0313e30d76
This commit started off by deleting the `cranelift_codegen::settings` reexport in the `wasmtime-environ` crate and then basically played whack-a-mole until everything compiled again. The main result of this is that the `wasmtime-*` family of crates have generally less of a dependency on the `TargetIsa` trait and type from Cranelift. While the dependency isn't entirely severed yet this is at least a significant start. This commit is intended to be largely refactorings, no functional changes are intended here. The refactorings are: * A `CompilerBuilder` trait has been added to `wasmtime_environ` which server as an abstraction used to create compilers and configure them in a uniform fashion. The `wasmtime::Config` type now uses this instead of cranelift-specific settings. The `wasmtime-jit` crate exports the ability to create a compiler builder from a `CompilationStrategy`, which only works for Cranelift right now. In a cranelift-less build of Wasmtime this is expected to return a trait object that fails all requests to compile. * The `Compiler` trait in the `wasmtime_environ` crate has been souped up with a number of methods that Wasmtime and other crates needed. * The `wasmtime-debug` crate is now moved entirely behind the `wasmtime-cranelift` crate. * The `wasmtime-cranelift` crate is now only depended on by the `wasmtime-jit` crate. * Wasm types in `cranelift-wasm` no longer contain their IR type, instead they only contain the `WasmType`. This is required to get everything to align correctly but will also be required in a future refactoring where the types used by `cranelift-wasm` will be extracted to a separate crate. * I moved around a fair bit of code in `wasmtime-cranelift`. * Some gdb-specific jit-specific code has moved from `wasmtime-debug` to `wasmtime-jit`.
Lightbeam
Lightbeam is an optimising one-pass streaming compiler for WebAssembly, intended for use in Wasmtime.
Quality of output
Already - with a very small number of relatively simple optimisation rules - Lightbeam produces surprisingly high-quality output considering how restricted it is. It even produces better code than Cranelift, Firefox or both for some workloads. Here's a very simple example, this recursive fibonacci function in Rust:
fn fib(n: i32) -> i32 {
if n == 0 || n == 1 {
1
} else {
fib(n - 1) + fib(n - 2)
}
}
When compiled with optimisations enabled, rustc will produce the following WebAssembly:
(module
(func $fib (param $p0 i32) (result i32)
(local $l1 i32)
(set_local $l1
(i32.const 1))
(block $B0
(br_if $B0
(i32.lt_u
(get_local $p0)
(i32.const 2)))
(set_local $l1
(i32.const 1))
(loop $L1
(set_local $l1
(i32.add
(call $fib
(i32.add
(get_local $p0)
(i32.const -1)))
(get_local $l1)))
(br_if $L1
(i32.gt_u
(tee_local $p0
(i32.add
(get_local $p0)
(i32.const -2)))
(i32.const 1)))))
(get_local $l1)))
Firefox's optimising compiler produces the following assembly (labels cleaned up somewhat):
fib:
sub rsp, 0x18
cmp qword ptr [r14 + 0x28], rsp
jae stack_overflow
mov dword ptr [rsp + 0xc], edi
cmp edi, 2
jae .Lelse
mov eax, 1
mov dword ptr [rsp + 8], eax
jmp .Lreturn
.Lelse:
mov dword ptr [rsp + 0xc], edi
mov eax, 1
mov dword ptr [rsp + 8], eax
.Lloop:
mov edi, dword ptr [rsp + 0xc]
add edi, -1
call 0
mov ecx, dword ptr [rsp + 8]
add ecx, eax
mov dword ptr [rsp + 8], ecx
mov ecx, dword ptr [rsp + 0xc]
add ecx, -2
mov dword ptr [rsp + 0xc], ecx
cmp ecx, 1
ja .Lloop
.Lreturn:
mov eax, dword ptr [rsp + 8]
nop
add rsp, 0x18
ret
Cranelift with optimisations enabled produces similar:
fib:
push rbp
mov rbp, rsp
sub rsp, 0x20
mov qword ptr [rsp + 0x10], rdi
mov dword ptr [rsp + 0x1c], esi
mov eax, 1
mov dword ptr [rsp + 0x18], eax
mov eax, dword ptr [rsp + 0x1c]
cmp eax, 2
jb .Lreturn
movabs rax, 0
mov qword ptr [rsp + 8], rax
.Lloop:
mov eax, dword ptr [rsp + 0x1c]
add eax, -1
mov rcx, qword ptr [rsp + 8]
mov rdx, qword ptr [rsp + 0x10]
mov rdi, rdx
mov esi, eax
call rcx
mov ecx, dword ptr [rsp + 0x18]
add eax, ecx
mov dword ptr [rsp + 0x18], eax
mov eax, dword ptr [rsp + 0x1c]
add eax, -2
mov dword ptr [rsp + 0x1c], eax
mov eax, dword ptr [rsp + 0x1c]
cmp eax, 1
ja .Lloop
.Lreturn:
mov eax, dword ptr [rsp + 0x18]
add rsp, 0x20
pop rbp
ret
Whereas Lightbeam produces smaller code with far fewer memory accesses than both (and fewer blocks than Firefox's output):
fib:
cmp esi, 2
mov eax, 1
jb .Lreturn
mov eax, 1
.Lloop:
mov rcx, rsi
add ecx, 0xffffffff
push rsi
push rax
push rax
mov rsi, rcx
call fib
add eax, [rsp + 8]
mov rcx, [rsp + 0x10]
add ecx, 0xfffffffe
cmp ecx, 1
mov rsi, rcx
lea rsp, [rsp + 0x18]
ja .Lloop
.Lreturn:
ret
Now obviously I'm not advocating for replacing Firefox's optimising compiler with Lightbeam since the latter can only really produce better code when receiving optimised WebAssembly (and so debug-mode or hand-written WebAssembly may produce much worse output). However, this shows that even with the restrictions of a streaming compiler it's absolutely possible to produce high-quality assembly output. For the assembly above, the Lightbeam output runs within 15% of native speed. This is paramount for one of Lightbeam's intended usecases for real-time systems that want good runtime performance but cannot tolerate compiler bombs.
Specification compliance
Lightbeam passes 100% of the specification test suite, but that doesn't necessarily mean that it's 100% specification-compliant. Hopefully as we run a fuzzer against it we can find any issues and get Lightbeam to a state where it can be used in production.
Getting involved
You can file issues in the Wasmtime issue tracker. If you want to get involved jump into the Bytecode Alliance Zulip and someone can direct you to the right place. I wish I could say "the most useful thing you can do is play with it and open issues where you find problems" but until it passes the spec suite that won't be very helpful.