Implement simple CopyPropagation based on SSA analysis

[cjgillot]

This PR extracts the "copy propagation" logic from #106285.

MIR may produce chains of assignment between locals, like _x = move? _y.
This PR attempts to remove such chains by unifying locals.

The current implementation is a bit overzealous in turning moves into copies, and in removing storage statements.

[rustbot]

r? @TaKO8Ki

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[oli-obk]

r? @oli-obk

[oli-obk]

@bors r+

[bors]

📌 Commit c1a924547eaecff4bc7af3c0de848429a7ac78f1 has been approved by oli-obk

It is now in the queue for this repository.

[JakobDegen]

The freeze check is not enough here, we should be bailing out on all references. The pass currently changes the bahavior of this code:

#![feature(custom_mir, core_intrinsics)]
#![allow(unused_assignments)]
extern crate core;
use core::intrinsics::mir::*;

#[custom_mir(dialect = "analysis", phase = "post-cleanup")]
fn f(c: bool) -> bool { mir!({
    let a = c;
    let p = core::ptr::addr_of!(a);
    let p2 = core::ptr::addr_of_mut!(*p);
    *p2 = false;
    RET = c;
    Return()
})}

fn main() {
    assert_eq!(true, f(true));
}

This is Miri UB today, but we should not depend on that on stable, especially because there is a real possibility that this may be allowed in the future.

[oli-obk]

@bors r-

[JakobDegen]

Sorry, should have thought of this before too. Banning raw pointers isn't enough, references cause problems too. Your pass changes the behavior of this code:

#![feature(custom_mir, core_intrinsics)]
#![allow(unused_assignments)]
extern crate core;
use core::intrinsics::mir::*;

fn cmp_ref(a: &u8, b: &u8) -> bool {
    std::ptr::eq(a as *const u8, b as *const u8)
}

#[custom_mir(dialect = "analysis", phase = "post-cleanup")]
fn f() -> bool { mir!(
    {
        let a = 5_u8;
        let r1 = &a;
        let b = a;
        let r2 = &b;
        Call(RET, next, cmp_ref(r1, r2))
    }
    next = {
        Return()
    }
)}

fn main() {
    assert!(!f());
}

Ideally this would actually be allowed, but we don't currently know how to do that, so need to be conservative in the meantime. See rust-lang/unsafe-code-guidelines#206 for discussion

[JakobDegen]

Ah, not the change I was expecting, but very nice. Technically this requires a T-opsem FCP these days (since it makes slightly more assumptions about reference immutability than we do already), but that team doesn't really exist yet, so should be fine to land

[oli-obk]

@bors r+

[bors]

📌 Commit cbfb98fb8bf1772e059df697c8d15a71b79256d3 has been approved by oli-obk

It is now in the queue for this repository.

[cjgillot]

@bors r=oli-obk

[bors]

📌 Commit 263da25 has been approved by oli-obk

It is now in the queue for this repository.

[bors]

⌛ Testing commit 263da25 with merge 2a4b00b...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 2a4b00b to master...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 2a4b00b to master...

[rust-timer]

Finished benchmarking commit (2a4b00b): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.8%	[3.8%, 3.8%]	1
Regressions ❌ (secondary)	2.8%	[2.8%, 2.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.8%	[3.8%, 3.8%]	1

Cycles

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