Fix MaybeUninit codegen using GVN

[saethlin]

This is an alternative to #142837, based on #146355 (comment).

The general approach I took here is to aggressively propagate anything that is entirely uninitialized. GVN generally takes the approach of only synthesizing small types, but we need to generate large consts to fix the codegen issue.

I also added a special case to MIR dumps for this where now an entirely uninit const is printed as const <uninit>, because otherwise we end up with extremely verbose dumps of the new consts.

After GVN though, we still end up with a lot of MIR that looks like this:

StorageLive(_1);
_1 = const <uninit>;
_2 = &raw mut _1;

Which will break tests/codegen-llvm/maybeuninit-rvo.rs with the naive lowering. I think the ideal fix here is to somehow omit these _1 = const <uninit> assignments that come directly after a StorageLive, but I'm not sure how to do that. For now at least, ignoring such assignments (even if they don't come right after a StorageLive) in codegen seems to work.

Note that since GVN is based on synthesizing a ConstValue which has a defined layout, this scenario still gets deoptimized by LLVM.

#![feature(rustc_attrs)]
#![crate_type = "lib"]
use std::mem::MaybeUninit;

#[unsafe(no_mangle)]
pub fn oof() -> [[MaybeUninit<u8>; 8]; 8] {
    #[rustc_no_mir_inline]
    pub fn inner<T: Copy>() -> [[MaybeUninit<T>; 8]; 8] {
        [[MaybeUninit::uninit(); 8]; 8]
    }

    inner()
}

This case can be handled correctly if enough inlining has happened, or it could be handled by post-mono GVN. Synthesizing UnevaluatedConst or some other special kind of const seems dubious.

[saethlin]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: e6fd12c (e6fd12c9ca883127b096afd871b40a5714f3d107, parent: f46475914de626785090a05ae037578aaa119fc8)

[rust-timer]

Finished benchmarking commit (e6fd12c): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	0.5%	[0.5%, 0.5%]	3
Improvements ✅ (primary)	-7.4%	[-7.4%, -7.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.3%	[-7.4%, 0.7%]	2

Max RSS (memory usage)

Results (primary 0.9%, secondary 0.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.9%, 0.9%]	1
Regressions ❌ (secondary)	3.0%	[1.8%, 4.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	0.9%	[0.9%, 0.9%]	1

Cycles

Results (primary -3.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.6%	[-3.6%, -3.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.6%	[-3.6%, -3.6%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 475.372s -> 474.527s (-0.18%)
Artifact size: 390.39 MiB -> 390.41 MiB (0.00%)

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

[rustbot]

r? @petrochenkov

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[saethlin]

r? scottmcm

[tmiasko]

I think the ideal fix here is to somehow omit these _1 = const assignments that come directly after a StorageLive, but I'm not sure how to do that.

If you go in that direction, please make sure #137936 is fixed first. Otherwise such a transformation would expose the issue in Rust, as opposed to being merely limited to a custom MIR (probably?).