Cranelift: Do not have any callee-save registers for the tail calling convention

[fitzgen]

That is, mark all allocatable registers as clobbered by tail calls.

Additionally, use (basically) all allocatable registers for passing arguments and returns. ("Basically" because we have to leave at least one register available to hold the callee address for indirect calls or else we'll get regalloc constraint errors.)

I could get three more argument registers on riscv if I refactored compute_arg_locs and/or did something similar to aarch64's compute_arg_locs_tail but it didn't seem worth the hassle.

[alexcrichton]

In the future it's expected that all wasm calls will move to the tail convention, right? If that's the case would it be possible to run a version of this through sightglass to see if it affects runtime benchmarks?

[fitzgen]

In the future it's expected that all wasm calls will move to the tail convention, right? If that's the case would it be possible to run a version of this through sightglass to see if it affects runtime benchmarks?

That is the eventual plan, and we will definitely do a sightglass run before switching over, but we aren't quite ready to do it yet.

[jameysharp]

I don't much like the structure of this code; I know you've said you don't either. But this PR doesn't make it worse, so let's go with it.

I spent some time trying to figure out if there was more we could share between the register allocator configuration and these ABI bits, given that all allocatable registers are supposed to be clobbered in this ABI, but that didn't look straightforward.

As future work, I think there's something we can do with RA2's PRegSet to represent all of these different ABI-relevant sets of registers compactly; there's a popcount-like trick for finding the index of the Nth set bit that I think covers most of the uses we need here. (See the "Select the bit position (from the most-significant bit) with the given count (rank)" section of https://graphics.stanford.edu/~seander/bithacks.html.) But that's not necessary to land this.