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Auto merge of #128742 - RalfJung:miri-vtable-uniqueness, r=saethlin

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miri: make vtable addresses not globally unique

Miri currently gives vtables a unique global address. That's not actually matching reality though. So this PR enables Miri to generate different addresses for the same type-trait pair.

To avoid generating an unbounded number of `AllocId` (and consuming unbounded amounts of memory), we use the "salt" technique that we also already use for giving constants non-unique addresses: the cache is keyed on a "salt" value n top of the actually relevant key, and Miri picks a random salt (currently in the range `0..16`) each time it needs to choose an `AllocId` for one of these globals -- that means we'll get up to 16 different addresses for each vtable. The salt scheme is integrated into the global allocation deduplication logic in `tcx`, and also used for functions and string literals. (So this also fixes the problem that casting the same function to a fn ptr over and over will consume unbounded memory.)

r? `@saethlin`
Fixes https://github.com/rust-lang/miri/issues/3737
This commit is contained in:
bors 2024-08-13 04:32:34 +00:00
commit 591ecb88df
24 changed files with 335 additions and 209 deletions
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9
library/core/tests/ptr.rs
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@ -810,9 +810,12 @@ fn ptr_metadata() {
assert_ne!(address_1, address_2);
// Different erased type => different vtable pointer
assert_ne!(address_2, address_3);
// Same erased type and same trait => same vtable pointer
assert_eq!(address_3, address_4);
assert_eq!(address_3, address_5);
// Same erased type and same trait => same vtable pointer.
// This is *not guaranteed*, so we skip it in Miri.
if !cfg!(miri) {
assert_eq!(address_3, address_4);
assert_eq!(address_3, address_5);
}
}
}