a1c9b06cea
Previously, the x64 backend's ABI code would generate a sign-extending load when loading a less-than-64-bit integer from a spillslot. This is incorrect: e.g., for i32s > 0x80000000, this would result in all high bits set. This interacts poorly with another optimization. Normally, the invariant is that the high bits of a register holding a value of a certain type, beyond that type's bits, are undefined. However, as an optimization, we recognize and use the fact that on x86-64, 32-bit instructions zero the upper 32 bits. This allows us to elide a 32-to-64-bit zero-extend op (turning it into just a move, which can then sometimes disappear entirely due to register coalescing). If a spill and reload happen between the production of a 32-bit value from an instruction known to zero the upper bits and its use, then we will rely on zero upper bits that might actually be set by a sign-extend. This will result in incorrect execution. As a fix, we stick to a simple invariant: we always spill and reload a full 64 bits when handling integer registers on x64. This ensures that no bits are mangled.
This crate contains the core Cranelift code generator. It translates code from an intermediate representation into executable machine code.