Fix bugs involving the combination of branching and subtyping #1403
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Addresses issues discussed in WebAssembly/gc#516
…ches When, for example, `[a i32]` is on the stack and we have a `br_if @label` where `@label`'s type is `[b]` and `a <: b`, then whole `br_if` is typed `[b i32] -> [b]`. Because `a <: b` the `br_if` is valid. However, `a <: b` does *not* mean that the instruction results in `[a]`. It "erases" the `a` and pushes a `b`. This addresses WebAssembly/gc#516 in `wasm-smith`.
alexcrichton
approved these changes
Feb 8, 2024
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This PR fixes similar bugs in both
wasmparserandwasm-smith.If we are conditionally branching to a label whose type is
[a b]and we have[a' b']on the stack wherea' <: aandb' <: b, we need to replace the stack's subtypes with the label's types after processing the instruction.That is, the following three operations, which used to be (mostly) equivalent before subtyping, are no longer equivalent:
pop_operands(label_types); push_operands(label_types)push_operands(pop_operands(label_types))We generally have to do (1) to properly erase subtyping, as called for by the spec. (2) is still used for
br_tablehowever. (3) should generally be avoided.See also WebAssembly/gc#516