Implement multiple return terminator optimization

[alarsyo]

Closes #72022

[rust-highfive]

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[oli-obk]

r? @oli-obk

[Dylan-DPC-zz]

@alarsyo any updates on this?

[alarsyo]

Found the time to work on this, should be ready for a first basic review :)

[alarsyo]

Rebased on master

[oli-obk]

@bors r+

[bors]

📌 Commit 779f52272d56c5ead356a1d17cb9cd6ca3fc304f has been approved by oli-obk

[tmandry]

There doesn't seem to be consensus on #72022 that this is a good idea, are we sure we want this?

Also, there may be perf impact to this change so it probably shouldn't be rolled up.

[oli-obk]

@bors rollup=never r-

[oli-obk]

@alarsyo there are multiple failing codegen tests, can you adjust them?

[alarsyo]

Just to make sure I haven't made any mistake: when generating the IR in this comment, I had to remove this line from the file for the test to run, presumably because I'm building the compiler in debug mode. That doesn't affect the optimization level of the generated IR, right?

So if I understand correctly, this test doesn't pass because something in libstd/libcore changed when building them with this new optimization, and somehow makes it so that the IR doesn't get optimized to a constant by LLVM/rustc (not sure which one should optimize this part)?

How would I go about putting this behind mir-opt-level 3 ? I'm seeing multiple checks for mir_opt_level > 0 in librustc_mir/transform/mod.rs, but nothing for level 3

[oli-obk]

Here's an example for mir-opt-level 3:

rust/src/librustc_mir/transform/unreachable_prop.rs

Line 16 in 6b2ca84

if tcx.sess.opts.debugging_opts.mir_opt_level < 3 {

Just to make sure I haven't made any mistake: when generating the IR in this comment, I had to remove this line from the file for the test to run, presumably because I'm building the compiler in debug mode. That doesn't affect the optimization level of the generated IR, right?

Oh, that's a curious situation. it's still release mode, but with debug assertions, can you try adding // compile-flags: -Cdebug-assertions=off to the test (and remove the // ignore-debug comment) and see if that works?

I won't be able to review again until the 7th, cc @rust-lang/wg-mir-opt can someone have a look at this?

[alarsyo]

Oh, that's a curious situation. it's still release mode, but with debug assertions, can you try adding // compile-flags: -Cdebug-assertions=off to the test (and remove the // ignore-debug comment) and see if that works?

That works, the test runs even though I'm in debug mode, thanks! I guess I shouldn't include this change in this PR though, right?

[alarsyo]

Whoops, forgot to bless the mir-opt tests with the opt behind opt-level 3.

[alarsyo]

Hmm, I can't reproduce the two failing tests by running ./x.py test src/test/mir-opt; they pass just fine on my computer.

[bors]

☔ The latest upstream changes (presumably #74862) made this pull request unmergeable. Please resolve the merge conflicts.

[oli-obk]

in order for the 32 bit versions to get updated you need to run with --target i686-unknown-linux-gnu

[crlf0710]

@alarsyo Ping from triage, any updates on this?

[alarsyo]

@crlf0710 I've been pretty busy the last few weeks, nothing new for now :( I have to get back on this, hopefully by the next weekend

[alarsyo]

@oli-obk rebased on master, I'll try to make progress :)

[alarsyo]

Everything passes 🎉

[oli-obk]

@bors r+

awesome

[bors]

📌 Commit 46c0bd3 has been approved by oli-obk

[bors]

⌛ Testing commit 46c0bd3 with merge 9cba260...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: oli-obk
Pushing 9cba260 to master...