-02T05:30:17Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

scottmcm · 2026-07-02T05:36:03Z

- StorageLive(_4);
- _4 = copy (*_3);
- _5 = copy _4 as T (Transmute);
- StorageDead(_4);
+ _4 = copy ((*_3).0: T);


The library change to use packed landed in #158427, so now this shows the critical change from this optimization...
View changes since the review

scottmcm · 2026-07-02T05:36:52Z

- // CHECK-NOT: !noundef
- // CHECK-NOT: !range
+ // CHECK-SAME: !range [[R16:![0-9]+]]
+ // CHECK-SAME: !noundef


...which then means these loads get metadata on them without codegen changes.
View changes since the review

scottmcm · 2026-07-03T11:18:04Z

The more I poked at this the less I'm confident that SRoA is the correct answer here, especially since I tried going a bit further and ended up just getting things like this

 bb0: {
StorageLive(_3);
_3 = PtrMetadata(copy _1);
StorageLive(_5);
StorageLive(_4);
_4 = &raw const (*_1);
- _5 = copy _4 as std::ptr::NonNull<[T]> (Transmute);+ _5 = copy _4 as (*const [T]) is !null (Transmute);
StorageDead(_4);
StorageLive(_7);
StorageLive(_6);
_6 = copy _5 as *mut [T] (Transmute);
_7 = copy _6 as *mut T (PtrToPtr);
StorageDead(_6);
- _8 = copy _7 as std::ptr::NonNull<T> (Transmute);+ _8 = copy _7 as (*const T) is !null (Transmute);
StorageDead(_7);
StorageDead(_5);
switchInt(const <T as std::mem::SizedTypeProperties>::IS_ZST) -> [0: bb1, otherwise: bb2];
}

which is kinda the opposite of the direction in #153085 which I think would be nicer to see when reading the MIR.

scottmcm · 2026-07-09T18:42:13Z

Closing this in favour of fixing the motivating example in layout instead, #159033

scottmcm · 2026-06-23T08:12:46Z

Inspired by #158202 (comment), this allows SRoA to expand a local consumed by a CastKind::Transmute if that transmute is to the type of a field, since if it's to the type of a field we can just have it take the local for that field like we would if it was a field projection.

Future PRs could do more complex stuff, like using layout to always allow SRoA of x in Transmute(x) when x is https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/layout/type.TyAndLayout.html#method.non_1zst_field, but this is enough for the specific case I needed.

r? mir-opt

rustbot · 2026-06-23T08:12:49Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

…d, r=aapoalas Implement `ptr::{read,write}_unaligned` via `repr(packed)` We already support doing unaligned reads & writes via `repr(packed)` fields, so this just uses that support from the backend rather than needing to thinks about `memcpy` and `intrinsics::forget` and such. Turns out in codegen this ends up essentially identical because the packed type read gets `BackendRepr::Memory` and thus the read/write of the packed type *is* an `align 1` memcpy, just like the library code before this PR. But doing this gives much simpler MIR and rust-lang#158291 will make it better than the previous version by not needing to emit the `memcpy` at all for things like `read_unaligned::<u32>`. First commit are some tests that pass on master; Second commit changes the implementation and shows the corresponding test changes. r? libs

rustbot · 2026-07-02T05:30:17Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

scottmcm · 2026-07-02T05:36:03Z

- StorageLive(_4);
- _4 = copy (*_3);
- _5 = copy _4 as T (Transmute);
- StorageDead(_4);
+ _4 = copy ((*_3).0: T);


The library change to use packed landed in #158427, so now this shows the critical change from this optimization...
View changes since the review

scottmcm · 2026-07-02T05:36:52Z

- // CHECK-NOT: !noundef
- // CHECK-NOT: !range
+ // CHECK-SAME: !range [[R16:![0-9]+]]
+ // CHECK-SAME: !noundef


...which then means these loads get metadata on them without codegen changes.
View changes since the review

scottmcm · 2026-07-03T11:18:04Z

The more I poked at this the less I'm confident that SRoA is the correct answer here, especially since I tried going a bit further and ended up just getting things like this

 bb0: {
StorageLive(_3);
_3 = PtrMetadata(copy _1);
StorageLive(_5);
StorageLive(_4);
_4 = &raw const (*_1);
- _5 = copy _4 as std::ptr::NonNull<[T]> (Transmute);+ _5 = copy _4 as (*const [T]) is !null (Transmute);
StorageDead(_4);
StorageLive(_7);
StorageLive(_6);
_6 = copy _5 as *mut [T] (Transmute);
_7 = copy _6 as *mut T (PtrToPtr);
StorageDead(_6);
- _8 = copy _7 as std::ptr::NonNull<T> (Transmute);+ _8 = copy _7 as (*const T) is !null (Transmute);
StorageDead(_7);
StorageDead(_5);
switchInt(const <T as std::mem::SizedTypeProperties>::IS_ZST) -> [0: bb1, otherwise: bb2];
}

which is kinda the opposite of the direction in #153085 which I think would be nicer to see when reading the MIR.

scottmcm · 2026-07-09T18:42:13Z

Closing this in favour of fixing the motivating example in layout instead, #159033

scottmcm · 2026-06-23T08:12:46Z

Inspired by #158202 (comment), this allows SRoA to expand a local consumed by a CastKind::Transmute if that transmute is to the type of a field, since if it's to the type of a field we can just have it take the local for that field like we would if it was a field projection.

Future PRs could do more complex stuff, like using layout to always allow SRoA of x in Transmute(x) when x is https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/layout/type.TyAndLayout.html#method.non_1zst_field, but this is enough for the specific case I needed.

r? mir-opt

rustbot · 2026-06-23T08:12:49Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

…d, r=aapoalas Implement `ptr::{read,write}_unaligned` via `repr(packed)` We already support doing unaligned reads & writes via `repr(packed)` fields, so this just uses that support from the backend rather than needing to thinks about `memcpy` and `intrinsics::forget` and such. Turns out in codegen this ends up essentially identical because the packed type read gets `BackendRepr::Memory` and thus the read/write of the packed type *is* an `align 1` memcpy, just like the library code before this PR. But doing this gives much simpler MIR and rust-lang#158291 will make it better than the previous version by not needing to emit the `memcpy` at all for things like `read_unaligned::<u32>`. First commit are some tests that pass on master; Second commit changes the implementation and shows the corresponding test changes. r? libs

rustbot · 2026-07-02T05:30:17Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

scottmcm · 2026-07-02T05:36:03Z

- StorageLive(_4);
- _4 = copy (*_3);
- _5 = copy _4 as T (Transmute);
- StorageDead(_4);
+ _4 = copy ((*_3).0: T);


The library change to use packed landed in #158427, so now this shows the critical change from this optimization...
View changes since the review

scottmcm · 2026-07-02T05:36:52Z

- // CHECK-NOT: !noundef
- // CHECK-NOT: !range
+ // CHECK-SAME: !range [[R16:![0-9]+]]
+ // CHECK-SAME: !noundef


...which then means these loads get metadata on them without codegen changes.
View changes since the review

scottmcm · 2026-07-03T11:18:04Z

The more I poked at this the less I'm confident that SRoA is the correct answer here, especially since I tried going a bit further and ended up just getting things like this

 bb0: {
StorageLive(_3);
_3 = PtrMetadata(copy _1);
StorageLive(_5);
StorageLive(_4);
_4 = &raw const (*_1);
- _5 = copy _4 as std::ptr::NonNull<[T]> (Transmute);+ _5 = copy _4 as (*const [T]) is !null (Transmute);
StorageDead(_4);
StorageLive(_7);
StorageLive(_6);
_6 = copy _5 as *mut [T] (Transmute);
_7 = copy _6 as *mut T (PtrToPtr);
StorageDead(_6);
- _8 = copy _7 as std::ptr::NonNull<T> (Transmute);+ _8 = copy _7 as (*const T) is !null (Transmute);
StorageDead(_7);
StorageDead(_5);
switchInt(const <T as std::mem::SizedTypeProperties>::IS_ZST) -> [0: bb1, otherwise: bb2];
}

which is kinda the opposite of the direction in #153085 which I think would be nicer to see when reading the MIR.

scottmcm · 2026-07-09T18:42:13Z

Closing this in favour of fixing the motivating example in layout instead, #159033

scottmcm · 2026-06-23T08:12:46Z

Inspired by #158202 (comment), this allows SRoA to expand a local consumed by a CastKind::Transmute if that transmute is to the type of a field, since if it's to the type of a field we can just have it take the local for that field like we would if it was a field projection.

Future PRs could do more complex stuff, like using layout to always allow SRoA of x in Transmute(x) when x is https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/layout/type.TyAndLayout.html#method.non_1zst_field, but this is enough for the specific case I needed.

r? mir-opt

rustbot · 2026-06-23T08:12:49Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

…d, r=aapoalas Implement `ptr::{read,write}_unaligned` via `repr(packed)` We already support doing unaligned reads & writes via `repr(packed)` fields, so this just uses that support from the backend rather than needing to thinks about `memcpy` and `intrinsics::forget` and such. Turns out in codegen this ends up essentially identical because the packed type read gets `BackendRepr::Memory` and thus the read/write of the packed type *is* an `align 1` memcpy, just like the library code before this PR. But doing this gives much simpler MIR and rust-lang#158291 will make it better than the previous version by not needing to emit the `memcpy` at all for things like `read_unaligned::<u32>`. First commit are some tests that pass on master; Second commit changes the implementation and shows the corresponding test changes. r? libs

rustbot · 2026-07-02T05:30:17Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

scottmcm · 2026-07-02T05:36:03Z

- StorageLive(_4);
- _4 = copy (*_3);
- _5 = copy _4 as T (Transmute);
- StorageDead(_4);
+ _4 = copy ((*_3).0: T);


The library change to use packed landed in #158427, so now this shows the critical change from this optimization...
View changes since the review

scottmcm · 2026-07-02T05:36:52Z

- // CHECK-NOT: !noundef
- // CHECK-NOT: !range
+ // CHECK-SAME: !range [[R16:![0-9]+]]
+ // CHECK-SAME: !noundef


...which then means these loads get metadata on them without codegen changes.
View changes since the review

scottmcm · 2026-07-03T11:18:04Z

The more I poked at this the less I'm confident that SRoA is the correct answer here, especially since I tried going a bit further and ended up just getting things like this

 bb0: {
StorageLive(_3);
_3 = PtrMetadata(copy _1);
StorageLive(_5);
StorageLive(_4);
_4 = &raw const (*_1);
- _5 = copy _4 as std::ptr::NonNull<[T]> (Transmute);+ _5 = copy _4 as (*const [T]) is !null (Transmute);
StorageDead(_4);
StorageLive(_7);
StorageLive(_6);
_6 = copy _5 as *mut [T] (Transmute);
_7 = copy _6 as *mut T (PtrToPtr);
StorageDead(_6);
- _8 = copy _7 as std::ptr::NonNull<T> (Transmute);+ _8 = copy _7 as (*const T) is !null (Transmute);
StorageDead(_7);
StorageDead(_5);
switchInt(const <T as std::mem::SizedTypeProperties>::IS_ZST) -> [0: bb1, otherwise: bb2];
}

which is kinda the opposite of the direction in #153085 which I think would be nicer to see when reading the MIR.

scottmcm · 2026-07-09T18:42:13Z

Closing this in favour of fixing the motivating example in layout instead, #159033

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 23, 2026

rustbot assigned saethlin Jun 23, 2026

scottmcm added the A-mir-opt Area: MIR optimizations label Jun 24, 2026

This was referenced Jun 24, 2026
[only tests] Add tests for current read/write unaligned behaviour #158341
Closed
Preserve metadata like !range and !nonnull in unaligned operations #158202
Closed
Implement ptr::{read,write}_unaligned via repr(packed)#158427
Merged

scottmcm force-pushed the even-better-read-unaligned branch from 7ed426a to fd33c1fCompare July 2, 2026 05:30

[mir-opt] Allow SRoA when an object is transmuted to a field type
a576ef9

scottmcm force-pushed the even-better-read-unaligned branch from fd33c1f to a576ef9Compare July 2, 2026 05:35

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rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 23, 2026

rustbot assigned saethlin Jun 23, 2026

scottmcm added the A-mir-opt Area: MIR optimizations label Jun 24, 2026

This was referenced Jun 24, 2026
[only tests] Add tests for current read/write unaligned behaviour #158341
Closed
Preserve metadata like !range and !nonnull in unaligned operations #158202
Closed
Implement ptr::{read,write}_unaligned via repr(packed)#158427
Merged

scottmcm force-pushed the even-better-read-unaligned branch from 7ed426a to fd33c1fCompare July 2, 2026 05:30

[mir-opt] Allow SRoA when an object is transmuted to a field type
a576ef9

scottmcm force-pushed the even-better-read-unaligned branch from fd33c1f to a576ef9Compare July 2, 2026 05:35

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scottmcm added the A-mir-opt Area: MIR optimizations label Jun 24, 2026

This was referenced Jun 24, 2026
[only tests] Add tests for current read/write unaligned behaviour #158341
Closed
Preserve metadata like !range and !nonnull in unaligned operations #158202
Closed
Implement ptr::{read,write}_unaligned via repr(packed)#158427
Merged

scottmcm force-pushed the even-better-read-unaligned branch from 7ed426a to fd33c1fCompare July 2, 2026 05:30

[mir-opt] Allow SRoA when an object is transmuted to a field type
a576ef9

scottmcm force-pushed the even-better-read-unaligned branch from fd33c1f to a576ef9Compare July 2, 2026 05:35

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scottmcm added the A-mir-opt Area: MIR optimizations label Jun 24, 2026

This was referenced Jun 24, 2026
[only tests] Add tests for current read/write unaligned behaviour #158341
Closed
Preserve metadata like !range and !nonnull in unaligned operations #158202
Closed
Implement ptr::{read,write}_unaligned via repr(packed)#158427
Merged

scottmcm force-pushed the even-better-read-unaligned branch from 7ed426a to fd33c1fCompare July 2, 2026 05:30

[mir-opt] Allow SRoA when an object is transmuted to a field type
a576ef9

scottmcm force-pushed the even-better-read-unaligned branch from fd33c1f to a576ef9Compare July 2, 2026 05:35

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scottmcm marked this pull request as draft July 3, 2026 11:18

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rustbot assigned saethlin Jun 23, 2026

scottmcm added the A-mir-opt Area: MIR optimizations label Jun 24, 2026

This was referenced Jun 24, 2026
[only tests] Add tests for current read/write unaligned behaviour #158341
Closed
Preserve metadata like !range and !nonnull in unaligned operations #158202
Closed
Implement ptr::{read,write}_unaligned via repr(packed)#158427
Merged

scottmcm force-pushed the even-better-read-unaligned branch from 7ed426a to fd33c1fCompare July 2, 2026 05:30

[mir-opt] Allow SRoA when an object is transmuted to a field type
a576ef9

scottmcm force-pushed the even-better-read-unaligned branch from fd33c1f to a576ef9Compare July 2, 2026 05:35

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scottmcm marked this pull request as draft July 3, 2026 11:18

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scottmcm added the A-mir-opt Area: MIR optimizations label Jun 24, 2026

This was referenced Jun 24, 2026
[only tests] Add tests for current read/write unaligned behaviour #158341
Closed
Preserve metadata like !range and !nonnull in unaligned operations #158202
Closed
Implement ptr::{read,write}_unaligned via repr(packed)#158427
Merged

scottmcm force-pushed the even-better-read-unaligned branch from 7ed426a to fd33c1fCompare July 2, 2026 05:30

[mir-opt] Allow SRoA when an object is transmuted to a field type
a576ef9

scottmcm force-pushed the even-better-read-unaligned branch from fd33c1f to a576ef9Compare July 2, 2026 05:35

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 3, 2026

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rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 23, 2026

rustbot assigned saethlin Jun 23, 2026

scottmcm added the A-mir-opt Area: MIR optimizations label Jun 24, 2026

This was referenced Jun 24, 2026
[only tests] Add tests for current read/write unaligned behaviour #158341
Closed
Preserve metadata like !range and !nonnull in unaligned operations #158202
Closed
Implement ptr::{read,write}_unaligned via repr(packed)#158427
Merged

scottmcm force-pushed the even-better-read-unaligned branch from 7ed426a to fd33c1fCompare July 2, 2026 05:30

[mir-opt] Allow SRoA when an object is transmuted to a field type
a576ef9

scottmcm force-pushed the even-better-read-unaligned branch from fd33c1f to a576ef9Compare July 2, 2026 05:35

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scottmcm marked this pull request as draft July 3, 2026 11:18

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 3, 2026

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rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 23, 2026

rustbot assigned saethlin Jun 23, 2026

scottmcm added the A-mir-opt Area: MIR optimizations label Jun 24, 2026

This was referenced Jun 24, 2026
[only tests] Add tests for current read/write unaligned behaviour #158341
Closed
Preserve metadata like !range and !nonnull in unaligned operations #158202
Closed
Implement ptr::{read,write}_unaligned via repr(packed)#158427
Merged

scottmcm force-pushed the even-better-read-unaligned branch from 7ed426a to fd33c1fCompare July 2, 2026 05:30

[mir-opt] Allow SRoA when an object is transmuted to a field type
a576ef9

scottmcm force-pushed the even-better-read-unaligned branch from fd33c1f to a576ef9Compare July 2, 2026 05:35

scottmcm commented Jul 2, 2026
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scottmcm marked this pull request as draft July 3, 2026 11:18

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