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Update 'threads-xform' for LLVM 9 #1675

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Jul 22, 2019
Merged

Update 'threads-xform' for LLVM 9 #1675

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106 changes: 81 additions & 25 deletions crates/threads-xform/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -78,25 +78,57 @@ impl Config {
///
/// More and/or less may happen here over time, stay tuned!
pub fn run(&self, module: &mut Module) -> Result<(), Error> {
let memory = update_memory(module, self.maximum_memory)?;
let segments = switch_data_segments_to_passive(module, memory)?;
let stack_pointer = find_stack_pointer(module)?;

let memory = find_memory(module)?;
let addr = inject_thread_id_counter(module, memory)?;
let zero = InitExpr::Value(Value::I32(0));
let globals = Globals {
thread_id: module.globals.add_local(ValType::I32, true, zero),
thread_tcb: module.globals.add_local(ValType::I32, true, zero),
};
let addr = inject_thread_id_counter(module, memory)?;
start_with_init_memory(

// There was an "inflection point" at the LLVM 9 release where LLD
// started having better support for producing binaries capable of being
// used with multi-threading. Prior to LLVM 9 (e.g. nightly releases
// before July 2019 basically) we had to sort of paper over a lot of
// support that hadn't been added to LLD. With LLVM 9 and onwards though
// we expect Rust binaries to be pretty well formed if prepared for
// threading when they come out of LLD. This `if` statement basically
// switches on these two cases, figuring out if we're "old style" or
// "new style".
let mem = module.memories.get_mut(memory);
let memory_init = if mem.shared {
let prev_max = mem.maximum.unwrap();
assert!(mem.import.is_some());
mem.maximum = Some(cmp::max(self.maximum_memory / PAGE_SIZE, prev_max));
assert!(mem.data.is_empty());

let init_memory = module
.exports
.iter()
.find(|e| e.name == "__wasm_init_memory")
.ok_or_else(|| format_err!("failed to find `__wasm_init_memory`"))?;
let init_memory_id = match init_memory.item {
walrus::ExportItem::Function(f) => f,
_ => bail!("`__wasm_init_memory` must be a function"),
};
let export_id = init_memory.id();
module.exports.delete(export_id);
InitMemory::Call(init_memory_id)
} else {
update_memory(module, memory, self.maximum_memory)?;
InitMemory::Segments(switch_data_segments_to_passive(module, memory)?)
};
inject_start(
module,
&segments,
memory_init,
&globals,
addr,
stack_pointer,
self.thread_stack_size,
memory,
);

implement_thread_intrinsics(module, &globals)?;
Ok(())
}
Expand Down Expand Up @@ -124,15 +156,20 @@ fn switch_data_segments_to_passive(
Ok(ret)
}

fn update_memory(module: &mut Module, max: u32) -> Result<MemoryId, Error> {
assert!(max % PAGE_SIZE == 0);
let mut memories = module.memories.iter_mut();
fn find_memory(module: &mut Module) -> Result<MemoryId, Error> {
let mut memories = module.memories.iter();
let memory = memories
.next()
.ok_or_else(|| format_err!("currently incompatible with no memory modules"))?;
if memories.next().is_some() {
bail!("only one memory is currently supported");
}
Ok(memory.id())
}

fn update_memory(module: &mut Module, memory: MemoryId, max: u32) -> Result<MemoryId, Error> {
assert!(max % PAGE_SIZE == 0);
let memory = module.memories.get_mut(memory);

// For multithreading if we want to use the exact same module on all
// threads we'll need to be sure to import memory, so switch it to an
Expand Down Expand Up @@ -245,9 +282,14 @@ fn find_stack_pointer(module: &mut Module) -> Result<Option<GlobalId>, Error> {
}
}

fn start_with_init_memory(
enum InitMemory {
Segments(Vec<PassiveSegment>),
Call(walrus::FunctionId),
}

fn inject_start(
module: &mut Module,
segments: &[PassiveSegment],
memory_init: InitMemory,
globals: &Globals,
addr: u32,
stack_pointer: Option<GlobalId>,
Expand Down Expand Up @@ -321,6 +363,15 @@ fn start_with_init_memory(
let sp = block.binop(BinaryOp::I32Add, sp_base, stack_size);
let set_stack_pointer = block.global_set(stack_pointer, sp);
block.expr(set_stack_pointer);

// FIXME(WebAssembly/tool-conventions#117) we probably don't want to
// duplicate drop with `if_zero_block` or otherwise just infer to drop
// all these data segments, this seems like something to synthesize in
// the linker...
for segment in module.data.iter() {
let drop = block.data_drop(segment.id());
block.expr(drop);
}
}
let if_nonzero_block = block.id();
drop(block);
Expand All @@ -330,27 +381,32 @@ fn start_with_init_memory(
// memory, however, so do that here.
let if_zero_block = {
let mut block = builder.if_else_block(Box::new([]), Box::new([]));
for segment in segments {
let zero = block.i32_const(0);
let offset = match segment.offset {
InitExpr::Global(id) => block.global_get(id),
InitExpr::Value(v) => block.const_(v),
};
let len = block.i32_const(segment.len as i32);
let init = block.memory_init(memory, segment.id, offset, zero, len);
block.expr(init);
match memory_init {
InitMemory::Segments(segments) => {
for segment in segments {
let zero = block.i32_const(0);
let offset = match segment.offset {
InitExpr::Global(id) => block.global_get(id),
InitExpr::Value(v) => block.const_(v),
};
let len = block.i32_const(segment.len as i32);
let init = block.memory_init(memory, segment.id, offset, zero, len);
block.expr(init);
let drop = block.data_drop(segment.id);
block.expr(drop);
}
}
InitMemory::Call(wasm_init_memory) => {
let call = block.call(wasm_init_memory, Box::new([]));
block.expr(call);
}
}
block.id()
};

let block = builder.if_else(thread_id_is_nonzero, if_nonzero_block, if_zero_block);
exprs.push(block);

// On all threads now memory segments are no longer needed
for segment in segments {
exprs.push(builder.data_drop(segment.id));
}

// If a start function previously existed we're done with our own
// initialization so delegate to them now.
if let Some(id) = module.start.take() {
Expand Down