Testing cranelift on RISC-V

[wakabat]

Hey, so we began testing wasmer's cranelift compiler on riscv64 platform as well. And just want to share something we find.

Unaligned load / store

We disallow unaligned loads / stores in our platform. So like the singlepass compiler, we hit on unaligned loads / stores immediately. I put together a fix that works for our tests. However I'm not sure if wasmer can simply include this fix. There are performance implications. Maybe it makes sense to add a feature that enables this optionally?

Difference in trapping

I notice trapping, or at least trapping triggered by some opcodes, are implemented by calling wasmer_vm_raise_trap in both LLVM compiler and singlepass compiler:

• wasmer/lib/compiler-llvm/src/translator/code.rs

  Lines 2471 to 2475 in bd91673

  	err!(self.builder.build_call(
  	self.intrinsics.throw_trap,
  	&[self.intrinsics.trap_unreachable.into()],
  	"throw",
  	));

• wasmer/lib/compiler-singlepass/src/codegen.rs

  Lines 3407 to 3433 in bd91673

  	self.machine.move_location(
  	Size::S64,
  	Location::Memory(
  	self.machine.get_vmctx_reg(),
  	self.vmoffsets
  	.vmctx_builtin_function(VMBuiltinFunctionIndex::get_raise_trap_index())
  	as i32,
  	),
  	Location::GPR(self.machine.get_gpr_for_call()),
  	)?;
  	self.emit_call_native(
  	|this| {
  	this.machine
  	.emit_call_register(this.machine.get_gpr_for_call())
  	},
  	// [trap_code]
  	[(
  	Location::Imm32(TrapCode::UnreachableCodeReached as u32),
  	CanonicalizeType::None,
  	)]
  	.iter()
  	.cloned(),
  	[WpType::I32].iter().cloned(),
  	iter::empty(),
  	NativeCallType::Unreachable,
  	)?;

But the cranelift compiler traps simply, and relies on OS-level signal handler to process the trap.

We are running wasmer in a special environment, which can be thought as a bare-metal embedded system. So OS signal handler is not available. So the question is: is there any reason the cranelift compiler simply traps? Can we change the cranelift compiler to call wasmer_vm_raise_trap as well?

As always, thanks for the help!