f22461b4b3
Switch to the new domtree.cfg_postorder() which returns a reference to a pre-computed post-order instead of allocating memory and computing a new post-order.
68 lines
2.2 KiB
Rust
68 lines
2.2 KiB
Rust
//! A simple GVN pass.
|
|
|
|
use flowgraph::ControlFlowGraph;
|
|
use dominator_tree::DominatorTree;
|
|
use ir::{Cursor, InstructionData, Function, Inst, Opcode};
|
|
use std::collections::HashMap;
|
|
|
|
/// Test whether the given opcode is unsafe to even consider for GVN.
|
|
fn trivially_unsafe_for_gvn(opcode: Opcode) -> bool {
|
|
opcode.is_call() || opcode.is_branch() || opcode.is_terminator() || opcode.is_return() ||
|
|
opcode.can_trap()
|
|
}
|
|
|
|
/// Perform simple GVN on `func`.
|
|
///
|
|
pub fn do_simple_gvn(func: &mut Function, cfg: &mut ControlFlowGraph) {
|
|
let mut visible_values: HashMap<InstructionData, Inst> = HashMap::new();
|
|
|
|
let domtree = DominatorTree::with_function(func, &cfg);
|
|
|
|
// Visit EBBs in a reverse post-order.
|
|
let mut pos = Cursor::new(&mut func.layout);
|
|
|
|
for &ebb in domtree.cfg_postorder().iter().rev() {
|
|
pos.goto_top(ebb);
|
|
|
|
while let Some(inst) = pos.next_inst() {
|
|
let opcode = func.dfg[inst].opcode();
|
|
|
|
// Resolve aliases, particularly aliases we created earlier.
|
|
func.dfg.resolve_aliases_in_arguments(inst);
|
|
|
|
if trivially_unsafe_for_gvn(opcode) {
|
|
continue;
|
|
}
|
|
|
|
// TODO: Implement simple redundant-load elimination.
|
|
if opcode.can_store() {
|
|
continue;
|
|
}
|
|
if opcode.can_load() {
|
|
continue;
|
|
}
|
|
|
|
let key = func.dfg[inst].clone();
|
|
let entry = visible_values.entry(key);
|
|
use std::collections::hash_map::Entry::*;
|
|
match entry {
|
|
Occupied(mut entry) => {
|
|
if domtree.dominates(*entry.get(), inst, &pos.layout) {
|
|
func.dfg.replace_with_aliases(inst, *entry.get());
|
|
pos.remove_inst_and_step_back();
|
|
} else {
|
|
// The prior instruction doesn't dominate inst, so it
|
|
// won't dominate any subsequent instructions we'll
|
|
// visit, so just replace it.
|
|
*entry.get_mut() = inst;
|
|
continue;
|
|
}
|
|
}
|
|
Vacant(entry) => {
|
|
entry.insert(inst);
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|