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Uninitialized bytes get copied to the buffer when creating an uninitialized tuple #83087

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Uninitialized bytes get copied to the buffer when creating an uninitialized tuple#83087
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nikic
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A-LLVMArea: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.I-slowIssue: Problems and improvements with respect to performance of generated code.P-highHigh priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.
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1.52.0

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@MSxDOS
MSxDOS
opened on Mar 13, 2021

In this example:

use std::mem::MaybeUninit;

extern {
    fn fill(p: *mut u16);
}

pub unsafe fn test() {
    let (mut buffer, mut buffer2): ([u16; 512], [u16; 256]) = MaybeUninit::uninit().assume_init();
    fill(buffer.as_mut_ptr());
    fill(buffer2.as_mut_ptr())
}

there is a call to memcpy for one of the buffers that copies uninitialized data to it.
https://rust.godbolt.org/z/z1zcor

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  1. MSxDOS commented on Mar 13, 2021

    @MSxDOS
    MSxDOS
    on Mar 13, 2021
    Author

    cc @nikic

    This started after #82806 was merged.

    Also, this is a regression from stable to nightly.

  2. rustbot
    added
    A-LLVMArea: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.
    I-slowIssue: Problems and improvements with respect to performance of generated code.
    regression-from-stable-to-nightlyPerformance or correctness regression from stable to nightly.
    I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}
    and removed
    I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}
    on Mar 13, 2021
  3. nikic
    self-assigned this
    on Mar 13, 2021

  4. nikic commented on Mar 13, 2021

    @nikic
    nikic
    on Mar 13, 2021
    Contributor

    Upstream fix: llvm/llvm-project@5556660

  5. emilyalbini
    added
    regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.
    and removed
    regression-from-stable-to-nightlyPerformance or correctness regression from stable to nightly.
    on May 4, 2021
  6. rustbot
    added
    I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}
    on May 4, 2021
  7. emilyalbini
    added this to the 1.52.0 milestone on May 4, 2021

  8. emilyalbini commented on May 4, 2021

    @emilyalbini
    emilyalbini
    on May 4, 2021
    Member

    This slipped up to stable 1.52.0, apparently we didn't label it regression-from-stable-to-beta losing track of it :/

  9. nikic commented on May 4, 2021

    @nikic
    nikic
    on May 4, 2021
    Contributor

    @pietroalbini We don't plan to address this prior to the next LLVM upgrade anyway. It's just an optimization issue.

  10. apiraino commented on May 6, 2021

    @apiraino
    apiraino
    on May 6, 2021
    Contributor

    Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

    @rustbot label -I-prioritize +P-high +T-compiler

  11. rustbot
    added
    P-highHigh priority
    T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.
    and removed
    I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}
    on May 6, 2021

  12. nikic commented on Aug 22, 2021

    @nikic
    nikic
    on Aug 22, 2021
    Contributor

    Fixed by #87570, new IR does not have memcpy:

    define void @_ZN5test24test17h0962739e00df972bE() unnamed_addr #0 {
start:
  %buffer2 = alloca [256 x i16], align 2
  %buffer = alloca [512 x i16], align 2
  %0 = bitcast [512 x i16]* %buffer to i8*
  call void @llvm.lifetime.start.p0i8(i64 1024, i8* nonnull %0)
  %1 = bitcast [256 x i16]* %buffer2 to i8*
  call void @llvm.lifetime.start.p0i8(i64 512, i8* nonnull %1)
  %2 = getelementptr inbounds [512 x i16], [512 x i16]* %buffer, i64 0, i64 0
  call void @fill(i16* nonnull %2)
  %3 = getelementptr inbounds [256 x i16], [256 x i16]* %buffer2, i64 0, i64 0
  call void @fill(i16* nonnull %3)
  call void @llvm.lifetime.end.p0i8(i64 512, i8* nonnull %1)
  call void @llvm.lifetime.end.p0i8(i64 1024, i8* nonnull %0)
  ret void
}
  13. nikic
    closed this as completedon Aug 22, 2021
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A-LLVMArea: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.I-slowIssue: Problems and improvements with respect to performance of generated code.P-highHigh priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.

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