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ConstProp miscompilation around references #73609

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RalfJung opened this issue Jun 22, 2020 · 13 comments · Fixed by #73613
Closed

ConstProp miscompilation around references #73609

RalfJung opened this issue Jun 22, 2020 · 13 comments · Fixed by #73613
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A-mir-opt Area: MIR optimizations C-bug Category: This is a bug. I-unsound Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness P-critical Critical priority regression-from-stable-to-nightly Performance or correctness regression from stable to nightly. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@RalfJung
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RalfJung commented Jun 22, 2020

#71911 introduced a (nightly-to-nightly) regression in mir-opt-level=3: since that landed, this assertion fails when running the test in Miri.

@jonas-schievink jonas-schievink added A-const-eval Area: Constant evaluation (MIR interpretation) A-mir-opt Area: MIR optimizations C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. requires-nightly This issue requires a nightly compiler in some way. I-unsound Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness labels Jun 22, 2020
@RalfJung

This comment has been minimized.

@RalfJung RalfJung changed the title Miscompilation with mir-opt-level=3 Miscompilation with mir-opt-level=2 Jun 22, 2020
@RalfJung
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In fact even level 2 is broken.

@RalfJung
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... and level 1, for the union issue (so let's treat the other one separately).

@RalfJung RalfJung added regression-from-stable-to-nightly Performance or correctness regression from stable to nightly. and removed requires-nightly This issue requires a nightly compiler in some way. labels Jun 22, 2020
@rustbot rustbot added the I-prioritize Issue: Indicates that prioritization has been requested for this issue. label Jun 22, 2020
@RalfJung
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RalfJung commented Jun 22, 2020

This doesn't require nightly after all, we have a stable-to-nightly regression when just running this code:

fn main() {
    #[allow(dead_code)]
    union U {
        f1: u32,
        f2: f32,
    }
    let mut u = U { f1: 1 };
    unsafe {
        let b1 = &mut u.f1;
        *b1 = 5;
    }
    assert_eq!(unsafe { u.f1 }, 5);
}

gives

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `5`', src/main.rs:12:5

@RalfJung RalfJung changed the title Miscompilation with mir-opt-level=2 ConstProp miscompilation around unions Jun 22, 2020
@RalfJung
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In fact we don't even need a union, nor an unsafe block:

fn main() {
    #[allow(dead_code)]
    struct U {
        f1: u32,
        f2: f32,
    }
    let mut u = U { f1: 1, f2: 1.0 };
    let b1 = &mut u.f1;
    *b1 = 5;
    assert_eq!( { u.f1 }, 5);
}

@RalfJung RalfJung changed the title ConstProp miscompilation around unions ConstProp miscompilation around references Jun 22, 2020
@RalfJung RalfJung removed the A-const-eval Area: Constant evaluation (MIR interpretation) label Jun 22, 2020
@jonas-schievink
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Minimized:

fn main() {
    let mut u = (1,);
    *&mut u.0 = 5;
    assert_eq!( { u.0 }, 5);
}

@oli-obk oli-obk self-assigned this Jun 22, 2020
@oli-obk
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oli-obk commented Jun 22, 2020

on it

@oli-obk oli-obk added regression-from-stable-to-beta Performance or correctness regression from stable to beta. and removed regression-from-stable-to-nightly Performance or correctness regression from stable to nightly. labels Jun 22, 2020
@Dylan-DPC-zz
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giving this p-critical based on our discussion

@wesleywiser
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Why is this regression-from-stable-to-beta when the offending PR only merged last night?

@Dylan-DPC-zz Dylan-DPC-zz added P-critical Critical priority and removed I-prioritize Issue: Indicates that prioritization has been requested for this issue. labels Jun 22, 2020
@Aaron1011
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@RalfJung: Why didn't the assertion failure cause a Miri toolstate change?

@RalfJung
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I think this is stable-to-nightly only... @oli-obk why did you change that?

@Aaron1011 rustc's CI currently only tests Miri with mir-opt-level=0.

@oli-obk
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oli-obk commented Jun 23, 2020

I tested it in the playground and I thought it also happened on beta?

@oli-obk oli-obk added regression-from-stable-to-nightly Performance or correctness regression from stable to nightly. and removed regression-from-stable-to-beta Performance or correctness regression from stable to beta. labels Jun 23, 2020
@oli-obk
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oli-obk commented Jun 23, 2020

Hmm... not sure what happened, must have mis-clicked on the playground

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Labels
A-mir-opt Area: MIR optimizations C-bug Category: This is a bug. I-unsound Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness P-critical Critical priority regression-from-stable-to-nightly Performance or correctness regression from stable to nightly. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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