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Runtime checks for occupied niches #624

Closed
1 of 3 tasks
saethlin opened this issue May 1, 2023 · 3 comments
Closed
1 of 3 tasks

Runtime checks for occupied niches #624

saethlin opened this issue May 1, 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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@saethlin
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saethlin commented May 1, 2023

Proposal

When executing a typed copy where the type contains a niche, the compiler may insert a runtime check that the type's niche(s) are unoccupied. In practical terms, these runtime checks will be inserted strategically in order to minimize runtime overhead (checking that every reference is not null every time they appear on the left side of a MIR assignment statement would have ridiculous overhead).

I have been slowly working on an implementation of this at rust-lang/rust#104862, following my work on #570

Mentors or Reviewers

I do not have any in mind

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.

@saethlin saethlin 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 May 1, 2023
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rustbot commented May 1, 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 May 1, 2023
@apiraino apiraino removed the to-announce Announce this issue on triage meeting label May 25, 2023
@oli-obk
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oli-obk commented Jun 22, 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 22, 2023
@apiraino
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apiraino commented Jul 6, 2023

@rustbot label -final-comment-period +major-change-accepted

@apiraino apiraino closed this as completed Jul 6, 2023
@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 6, 2023
@apiraino apiraino removed the to-announce Announce this issue on triage meeting label Jul 6, 2023
bors added a commit to rust-lang-ci/rust that referenced this issue Oct 20, 2023
bors added a commit to rust-lang-ci/rust that referenced this issue Oct 27, 2023
Check for occupied niches

Implementation of rust-lang/compiler-team#624

Crater run has 62 crates that hit the check, 43 of those are published to crates.io. I see a lot of null function pointers and use of `mem::uninitialized` where the 0x1-filling collides with an enum niche.

But that is with full niche checks; checking transmute, plus any place where that we Copy, Move, or Inspect. Such checking is definitely too thorough to be on by default because it is 2x compile time overhead.

r? `@ghost`
bors added a commit to rust-lang-ci/rust that referenced this issue Oct 27, 2023
Check for occupied niches

Implementation of rust-lang/compiler-team#624

Crater run has 62 crates that hit the check, 43 of those are published to crates.io. I see a lot of null function pointers and use of `mem::uninitialized` where the 0x1-filling collides with an enum niche.

But that is with full niche checks; checking transmute, plus any place where that we Copy, Move, or Inspect. Such checking is definitely too thorough to be on by default because it is 2x compile time overhead.

---

During implementation, this ran into llvm/llvm-project#68381

r? `@ghost`
bors added a commit to rust-lang-ci/rust that referenced this issue Oct 28, 2023
Check for occupied niches

Implementation of rust-lang/compiler-team#624

Crater run has 62 crates that hit the check, 43 of those are published to crates.io. I see a lot of null function pointers and use of `mem::uninitialized` where the 0x1-filling collides with an enum niche.

But that is with full niche checks; checking transmute, plus any place where that we Copy, Move, or Inspect. Such checking is definitely too thorough to be on by default because it is 2x compile time overhead.

---

During implementation, this ran into llvm/llvm-project#68381

r? `@ghost`
bors added a commit to rust-lang-ci/rust that referenced this issue Oct 28, 2023
Check for occupied niches

Implementation of rust-lang/compiler-team#624

Crater run has 62 crates that hit the check, 43 of those are published to crates.io. I see a lot of null function pointers and use of `mem::uninitialized` where the 0x1-filling collides with an enum niche.

But that is with full niche checks; checking transmute, plus any place where that we Copy, Move, or Inspect. Such checking is definitely too thorough to be on by default because it is 2x compile time overhead.

---

During implementation, this ran into llvm/llvm-project#68381

r? `@ghost`
bors added a commit to rust-lang-ci/rust that referenced this issue Oct 29, 2023
Check for occupied niches

Implementation of rust-lang/compiler-team#624

Crater run has 62 crates that hit the check, 43 of those are published to crates.io. I see a lot of null function pointers and use of `mem::uninitialized` where the 0x1-filling collides with an enum niche.

But that is with full niche checks; checking transmute, plus any place where that we Copy, Move, or Inspect. Such checking is definitely too thorough to be on by default because it is 2x compile time overhead.

---

During implementation, this ran into llvm/llvm-project#68381

r? `@ghost`
bors added a commit to rust-lang-ci/rust that referenced this issue Oct 29, 2023
Check for occupied niches

Implementation of rust-lang/compiler-team#624

Crater run has 62 crates that hit the check, 43 of those are published to crates.io. I see a lot of null function pointers and use of `mem::uninitialized` where the 0x1-filling collides with an enum niche.

But that is with full niche checks; checking transmute, plus any place where that we Copy, Move, or Inspect. Such checking is definitely too thorough to be on by default because it is 2x compile time overhead.

---

During implementation, this ran into llvm/llvm-project#68381

r? `@ghost`
bors added a commit to rust-lang-ci/rust that referenced this issue Nov 10, 2023
Check for occupied niches

Implementation of rust-lang/compiler-team#624

Crater run has 62 crates that hit the check, 43 of those are published to crates.io. I see a lot of null function pointers and use of `mem::uninitialized` where the 0x1-filling collides with an enum niche.

But that is with full niche checks; checking transmute, plus any place where that we Copy, Move, or Inspect. Such checking is definitely too thorough to be on by default because it is 2x compile time overhead.

---

During implementation, this ran into llvm/llvm-project#68381

r? `@ghost`
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