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optimize refcount inc/dec (tail recursion)

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Folkert 2021-01-22 15:24:49 +01:00
parent 191dee73df
commit 6344771a0c
1 changed files with 35 additions and 11 deletions
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46
compiler/gen/src/llvm/refcounting.rs
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@ -926,6 +926,10 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
false
})();
// to increment/decrement the cons-cell itself
let refcount_ptr = PointerToRefcount::from_ptr_to_data(env, value_ptr);
let call_mode = mode_to_call_mode(fn_val, mode);
let ctx = env.context;
let cont_block = ctx.append_basic_block(parent, "cont");
if is_nullable {
@ -988,6 +992,11 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
)
.into_pointer_value();
// defer actually performing the refcount modifications until after the current cell has
// been decremented, see below
let mut deferred_rec = Vec::new_in(env.arena);
let mut deferred_nonrec = Vec::new_in(env.arena);
for (i, field_layout) in field_layouts.iter().enumerate() {
if let Layout::RecursivePointer = field_layout {
// this field has type `*i64`, but is really a pointer to the data we want
@ -1010,13 +1019,8 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
union_type.ptr_type(AddressSpace::Generic).into(),
);
// recursively decrement the field
let call_name = pick("recursive_tag_increment", "recursive_tag_decrement");
call_help(env, fn_val, mode, recursive_field_ptr, call_name);
deferred_rec.push(recursive_field_ptr);
} else if field_layout.contains_refcounted() {
// TODO this loads the whole field onto the stack;
// that's wasteful if e.g. the field is a big record, where only
// some fields are actually refcounted.
let elem_pointer = env
.builder
.build_struct_gep(struct_ptr, i as u32, "gep_recursive_pointer")
@ -1027,11 +1031,31 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
pick("increment_struct_field", "decrement_struct_field"),
);
modify_refcount_layout(env, parent, layout_ids, mode, field, field_layout);
deferred_nonrec.push((field, field_layout));
}
}
env.builder.build_unconditional_branch(merge_block);
// OPTIMIZATION
//
// We really would like `inc/dec` to be tail-recursive; it gives roughly a 2X speedup on linked
// lists. To achieve it, we must first load all fields that we want to inc/dec (done above)
// and store them on the stack, then modify (and potentially free) the current cell, then
// actually inc/dec the fields.
refcount_ptr.modify(call_mode, &layout, env);
for (field, field_layout) in deferred_nonrec {
modify_refcount_layout(env, parent, layout_ids, mode, field, field_layout);
}
let call_name = pick("recursive_tag_increment", "recursive_tag_decrement");
for ptr in deferred_rec {
// recursively decrement the field
let call = call_help(env, fn_val, mode, ptr, call_name);
call.set_tail_call(true);
}
// this function returns void
builder.build_return(None);
cases.push((
env.context.i64_type().const_int(tag_id as u64, false),
@ -1062,8 +1086,6 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
env.builder.position_at_end(merge_block);
// increment/decrement the cons-cell itself
let refcount_ptr = PointerToRefcount::from_ptr_to_data(env, value_ptr);
let call_mode = mode_to_call_mode(fn_val, mode);
refcount_ptr.modify(call_mode, &layout, env);
// this function returns void
@ -1093,7 +1115,7 @@ fn call_help<'a, 'ctx, 'env>(
mode: Mode,
value: BasicValueEnum<'ctx>,
call_name: &str,
) {
) -> inkwell::values::CallSiteValue<'ctx> {
let call = match mode {
Mode::Inc(inc_amount) => {
let rc_increment = ptr_int(env.context, env.ptr_bytes).const_int(inc_amount, false);
@ -1105,6 +1127,8 @@ fn call_help<'a, 'ctx, 'env>(
};
call.set_call_convention(FAST_CALL_CONV);
call
}
fn modify_refcount_union<'a, 'ctx, 'env>(