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//! Optimisations related to mem_copy.
//! - replace a `store` directly from a `load` with a `mem_copy_val`.
use rustc_hash::{FxHashMap, FxHashSet};
use crate::{
AnalysisResults, Block, Context, Function, Instruction, IrError, LocalVar, Pass,
PassMutability, ScopedPass, Value, ValueDatum,
};
pub const MEMCPYOPT_NAME: &str = "memcpyopt";
pub fn create_memcpyopt_pass() -> Pass {
Pass {
name: MEMCPYOPT_NAME,
descr: "Memcopy optimization.",
deps: vec![],
runner: ScopedPass::FunctionPass(PassMutability::Transform(mem_copy_opt)),
}
}
pub fn mem_copy_opt(
context: &mut Context,
_analyses: &AnalysisResults,
function: Function,
) -> Result<bool, IrError> {
let mut modified = false;
modified |= local_copy_prop(context, function)?;
modified |= load_store_to_memcopy(context, function)?;
Ok(modified)
}
struct InstInfo {
// The block in which an instruction is
block: Block,
// Relative (use only for comparison) position of instruction in `block`.
pos: usize,
}
/// Copy propagation of loads+store (i.e., a memory copy) requires
/// a data-flow analysis. Until then, we do a safe approximation,
/// restricting to when every related instruction is in the same block.
fn local_copy_prop(context: &mut Context, function: Function) -> Result<bool, IrError> {
let mut loads_map = FxHashMap::<LocalVar, Vec<Value>>::default();
let mut stores_map = FxHashMap::<LocalVar, Vec<Value>>::default();
let mut instr_info_map = FxHashMap::<Value, InstInfo>::default();
let mut asm_uses = FxHashSet::<LocalVar>::default();
fn get_local(context: &Context, val: Value) -> Option<LocalVar> {
match val.get_instruction(context) {
Some(Instruction::GetLocal(local)) => Some(*local),
Some(Instruction::GetElemPtr { base, .. }) => get_local(context, *base),
_ => None,
}
}
for (pos, (block, inst)) in function.instruction_iter(context).enumerate() {
let info = || InstInfo { block, pos };
let inst_e = inst.get_instruction(context).unwrap();
match inst_e {
Instruction::Load(src_val_ptr) => {
if let Some(local) = get_local(context, *src_val_ptr) {
loads_map
.entry(local)
.and_modify(|loads| loads.push(inst))
.or_insert(vec![inst]);
instr_info_map.insert(inst, info());
}
}
Instruction::Store { dst_val_ptr, .. } => {
if let Some(local) = get_local(context, *dst_val_ptr) {
stores_map
.entry(local)
.and_modify(|stores| stores.push(inst))
.or_insert(vec![inst]);
instr_info_map.insert(inst, info());
}
}
Instruction::AsmBlock(_, args) => {
for arg in args {
if let Some(arg) = arg.initializer {
if let Some(local) = get_local(context, arg) {
asm_uses.insert(local);
}
}
}
}
_ => (),
}
}
let mut to_delete = FxHashSet::<Value>::default();
let candidates: FxHashMap<LocalVar, LocalVar> = function
.instruction_iter(context)
.enumerate()
.filter_map(|(pos, (block, instr_val))| {
instr_val
.get_instruction(context)
.and_then(|instr| {
// Is the instruction a Store?
if let Instruction::Store {
dst_val_ptr,
stored_val,
} = instr
{
get_local(context, *dst_val_ptr).and_then(|dst_local| {
stored_val
.get_instruction(context)
.map(|src_instr| (src_instr, stored_val, dst_local))
})
} else {
None
}
})
.and_then(|(src_instr, stored_val, dst_local)| {
// Is the Store source a Load?
if let Instruction::Load(src_val_ptr) = src_instr {
get_local(context, *src_val_ptr)
.map(|src_local| (stored_val, dst_local, src_local))
} else {
None
}
})
.and_then(|(src_load, dst_local, src_local)| {
let (temp_empty1, temp_empty2, temp_empty3) = (vec![], vec![], vec![]);
let dst_local_stores = stores_map.get(&dst_local).unwrap_or(&temp_empty1);
let src_local_stores = stores_map.get(&src_local).unwrap_or(&temp_empty2);
let dst_local_loads = loads_map.get(&dst_local).unwrap_or(&temp_empty3);
// This must be the only store of dst_local.
if dst_local_stores.len() != 1 || dst_local_stores[0] != instr_val
||
// All stores of src_local must be in the same block, prior to src_load.
!src_local_stores.iter().all(|store_val|{
let instr_info = instr_info_map.get(store_val).unwrap();
let src_load_info = instr_info_map.get(src_load).unwrap();
instr_info.block == block && instr_info.pos < src_load_info.pos
})
||
// All loads of dst_local must be after this instruction, in the same block.
!dst_local_loads.iter().all(|load_val| {
let instr_info = instr_info_map.get(load_val).unwrap();
instr_info.block == block && instr_info.pos > pos
})
// We don't deal with ASM blocks.
|| asm_uses.contains(&dst_local)
// We don't deal part copies.
|| dst_local.get_type(context) != src_local.get_type(context)
{
None
} else {
to_delete.insert(instr_val);
Some((dst_local, src_local))
}
})
})
.collect();
// if we have A replaces B and B replaces C, then A must replace C also.
fn closure(
candidates: &FxHashMap<LocalVar, LocalVar>,
src_local: &LocalVar,
) -> Option<LocalVar> {
candidates
.get(src_local)
.map(|replace_with| closure(candidates, replace_with).unwrap_or(*replace_with))
}
// Because we can't borrow context for both iterating and replacing, do it in 2 steps.
let replaces: Vec<_> = function
.instruction_iter(context)
.filter_map(|(_block, value)| match value.get_instruction(context) {
Some(Instruction::GetLocal(local)) => {
closure(&candidates, local).map(|replace_with| (value, *local, replace_with))
}
_ => None,
})
.collect();
for (value, redundant_var, replacement_var) in replaces.into_iter() {
// Be sure to propagate the mutability of the original local variable to the copy.
if redundant_var.is_mutable(context) {
replacement_var.set_mutable(context, true);
}
value.replace(
context,
ValueDatum::Instruction(Instruction::GetLocal(replacement_var)),
);
}
// Delete stores to the replaced local.
let blocks: Vec<Block> = function.block_iter(context).collect();
for block in blocks {
block.remove_instructions(context, |value| to_delete.contains(&value));
}
Ok(true)
}
fn load_store_to_memcopy(context: &mut Context, function: Function) -> Result<bool, IrError> {
// Find any `store`s of `load`s. These can be replaced with `mem_copy` and are especially
// important for non-copy types on architectures which don't support loading them.
let candidates = function
.instruction_iter(context)
.filter_map(|(block, instr_val)| {
instr_val
.get_instruction(context)
.and_then(|instr| {
// Is the instruction a Store?
if let Instruction::Store {
dst_val_ptr,
stored_val,
} = instr
{
stored_val
.get_instruction(context)
.map(|src_instr| (src_instr, dst_val_ptr))
} else {
None
}
})
.and_then(|(src_instr, dst_val_ptr)| {
// Is the Store source a Load?
if let Instruction::Load(src_val_ptr) = src_instr {
Some((block, instr_val, *dst_val_ptr, *src_val_ptr))
} else {
None
}
})
.and_then(|candidate @ (_block, _store_val, dst_ptr, _src_ptr)| {
// XXX TEMPORARY 'FIX':
//
// We need to do proper aliasing analysis for this pass. It's possible to have
// the following:
//
// X = load ptr A -- dereference A
// store Y to ptr A -- mutate A
// store X to ptr B -- store original A to B
//
// Which this pass would convert to:
//
// -- DCE the load
// store Y to ptr A -- mutate A
// memcpy ptr B, ptr A -- copy _mutated_ A to B
//
// To temporarily avoid this problem we're not going to mem_copy copy types and
// assume (oh, no) that larger types subject to this pass aren't mutated. This
// only works for now because it has always worked in the past, but there are
// no guarantees this couldn't flare up somewhere.
dst_ptr
.get_type(context)
.and_then(|ptr_ty| ptr_ty.get_pointee_type(context))
.map(|ty| super::target_fuel::is_demotable_type(context, &ty))?
.then_some(candidate)
})
})
.collect::<Vec<_>>();
if candidates.is_empty() {
return Ok(false);
}
for (block, store_val, dst_val_ptr, src_val_ptr) in candidates {
let mem_copy_val = Value::new_instruction(
context,
Instruction::MemCopyVal {
dst_val_ptr,
src_val_ptr,
},
);
block.replace_instruction(context, store_val, mem_copy_val)?;
}
Ok(true)
}