bbe056bf9d
* Make passes assert their dependencies consistently. This avoids ambiguity about whose responsibility it is to run to compute cfg, domtree, and loop_analysis data. * Reset the `valid` flag in DominatorTree's `clear()`. * Remove the redundant assert from DominatorTree::with_function. * Remove the message strings from obvious asserts. This avoids having them spill out into multiple lines. * Refactor calls to `compute` on `Context` objects into helper functions.
70 lines
2.4 KiB
Rust
70 lines
2.4 KiB
Rust
//! A simple GVN pass.
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use flowgraph::ControlFlowGraph;
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use dominator_tree::DominatorTree;
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use ir::{Cursor, CursorBase, InstructionData, Function, Inst, Opcode, Type};
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use std::collections::HashMap;
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/// Test whether the given opcode is unsafe to even consider for GVN.
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fn trivially_unsafe_for_gvn(opcode: Opcode) -> bool {
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opcode.is_call() || opcode.is_branch() || opcode.is_terminator() || opcode.is_return() ||
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opcode.can_trap() || opcode.other_side_effects()
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}
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/// Perform simple GVN on `func`.
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///
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pub fn do_simple_gvn(func: &mut Function, cfg: &mut ControlFlowGraph, domtree: &mut DominatorTree) {
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debug_assert!(cfg.is_valid());
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debug_assert!(domtree.is_valid());
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let mut visible_values: HashMap<(InstructionData, Type), Inst> = HashMap::new();
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// Visit EBBs in a reverse post-order.
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let mut pos = Cursor::new(&mut func.layout);
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for &ebb in domtree.cfg_postorder().iter().rev() {
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pos.goto_top(ebb);
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while let Some(inst) = pos.next_inst() {
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let opcode = func.dfg[inst].opcode();
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let ctrl_typevar = func.dfg.ctrl_typevar(inst);
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// Resolve aliases, particularly aliases we created earlier.
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func.dfg.resolve_aliases_in_arguments(inst);
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if trivially_unsafe_for_gvn(opcode) {
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continue;
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}
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// TODO: Implement simple redundant-load elimination.
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if opcode.can_store() {
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continue;
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}
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if opcode.can_load() {
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continue;
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}
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let key = (func.dfg[inst].clone(), ctrl_typevar);
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let entry = visible_values.entry(key);
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use std::collections::hash_map::Entry::*;
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match entry {
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Occupied(mut entry) => {
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if domtree.dominates(*entry.get(), inst, pos.layout) {
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func.dfg.replace_with_aliases(inst, *entry.get());
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pos.remove_inst_and_step_back();
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} else {
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// The prior instruction doesn't dominate inst, so it
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// won't dominate any subsequent instructions we'll
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// visit, so just replace it.
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*entry.get_mut() = inst;
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continue;
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}
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}
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Vacant(entry) => {
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entry.insert(inst);
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}
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}
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}
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}
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}
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