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Emit noalias on &mut parameters by default #50744

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merged 1 commit into from
May 19, 2018
Merged

Emit noalias on &mut parameters by default #50744

merged 1 commit into from
May 19, 2018

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nikic
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@nikic nikic commented May 14, 2018

This used to be disabled due to LLVM bugs in the handling of
noalias information in conjunction with unwinding. However,
according to #31681 all known LLVM bugs have been fixed by
LLVM 6.0, so it's probably time to reenable this optimization.

-Z no-mutable-noalias is left as an escape-hatch to debug problems
suspected to stem from this change.

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r? @eddyb

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@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label May 14, 2018
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eddyb commented May 14, 2018

Shouldn't this be gated on the LLVM version? I think we still support distros using 3.9.
cc @alexcrichton @gankro

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nikic commented May 14, 2018

@eddyb Good point, I've conditioned the default behavior on LLVM >= 6.0 now.

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@bors: r+

Nice!

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bors commented May 14, 2018

📌 Commit 0c965a6 has been approved by alexcrichton

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 14, 2018
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It'd be very cool to see any before/after benchmarks for this. I'm really curious as to how much of an impact it actually has.

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Gankra commented May 14, 2018

This is removing the panic=abort stable opt in. Is firefox on a new enough llvm that it won't regress?

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nikic commented May 14, 2018

@gankro I've added back the panic=abort check just in case. Apparently Firefox is stuck on old Rust due to LLVM 6 regression in #49873.

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Gankra commented May 14, 2018

Thanks!

@kennytm kennytm added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels May 14, 2018
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bors commented May 14, 2018

📌 Commit 8acc992 has been approved by alexcrichton

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 14, 2018
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bors commented May 14, 2018

💡 This pull request was already approved, no need to approve it again.

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bors commented May 14, 2018

📌 Commit 8acc992 has been approved by alexcrichton

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bors commented May 17, 2018

☔ The latest upstream changes (presumably #50807) made this pull request unmergeable. Please resolve the merge conflicts.

@bors bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels May 17, 2018
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bors commented May 17, 2018

☔ The latest upstream changes (presumably #50615) made this pull request unmergeable. Please resolve the merge conflicts.

@nikic
Emit noalias on &mut parameters by default
1230813 
This used to be disabled due to LLVM bugs in the handling of
noalias information in conjunction with unwinding. However,
according to rust-lang#31681 all known LLVM bugs have been fixed by
LLVM 6.0, so it's probably time to reenable this optimization.

Noalias annotations will not be emitted by default if either
-C panic=abort (as previously) or LLVM >= 6.0 (new).

-Z mutable-noalias=no is left as an escape-hatch to allow
debugging problems suspected to stem from this change.
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@bors r=alexcrichton

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bors commented May 18, 2018

📌 Commit 1230813 has been approved by alexcrichton

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 18, 2018
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bors commented May 19, 2018

⌛ Testing commit 1230813 with merge d4cb245d65e63ab49eef3db792070fafd6c7d048...

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bors commented May 19, 2018

💔 Test failed - status-travis

@bors bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels May 19, 2018
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@kennytm kennytm mentioned this pull request May 19, 2018
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kennytm commented May 19, 2018

@bors retry #50887

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 19, 2018
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bors commented May 19, 2018

⌛ Testing commit 1230813 with merge bdace29...

bors added a commit that referenced this pull request May 19, 2018
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Auto merge of #50744 - nikic:mutable-noalias, r=alexcrichton
bdace29 
Emit noalias on &mut parameters by default

This used to be disabled due to LLVM bugs in the handling of
noalias information in conjunction with unwinding. However,
according to #31681 all known LLVM bugs have been fixed by
LLVM 6.0, so it's probably time to reenable this optimization.

-Z no-mutable-noalias is left as an escape-hatch to debug problems
suspected to stem from this change.
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bors commented May 19, 2018

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing bdace29 to master...

@bors bors merged commit 1230813 into rust-lang:master May 19, 2018
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Is this changing the performance of some standard benchmark?

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http://perf.rust-lang.org/compare.html?start=8319ef5b78a10b3a8de4109bb8b0e6d23fbe4de1&end=bdace29de04af4fe9e4317b73c3f7d6418a33de1&stat=instructions%3Au

Looks like the Rust part of the compiler is faster, but the LLVM part spends more time optimizing. So it seems promising for runtime perf; hopefully lolbench will confirm that.

@nikic nikic mentioned this pull request May 21, 2018
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@daboross daboross mentioned this pull request Jun 19, 2018
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