Incorrectly unified locals at -Zmir-opt-level=2

fn main() {
    if f(false) != 1 {
        panic!();
    }
}

#[inline(always)]
fn f(use_y: bool) -> u8 {
    let (x, y) = (1, 2);
    if use_y { y } else { x }
}

$ rustc -Zmir-opt-level=1 a.rs && ./a
$ rustc -Zmir-opt-level=2 a.rs && ./a
thread 'main' panicked at 'explicit panic', a.rs:3:9

--- a.main.005-011.DestinationPropagation.before.mir
+++ a.main.005-011.DestinationPropagation.after.mir
@@ -1,70 +1,70 @@
-// MIR for `main` before DestinationPropagation
+// MIR for `main` after DestinationPropagation
 
 fn main() -> () {
     let mut _0: ();
     let mut _1: bool;
     let mut _2: u8;
     let mut _3: !;
     let _4: ();
     let mut _5: !;
     let mut _6: bool;
     scope 1 (inlined f) {
         debug use_y => _6;
         let _7: u8;
         let mut _8: bool;
         scope 2 {
             debug x => _2;
-            debug y => _7;
+            debug y => _2;
         }
     }
 
     bb0: {
         StorageLive(_1);
-        StorageLive(_2);
+        nop;
         StorageLive(_6);
         _6 = const false;
         _2 = const 1_u8;
-        StorageLive(_7);
-        _7 = const 2_u8;
+        nop;
+        _2 = const 2_u8;
         StorageLive(_8);
         _8 = const false;
         goto -> bb4;
     }
 
     bb1: {
-        StorageDead(_2);
+        nop;
         _0 = const ();
         StorageDead(_1);
         return;
     }
 
     bb2: {
-        StorageDead(_2);
+        nop;
         StorageLive(_4);
         StorageLive(_5);
         begin_panic::<&str>(const "explicit panic");
     }
 
     bb3: {
-        _2 = _7;
+        nop;
         goto -> bb4;
     }
 
     bb4: {
-        StorageDead(_7);
+        nop;
         StorageDead(_8);
         StorageDead(_6);
         _1 = Ne(_2, const 1_u8);
         nop;
         switchInt(move _2) -> [1_u8: bb1, otherwise: bb2];
     }
 }



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Hmm, seems like it doesn't see the conflict because the assignment is unreachable:

    // init: <unreachable>
    // live: <unreachable>
    bb3: {
        // init: <unreachable>
        // live: <unreachable>
        _2 = _7;                         // scope 2 at borken.rs:2:8: 2:16
        // init: <unreachable>
        // live: <unreachable>
        goto -> bb4;                     // scope 2 at borken.rs:2:8: 2:16
        // init: <unreachable>
        // live: <unreachable>
    }

Seems like we should consider assignments in dead blocks as propagation candidates. But does that fix the whole bug?

I think that the main issue is that the notion of conflicts is based on liveness. A local could be never live but still conflict with others merely because there are dead stores to it.

For example, in code below a will be unified with b (after running destination propagation for the second time after inlining):

#![allow(unused)]

#[inline(never)]
pub fn id<T>(x: T) -> T { x }

pub fn main() {
    assert_eq!(1, g(1));
}

#[inline(never)]
pub fn g(x: usize) -> usize {
    f(x)
}

#[inline(always)]
pub fn f(a: usize) -> usize {
    let mut b = 3;
    b = a;
    b = 4;
    id(a)
}

Right. I think this briefly came up when implementing the pass, but I never wrote it down anywhere, nor could I produce an example.

Sounds like we need to run a dead store elimination pass before destination propagation, or change the analysis it uses to account for dead stores.

@rustbot

@rustbot claim

@JakobDegen

@JakobDegen I'm interested in this, too. Have you done some work? If not, I should be able to submit a PR soon

Yeah, I have a local branch up. Looking to open a PR today. The PR will be a draft though - the opt actually has significantly more problems than are reported in issues, and some of those problems being resolved is blocked on open questions about MIR semantics

Auto merge of rust-lang#96451 - JakobDegen:dest-prop, r=tmiasko

Fixed in #96451.

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Incorrectly unified locals at -Zmir-opt-level=2 #79191

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Incorrectly unified locals at -Zmir-opt-level=2 #79191

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jonas-schievink commented on Nov 26, 2020

tmiasko commented on Nov 27, 2020

jonas-schievink commented on Nov 28, 2020

JakobDegen commented on Feb 23, 2022

SparrowLii commented on Apr 25, 2022

JakobDegen commented on Apr 25, 2022

tmiasko commented on Dec 5, 2022

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