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Implement alignment and size niches for references #641

Closed
1 of 3 tasks
moulins opened this issue Jun 22, 2023 · 3 comments
Closed
1 of 3 tasks

Implement alignment and size niches for references #641

moulins opened this issue Jun 22, 2023 · 3 comments
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major-change A proposal to make a major change to rustc major-change-accepted A major change proposal that was accepted T-compiler Add this label so rfcbot knows to poll the compiler team

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@moulins
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moulins commented Jun 22, 2023

Proposal

Implement compiler support for alignment and size niches, as outlined in RFC 3204. This does not propose adding the RFC's WellFormed<T> type to std; only &T and &mut T are eligible for now.

The rough plan is:

  • Add a new naive_layout_of query (this was named min_layout_of in the RFC) that can be used to (under)estimate the size/align of a type behind an indirection without causing unwanted query cycles;
  • Fix the various places in the compiler assuming references and pointers only ever have null niches:
    • the niching logic in rustc_abi (to maintain the null-pointer optimization guarantee in the presence of non-null niches);
    • scalar checks in CTFE;
    • some checks in the improper_ctypes lint;
    • SizeSkeleton logic for transmute checks;
    • Ty::ty_and_layout_pointee_info_at;
    • probably others I missed...
  • Enable niches for references (maybe behind some unstable -Z flag), using the size and alignment estimations provided by naive_layout_of.

Mentors or Reviewers

I already have a draft implementation of the naive_layout_of query (which I'll make into a PR once it's cleaned up), and got some good advice from @scottmcm (and others) on the Rust Discord server; but this is my first contribution to the rustc codebase and mentoring help for the trickier changes would be greatly appreciated.

Process

The main points of the Major Change Process are as follows:

  • File an issue describing the proposal.
  • A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
    • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
    • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
  • Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.
@moulins moulins added major-change A proposal to make a major change to rustc T-compiler Add this label so rfcbot knows to poll the compiler team labels Jun 22, 2023
@rustbot
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rustbot commented Jun 22, 2023

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors

@rustbot rustbot added the to-announce Announce this issue on triage meeting label Jun 22, 2023
@oli-obk
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oli-obk commented Jun 23, 2023

@rustbot second

@rustbot rustbot added the final-comment-period The FCP has started, most (if not all) team members are in agreement label Jun 23, 2023
@apiraino apiraino removed the to-announce Announce this issue on triage meeting label Jun 23, 2023
@moulins moulins mentioned this issue Jun 29, 2023
Merged
bors added a commit to rust-lang-ci/rust that referenced this issue Jul 21, 2023
@bors
Auto merge of rust-lang#113166 - moulins:ref-niches-initial, r=oli-obk
557359f 
Prototype: Add unstable `-Z reference-niches` option

MCP: rust-lang/compiler-team#641
Relevant RFC: rust-lang/rfcs#3204

This prototype adds a new `-Z reference-niches` option, controlling the range of valid bit-patterns for reference types (`&T` and `&mut T`), thereby enabling new enum niching opportunities. Like `-Z randomize-layout`, this setting is crate-local; as such, references to built-in types (primitives, tuples, ...) are not affected.

The possible settings are (here, `MAX` denotes the all-1 bit-pattern):
| `-Z reference-niches=` | Valid range |
|:---:|:---:|
| `null` (the default) | `1..=MAX` |
| `size` | `1..=(MAX- size)` |
| `align` | `align..=MAX.align_down_to(align)` |
| `size,align` | `align..=(MAX-size).align_down_to(align)` |

------

This is very WIP, and I'm not sure the approach I've taken here is the best one, but stage 1 tests pass locally; I believe this is in a good enough state to unleash this upon unsuspecting 3rd-party code, and see what breaks.
github-actions bot pushed a commit to rust-lang/miri that referenced this issue Jul 22, 2023
@bors
Auto merge of #113166 - moulins:ref-niches-initial, r=oli-obk
be49206 
Prototype: Add unstable `-Z reference-niches` option

MCP: rust-lang/compiler-team#641
Relevant RFC: rust-lang/rfcs#3204

This prototype adds a new `-Z reference-niches` option, controlling the range of valid bit-patterns for reference types (`&T` and `&mut T`), thereby enabling new enum niching opportunities. Like `-Z randomize-layout`, this setting is crate-local; as such, references to built-in types (primitives, tuples, ...) are not affected.

The possible settings are (here, `MAX` denotes the all-1 bit-pattern):
| `-Z reference-niches=` | Valid range |
|:---:|:---:|
| `null` (the default) | `1..=MAX` |
| `size` | `1..=(MAX- size)` |
| `align` | `align..=MAX.align_down_to(align)` |
| `size,align` | `align..=(MAX-size).align_down_to(align)` |

------

This is very WIP, and I'm not sure the approach I've taken here is the best one, but stage 1 tests pass locally; I believe this is in a good enough state to unleash this upon unsuspecting 3rd-party code, and see what breaks.
@apiraino
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@rustbot label -final-comment-period +major-change-accepted

@rustbot rustbot added major-change-accepted A major change proposal that was accepted to-announce Announce this issue on triage meeting and removed final-comment-period The FCP has started, most (if not all) team members are in agreement labels Jul 26, 2023
@apiraino apiraino removed the to-announce Announce this issue on triage meeting label Aug 4, 2023
antoyo pushed a commit to rust-lang/rustc_codegen_gcc that referenced this issue Aug 13, 2023
@bors
Auto merge of #113166 - moulins:ref-niches-initial, r=oli-obk
e865542 
Prototype: Add unstable `-Z reference-niches` option

MCP: rust-lang/compiler-team#641
Relevant RFC: rust-lang/rfcs#3204

This prototype adds a new `-Z reference-niches` option, controlling the range of valid bit-patterns for reference types (`&T` and `&mut T`), thereby enabling new enum niching opportunities. Like `-Z randomize-layout`, this setting is crate-local; as such, references to built-in types (primitives, tuples, ...) are not affected.

The possible settings are (here, `MAX` denotes the all-1 bit-pattern):
| `-Z reference-niches=` | Valid range |
|:---:|:---:|
| `null` (the default) | `1..=MAX` |
| `size` | `1..=(MAX- size)` |
| `align` | `align..=MAX.align_down_to(align)` |
| `size,align` | `align..=(MAX-size).align_down_to(align)` |

------

This is very WIP, and I'm not sure the approach I've taken here is the best one, but stage 1 tests pass locally; I believe this is in a good enough state to unleash this upon unsuspecting 3rd-party code, and see what breaks.
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