T0b1/wasmtime
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Change wasm-to-host trampolines to take the values_vec size (#4192)

Browse Source 
* Change wasm-to-host trampolines to take the values_vec size

This commit changes the ABI of wasm-to-host trampolines, which are
only used right now for functions created with `Func::new`, to pass
along the size of the `values_vec` argument. Previously the trampoline
simply received `*mut ValRaw` and assumed that it was the appropriate
size. By receiving a size as well we can thread through `&mut [ValRaw]`
internally instead of `*mut ValRaw`.

The original motivation for this is that I'm planning to leverage these
trampolines for the component model for host-defined functions. Out of
an abundance of caution of making sure that everything lines up I wanted
to be able to write down asserts about the size received at runtime
compared to the size expected. This overall led me to the desire to
thread this size parameter through on the assumption that it would not
impact performance all that much.

I ran two benchmarks locally from the `call.rs` benchmark and got:

* `sync/no-hook/wasm-to-host - nop - unchecked` - no change
* `sync/no-hook/wasm-to-host - nop-params-and-results - unchecked` - 5%
  slower

This is what I roughly expected in that if nothing actually reads the
new parameter (e.g. no arguments) then threading through the parameter
is effectively otherwise free. Otherwise though accesses to the `ValRaw`
storage is now bounds-checked internally in Wasmtime instead of assuming
it's valid, leading to the 5% slowdown (~9.6ns to ~10.3ns). If this
becomes a peformance bottleneck for a particular use case then we should
be fine to remove the bounds checking here or otherwise only bounds
check in debug mode, otherwise I plan on leaving this as-is.

Of particular note this also changes the C API for `*_unchecked`
functions where the C callback now receives the size of the array as
well.

* Add docs
This commit is contained in:
Alex Crichton
2022-06-01 09:05:37 -05:00
committed by GitHub
parent 0bdd8e3510
commit f4b9020913
8 changed files with 46 additions and 26 deletions
Download Patch File Download Diff File Expand all files Collapse all files

21
crates/cranelift/src/compiler.rs
View File

@@ -516,14 +516,18 @@ impl Compiler {
let isa = &*self.isa;
let pointer_type = isa.pointer_type();
let wasm_signature = indirect_signature(isa, ty);
// The host signature has an added parameter for the `values_vec` input
// and output.
let mut host_signature = blank_sig(isa, wasmtime_call_conv(isa));
// The host signature has an added parameter for the `values_vec`
// input/output buffer in addition to the size of the buffer, in units
// of `ValRaw`.
host_signature.params.push(ir::AbiParam::new(pointer_type));
host_signature.params.push(ir::AbiParam::new(pointer_type));
// Compute the size of the values vector. The vmctx and caller vmctx are passed separately.
let value_size = mem::size_of::<u128>();
let values_vec_len = (value_size * cmp::max(ty.params().len(), ty.returns().len())) as u32;
let values_vec_len = cmp::max(ty.params().len(), ty.returns().len());
let values_vec_byte_size = u32::try_from(value_size * values_vec_len).unwrap();
let values_vec_len = u32::try_from(values_vec_len).unwrap();
let CompilerContext {
mut func_translator,
@@ -535,7 +539,7 @@ impl Compiler {
let ss = context.func.create_stack_slot(ir::StackSlotData::new(
ir::StackSlotKind::ExplicitSlot,
values_vec_len,
values_vec_byte_size,
));
let mut builder = FunctionBuilder::new(&mut context.func, func_translator.context());
@@ -559,7 +563,14 @@ impl Compiler {
let vmctx_ptr_val = block_params[0];
let caller_vmctx_ptr_val = block_params[1];
let callee_args = vec![vmctx_ptr_val, caller_vmctx_ptr_val, values_vec_ptr_val];
let callee_args = vec![
vmctx_ptr_val,
caller_vmctx_ptr_val,
values_vec_ptr_val,
builder
.ins()
.iconst(pointer_type, i64::from(values_vec_len)),
];
let new_sig = builder.import_signature(host_signature);