1672813 - Rust 1.47+ (LLVM11) miscompilation on MSVC

Status: RESOLVED FIXED

(Firefox Build System :: Toolchains, defect)

Description

[Jeff Muizelaar [:jrmuizel]]

This version of the test passes:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=a3ebf0f1f1c1dbb8473e9118c41f1d397c897ec4

This version doesn't:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=62432b7209ebfc51cbb8d5bc6dfbc85972c499e0

The difference is:
https://hg.mozilla.org/try/rev/ef805b7366cbc73ebbba84b0c9fc5f01a60fac4a

which shouldn't make any difference.

Unfortunately, I can't reproduce the test failure locally.

Comment 1

[Jeff Muizelaar [:jrmuizel]]

The test is passing with rustc 1.43.0
https://treeherder.mozilla.org/#/jobs?repo=try&revision=3352ef8843aaffc42802ba162d1bac1b8cb5d171&selectedTaskRun=HI-U8NyqQx66cucdH6NvJg.0

Comment 2

[Jeff Muizelaar [:jrmuizel]]

And it passes with rust 1.46.0
https://treeherder.mozilla.org/#/jobs?repo=try&revision=2f3e81137bfba71e5fab96c39b439a1247cfef7c&selectedTaskRun=MA9ac3jpSjK2HusYuDwD_g.0

Comment 3

[Jeff Muizelaar [:jrmuizel]]

nightly-2020-08-20 - good - https://treeherder.mozilla.org/#/jobs?repo=try&revision=c3ebb9ef160130a23d4f20b4d5a155b44997bed5
nightly-2020-08-27 - bad - https://treeherder.mozilla.org/#/jobs?repo=try&revision=20da5949f689e330f985ff7a3231e751baac4607

Comment 4

[Jeff Muizelaar [:jrmuizel]]

My guess is that it's caused by the upgrade to LLVM 11 :(

Comment 5

[Jeff Muizelaar [:jrmuizel]]

Here are 08-24: https://treeherder.mozilla.org/#/jobs?repo=try&revision=75c40b737d490b1d4ee44187eee1551bdf81a4f8
and 08-23: https://treeherder.mozilla.org/#/jobs?repo=try&revision=cb49bc57532fcd3ce2726a829ec27f809c6e0bce
which surround the LLVM update.

Comment 6

[Mike Hommey [:glandium]]

(In reply to Jeff Muizelaar [:jrmuizel] from comment #5)

Here are 08-24: https://treeherder.mozilla.org/#/jobs?repo=try&revision=75c40b737d490b1d4ee44187eee1551bdf81a4f8
and 08-23: https://treeherder.mozilla.org/#/jobs?repo=try&revision=cb49bc57532fcd3ce2726a829ec27f809c6e0bce
which surround the LLVM update.

If you apply bug 1672328, you can replace nightly-yyyy-mm-dd with bors-sha1 where sha1 is a merge commit authored by bors in the rust repo, so you could check for sure that it's the LLVM update by testing that merge and its parent.

Comment 7

[Jeff Muizelaar [:jrmuizel]]

(In reply to Mike Hommey [:glandium] from comment #6)

If you apply bug 1672328, you can replace nightly-yyyy-mm-dd with bors-sha1 where sha1 is a merge commit authored by bors in the rust repo, so you could check for sure that it's the LLVM update by testing that merge and its parent.

I'll try that tomorrow. In the mean time let's see if it's fixed in Nightly:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=8fdc9dcf7b31843950da162fbaec31ce3b23d0a3

Comment 8

[Mike Hommey [:glandium]]

(In reply to Jeff Muizelaar [:jrmuizel] from comment #7)

(In reply to Mike Hommey [:glandium] from comment #6)

If you apply bug 1672328, you can replace nightly-yyyy-mm-dd with bors-sha1 where sha1 is a merge commit authored by bors in the rust repo, so you could check for sure that it's the LLVM update by testing that merge and its parent.

I'll try that tomorrow. In the mean time let's see if it's fixed in Nightly:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=8fdc9dcf7b31843950da162fbaec31ce3b23d0a3

This won't work because of a) bug 1428197 and b) bug 1672837. You want nightly-2020-10-22 and to apply bug 1672837

Comment 9

[Jeff Muizelaar [:jrmuizel]]

Here's that https://treeherder.mozilla.org/#/jobs?repo=try&revision=c58dda65ef57f5efe4c9aa1c4b7e38fb2ef8fdb1

Comment 10

[Mike Hommey [:glandium]]

I confirmed it comes from the merge of LLVM11:
before: https://treeherder.mozilla.org/#/jobs?repo=try&revision=52c6d14a571ab093e7210db13a032e3d676d7912
after: https://treeherder.mozilla.org/#/jobs?repo=try&revision=c2c46cec0ee554963b78e782f55a2d749b72ef98

Comment 11

[Jeff Muizelaar [:jrmuizel]]

I filed: https://github.com/rust-lang/rust/issues/78283

Comment 12

[Jeff Muizelaar [:jrmuizel]]

Adding codegen-units=1 let's me reproduce this locally.

Comment 13

[(Away)]

Here are two other LLVM 11 bugs reported through rust: https://bugs.llvm.org/show_bug.cgi?id=47444 https://bugs.llvm.org/show_bug.cgi?id=47090

It's a long shot, but any chance their patches fix this?

Comment 14

[Mike Hommey [:glandium]]

We're now on LLVM 15, this was likely fixed a long time ago.