Rust 1.34 gets errors like this:
```
$ cargo +1.34.0 build --release
Compiling cranelift-wasm v0.32.0 (cranelift/cranelift-wasm)
error[E0259]: the name `std` is defined multiple times
|
= note: `std` must be defined only once in the type namespace of this module
error: aborting due to previous error
For more information about this error, try `rustc --explain E0259`.
error: Could not compile `cranelift-wasm`.
To learn more, run the command again with --verbose.
```
Updating to Rust 1.35 fixes this.
PR #773 detects, at reload time, `copy` instructions that copy a value from stack
slot back to the same stack slot. It replaces them with `copy_nop` instructions
that have a null encoding (hence producing no code).
For x86_64, `copy_nop` encodings for the types I64, I32, F64 and F32 are
provided. Unfortunately the code that detects the redundant copy doesn't
check the type of the copied value, hence leaving itself open to the danger of
creating a `copy_nop` instruction cannot be encoded (which is different from
saying it has a null encoding).
This patch:
* Expands the x86_64 set of `copy_nop` encodings to: I64 I32 I16 I8 F64 and F32
* Adds encodings for the same for x86_32, rv64 and rv32.
* In `visit_inst()` in `reload.rs`, checks the type of the copied value accordingly.
* Adds comments explaining the above.
The result of the emitter is a vector of bytes holding machine code,
jump tables, and (in the future) other read-only data. Some clients,
notably Firefox's Wasm compiler, needs to separate the machine code
from the data in order to insert more code directly after the code
generated by Cranelift.
To make such separation possible, we record more information about the
emitted bytes: the sizes of each of the sections of code, jump tables,
and read-only data, as well as the locations within the code that
reference (PC-relatively) the jump tables and read-only data.
This follows the rest of the code base data structures, where we have a
mutable data structure builder; once the data structure is constructed,
it's immutable.
This also makes the definition of instructions easier, and it paves the
way for defining immediate variants.
