Define a new "wasm-multivalue" calling convention

[alexcrichton]

This commit is an attempt to define a new calling convention for WebAssembly which I've provisionally decided to call "wasm-multivalue". This new calling convention is inspired by discussions on #247 and #88, and the primary motivation of this calling convention is to be able to use multi-return signatures for intrinsics/functions in the component model.

This is a definition of a new, parallel, calling convention to the existing, now called "C", calling convention. The intention is that this avoids breakage to existing programs. The new calling convention is opt-in, requiring an annotation. The multivalue target feature enables usage of "wasm-multivalue", but enabling or disabling multivalue-the-target-feature has no effect on the "C" calling convention.

The technical definition of this new "wasm-multivalue" calling convention is to alter the previous calling convention in three ways:

• Primarily, struct returns are now returned directly if all fields are (optionally recursively) scalars. This means that returning a struct-by-value gets translated to multiple return values.

• To account for Pass small structs in parameters instead of memory #88 this new calling convention additionally passes two-scalar-argument struct definitions directly instead of indirectly, matching what native calling conventions do for example.

• Finally, __int128 and long double return values are now returned directly instead of indirectly.

On an implementation side of things, what I'd like to do is to gain consensus on this calling convention definition in the tool-conventions repository as a first step. Once the definition is settled I plan to go to LLVM/Clang to implement this calling convention, and after that I plan to go to the Rust compiler and implement the calling convention there. My plan for Rust is to implement extern "wasm-multivalue" fn(..) syntactically, and for C to use
__attribute__((wasm_multivalue)) as the opt-in. Note that this is all bikesheddable, of course.

[tlively]

SGTM! I like that making this opt-in lets us avoid the question of how large a struct should be allowed to be returned by value.