GVN causes moved function arguments to be used again

[cbeuw]

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

#[inline(never)]
fn dump_var(x: [u128; 6]) {
    println!("{x:?}");
}

#[custom_mir(dialect = "runtime", phase = "initial")]
pub fn fn0() {
    mir! {
    let _12: usize;
    let _14: [u128; 6];
    let _26: ([u128; 6],);
    let _29: ([u128; 6],);
    let x: ();
    {
    _12 = 1_usize;
    _14 = [42; 6];
    _14[_12] = 1;
    _29 = (_14,);
    _26 = _29;
    Call(x = fn1(_29.0, _26), ReturnTo(bb14), UnwindUnreachable())
    }
    bb14 = {
    Return()
    }
    }
}

pub fn fn1(mut _13: [u128; 6], mut _14: ([u128; 6],)) {
    _14.0 = [0; 6];
    dump_var(_13);
    dump_var(_14.0);
}
pub fn main() {
    fn0();
}

Right:

$ rustc -Zmir-opt-level=2 -Zmir-enable-passes=-GVN -Copt-level=0 1717541-debug.rs && ./1717541-debug
[42, 1, 42, 42, 42, 42]
[0, 0, 0, 0, 0, 0]

Wrong:

$ rustc -Zmir-opt-level=2 -Copt-level=0 1717541-debug.rs && ./1717541-debug
[0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0]

The first argument to fn1 is partially moved. GVN causes the second argument to use the partially moved value:

--- a/mir_dump/1717541_debug.fn0.005-015.GVN.before.mir
+++ b/mir_dump/1717541_debug.fn0.005-015.GVN.after.mir
@@ -1,4 +1,4 @@
-// MIR for `fn0` before GVN
+// MIR for `fn0` after GVN
 
 fn fn0() -> () {
     let mut _0: ();
@@ -11,10 +11,10 @@ fn fn0() -> () {
     bb0: {
         _1 = const 1_usize;
         _2 = [const 42_u128; 6];
-        _2[_1] = const 1_u128;
+        _2[1 of 2] = const 1_u128;
         _4 = (_2,);
         _3 = _4;
-        _5 = fn1(move (_4.0: [u128; 6]), move _3) -> [return: bb1, unwind unreachable];
+        _5 = fn1(move (_4.0: [u128; 6]), _4) -> [return: bb1, unwind unreachable];
     }
 
     bb1: {

rustc 1.77.0-nightly (11f32b73e 2024-01-31)
binary: rustc
commit-hash: 11f32b73e0dc9287e305b5b9980d24aecdc8c17f
commit-date: 2024-01-31
host: x86_64-unknown-linux-gnu
release: 1.77.0-nightly
LLVM version: 17.0.6

[saethlin]

I can't provoke the miscompilation on beta using this example (just had to replace ReturnTo(bb14) with bb14 to make it build), so I don't think there's any need to rush to fix this.

(Some part of GVN is used in the beta that's about to become stable, but it looks like it doesn't have this bug in it)

[RalfJung]

Cc @cjgillot

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium