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Pre-generate trampoline functions (#957)

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* Refactor wasmtime_runtime::Export

Instead of an enumeration with variants that have data fields have an
enumeration where each variant has a struct, and each struct has the
data fields. This allows us to store the structs in the `wasmtime` API
and avoid lots of `panic!` calls and various extraneous matches.

* Pre-generate trampoline functions

The `wasmtime` crate supports calling arbitrary function signatures in
wasm code, and to do this it generates "trampoline functions" which have
a known ABI that then internally convert to a particular signature's ABI
and call it. These trampoline functions are currently generated
on-the-fly and are cached in the global `Store` structure. This,
however, is suboptimal for a few reasons:

* Due to how code memory is managed each trampoline resides in its own
  64kb allocation of memory. This means if you have N trampolines you're
  using N * 64kb of memory, which is quite a lot of overhead!

* Trampolines are never free'd, even if the referencing module goes
  away. This is similar to #925.

* Trampolines are a source of shared state which prevents `Store` from
  being easily thread safe.

This commit refactors how trampolines are managed inside of the
`wasmtime` crate and jit/runtime internals. All trampolines are now
allocated in the same pass of `CodeMemory` that the main module is
allocated into. A trampoline is generated per-signature in a module as
well, instead of per-function. This cache of trampolines is stored
directly inside of an `Instance`. Trampolines are stored based on
`VMSharedSignatureIndex` so they can be looked up from the internals of
the `ExportFunction` value.

The `Func` API has been updated with various bits and pieces to ensure
the right trampolines are registered in the right places. Overall this
should ensure that all trampolines necessary are generated up-front
rather than lazily. This allows us to remove the trampoline cache from
the `Compiler` type, and move one step closer to making `Compiler`
threadsafe for usage across multiple threads.

Note that as one small caveat the `Func::wrap*` family of functions
don't need to generate a trampoline at runtime, they actually generate
the trampoline at compile time which gets passed in.

Also in addition to shuffling a lot of code around this fixes one minor
bug found in `code_memory.rs`, where `self.position` was loaded before
allocation, but the allocation may push a new chunk which would cause
`self.position` to be zero instead.

* Pass the `SignatureRegistry` as an argument to where it's needed.

This avoids the need for storing it in an `Arc`.

* Ignore tramoplines for functions with lots of arguments

Co-authored-by: Dan Gohman <sunfish@mozilla.com>
This commit is contained in:
Alex Crichton
2020-03-12 16:17:48 -05:00
committed by GitHub
parent 34f768ddd5
commit 3e2be43502
23 changed files with 599 additions and 447 deletions
Download Patch File Download Diff File Expand all files Collapse all files

90
crates/jit/src/compiler.rs
View File

@@ -23,7 +23,7 @@ use wasmtime_environ::{
use wasmtime_profiling::ProfilingAgent;
use wasmtime_runtime::{
InstantiationError, SignatureRegistry, TrapRegistration, TrapRegistry, VMFunctionBody,
VMSharedSignatureIndex,
VMSharedSignatureIndex, VMTrampoline,
};
/// Select which kind of compilation to use.
@@ -53,13 +53,9 @@ pub struct Compiler {
code_memory: CodeMemory,
trap_registry: TrapRegistry,
trampoline_park: HashMap<VMSharedSignatureIndex, *const VMFunctionBody>,
signatures: SignatureRegistry,
strategy: CompilationStrategy,
cache_config: CacheConfig,
/// The `FunctionBuilderContext`, shared between trampline function compilations.
fn_builder_ctx: FunctionBuilderContext,
}
impl Compiler {
@@ -72,9 +68,7 @@ impl Compiler {
Self {
isa,
code_memory: CodeMemory::new(),
trampoline_park: HashMap::new(),
signatures: SignatureRegistry::new(),
fn_builder_ctx: FunctionBuilderContext::new(),
strategy,
trap_registry: TrapRegistry::default(),
cache_config,
@@ -103,6 +97,7 @@ impl Compiler {
) -> Result<
(
PrimaryMap<DefinedFuncIndex, *mut [VMFunctionBody]>,
HashMap<VMSharedSignatureIndex, VMTrampoline>,
PrimaryMap<DefinedFuncIndex, ir::JumpTableOffsets>,
Relocations,
Option<Vec<u8>>,
@@ -145,6 +140,8 @@ impl Compiler {
}
.map_err(SetupError::Compile)?;
// Allocate all of the compiled functions into executable memory,
// copying over their contents.
let allocated_functions =
allocate_functions(&mut self.code_memory, &compilation).map_err(|message| {
SetupError::Instantiate(InstantiationError::Resource(format!(
@@ -153,8 +150,45 @@ impl Compiler {
)))
})?;
// Create a registration value for all traps in our allocated
// functions. This registration will allow us to map a trapping PC
// value to what the trap actually means if it came from JIT code.
let trap_registration = register_traps(&allocated_functions, &traps, &self.trap_registry);
// Eagerly generate a entry trampoline for every type signature in the
// module. This should be "relatively lightweight" for most modules and
// guarantees that all functions (including indirect ones through
// tables) have a trampoline when invoked through the wasmtime API.
let mut cx = FunctionBuilderContext::new();
let mut trampolines = HashMap::new();
for sig in module.local.signatures.values() {
let index = self.signatures.register(sig);
if trampolines.contains_key(&index) {
continue;
}
// FIXME(#1303) we should be generating a trampoline for all
// functions in a module, not just those with less than 40
// arguments. Currently though cranelift dies in spec tests when one
// function has 100 arguments. This looks to be a cranelift bug, so
// let's work around it for now by skipping generating a trampoline
// for that massive function. The trampoline isn't actually needed
// at this time, and we'll hopefully get the cranelift bug fixed
// soon enough to remove this condition.
if sig.params.len() > 40 {
continue;
}
trampolines.insert(
index,
make_trampoline(
&*self.isa,
&mut self.code_memory,
&mut cx,
sig,
std::mem::size_of::<u128>(),
)?,
);
}
// Translate debug info (DWARF) only if at least one function is present.
let dbg = if debug_data.is_some() && !allocated_functions.is_empty() {
let target_config = self.isa.frontend_config();
@@ -199,6 +233,7 @@ impl Compiler {
Ok((
allocated_functions,
trampolines,
jt_offsets,
relocations,
dbg,
@@ -206,38 +241,6 @@ impl Compiler {
))
}
/// Create a trampoline for invoking a function.
pub(crate) fn get_trampoline(
&mut self,
signature: &ir::Signature,
value_size: usize,
) -> Result<*const VMFunctionBody, SetupError> {
let index = self.signatures.register(signature);
if let Some(trampoline) = self.trampoline_park.get(&index) {
return Ok(*trampoline);
}
let body = make_trampoline(
&*self.isa,
&mut self.code_memory,
&mut self.fn_builder_ctx,
signature,
value_size,
)?;
self.trampoline_park.insert(index, body);
return Ok(body);
}
/// Create and publish a trampoline for invoking a function.
pub fn get_published_trampoline(
&mut self,
signature: &ir::Signature,
value_size: usize,
) -> Result<*const VMFunctionBody, SetupError> {
let result = self.get_trampoline(signature, value_size)?;
self.publish_compiled_code();
Ok(result)
}
/// Make memory containing compiled code executable.
pub(crate) fn publish_compiled_code(&mut self) {
self.code_memory.publish();
@@ -265,13 +268,13 @@ impl Compiler {
}
/// Create a trampoline for invoking a function.
fn make_trampoline(
pub fn make_trampoline(
isa: &dyn TargetIsa,
code_memory: &mut CodeMemory,
fn_builder_ctx: &mut FunctionBuilderContext,
signature: &ir::Signature,
value_size: usize,
) -> Result<*const VMFunctionBody, SetupError> {
) -> Result<VMTrampoline, SetupError> {
let pointer_type = isa.pointer_type();
let mut wrapper_sig = ir::Signature::new(isa.frontend_config().default_call_conv);
@@ -373,14 +376,15 @@ fn make_trampoline(
let unwind_info = CompiledFunctionUnwindInfo::new(isa, &context);
Ok(code_memory
let ptr = code_memory
.allocate_for_function(&CompiledFunction {
body: code_buf,
jt_offsets: context.func.jt_offsets,
unwind_info,
})
.map_err(|message| SetupError::Instantiate(InstantiationError::Resource(message)))?
.as_ptr())
.as_ptr();
Ok(unsafe { std::mem::transmute::<*const VMFunctionBody, VMTrampoline>(ptr) })
}
fn allocate_functions(