Add Windows support for the single-pass backend.

[losfair]

Currently the single-pass backend assumes a System V calling convention, but Windows has its own one instead of System V.

It shouldn't be hard to support Windows' calling convention on x64. The following changes need to be made:

1. In machine.rs:

   • Add a Windows version of get_param_location that defines where function arguments should be located.
   • Add Windows definitions for REGS in pick_gpr and pick_temp_gpr.
   • Remove the use of XMM6 and XMM7 in pick_xmm (since Windows requires them to be preserved, while we cannot decide whether this needs to be done in a single pass).

2. In codegen_x64.rs:

   • Add a Windows version of CONSTRUCT_STACK_AND_CALL_WASM.
   • Rename emit_call_sysv to emit_call.
   • Allocate 32 bytes of "shadow space" before executing call in emit_call.

EDIT: Windows support is more complex to implement than I thought before because the Windows x64 ABI does not guarantee a 128-byte red zone, which the code generator currently uses everywhere. Fixed

[CryZe]

What's the status of this? I'm at least partially blocked by this. (With enough mentoring I could maybe do this, but I already looked at this a while ago and it seems like there's one huge asm block (for setup or something, I don't remember), where I did have absolutely no clue how that would need to be modified for Windows compatibility).

[syrusakbary]

@CryZe we will be super happy to mentor 🙂

@losfair can you provide some assistance on what would be needed to be modified on the asm block to support Windows, so we can unblock this issue?

[losfair]

@CryZe

Thanks for your interest in helping with this issue!

For the CONSTRUCT_STACK_AND_CALL_WASM ASM block, what it does is basically loading the 8-byte arguments from stack_top to stack_base (the names are a bit misleading though) into the corresponding calling convention argument locations (either register or stack), ensuring ABI constraints (on x86_64 Linux for example it is to ensure the stack is 16-byte aligned), and then calling the provided compiled WebAssembly function with a Ctx as the first parameter.

To support windows, what needs to be done is to figure out where each argument should be placed before a function call and what extra ABI constraints need to be satisfied. So instead of mov-ing arguments to rdi, rsi, rdx, rcx, r8, r9 and then to the stack in order, the locations defined by the Windows ABI (https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention?view=vs-2019) must be used. And also, the 32-byte shadow space needs to be allocated.

[repi]

Any updates for this? Would be nice to have it available on Windows

[syrusakbary]

Our work in the refactor will make this much easier to happen

[syrusakbary]

We have done this in #2574. Closing the issue