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Bug 1837660 - Miscompilation of rustc 1.43 leads to LTO failure
Summary: Miscompilation of rustc 1.43 leads to LTO failure
Alias: None
Product: Fedora
Classification: Fedora
Component: llvm
Version: 31
Hardware: s390x
OS: Unspecified
Target Milestone: ---
Assignee: Josh Stone
QA Contact: Fedora Extras Quality Assurance
Depends On:
Blocks: 1840305
TreeView+ depends on / blocked
Reported: 2020-05-19 18:49 UTC by Josh Stone
Modified: 2020-06-07 19:45 UTC (History)
8 users (show)

Fixed In Version: llvm-9.0.1-6.fc31
Doc Type: If docs needed, set a value
Doc Text:
Clone Of:
: 1840305 (view as bug list)
Last Closed: 2020-06-07 19:45:23 UTC
Type: Bug

Attachments (Terms of Use)

System ID Private Priority Status Summary Last Updated
Fedora Pagure fedora-rust/sig issue 12 0 None None None 2020-05-19 18:49:52 UTC

Description Josh Stone 2020-05-19 18:49:53 UTC
Description of problem:
Starting with Rust 1.43, we found that programs configured to build with full LTO were crashing rustc on s390x with a segmentation fault. It also happens with upstream Rust binaries. This is locally reproducible with a stage2 rustc, but stage1 (bootstrapped from 1.42) is fine.

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:
1. Rebuild an LTO rust package, like rust-afterburn.

Actual results:

Expected results:
Successful compilation.

Additional info:
The code which processes FatLTO modules was seeing a 0x1 data pointer for the name String. Ulrich Weigand (IBM) found a miscompilation in "Drop for Bomb" in src/librustc_codegen_ssa/back/, where it copied the wrong range into the newly Boxed value.

I bisected the problem in Rust to commit e82ec2315e5a, but the effect of that only simplified the LLVM IR. This did lead SROA to break up those memcpys more than they were before. Since F32 with LLVM 10 is working, I compared "opt" from 9 and 10 output and found a difference in shufflevector arguments. Bisecting LLVM led me to commit a9d6b0e5, "[InstCombine] Fix big-endian miscompile of (bitcast (zext/trunc (bitcast)))".

I have confirmed that backporting that change to LLVM 9 fixes the build for rust-afterburn, and the asm of that rustc code looks correct to me.

Comment 2 Fedora Update System 2020-05-29 16:50:29 UTC
FEDORA-2020-d8bba51d6a has been submitted as an update to Fedora 31.

Comment 3 Fedora Update System 2020-05-30 02:03:51 UTC
FEDORA-2020-d8bba51d6a has been pushed to the Fedora 31 testing repository.
In short time you'll be able to install the update with the following command:
`sudo dnf upgrade --enablerepo=updates-testing --advisory=FEDORA-2020-d8bba51d6a`
You can provide feedback for this update here:

See also for more information on how to test updates.

Comment 4 Fedora Update System 2020-06-07 19:45:23 UTC
FEDORA-2020-d8bba51d6a has been pushed to the Fedora 31 stable repository.
If problem still persists, please make note of it in this bug report.

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