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A couple of small refactorings to the egraph elaboration pass #7304

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Oct 20, 2023
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A couple of small refactorings to the egraph elaboration pass #7304

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6 changes: 0 additions & 6 deletions cranelift/codegen/src/egraph/cost.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -33,12 +33,6 @@ use crate::ir::Opcode;
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub(crate) struct Cost(u32);
impl Cost {
pub(crate) fn at_level(&self, loop_level: usize) -> Cost {
let loop_level = std::cmp::min(2, loop_level);
let multiplier = 1u32 << ((10 * loop_level) as u32);
Cost(self.0.saturating_mul(multiplier)).finite()
}

pub(crate) fn infinity() -> Cost {
// 2^32 - 1 is, uh, pretty close to infinite... (we use `Cost`
// only for heuristics and always saturate so this suffices!)
Expand Down
49 changes: 21 additions & 28 deletions cranelift/codegen/src/egraph/elaborate.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ use super::Stats;
use crate::dominator_tree::DominatorTree;
use crate::fx::FxHashSet;
use crate::ir::{Block, Function, Inst, Value, ValueDef};
use crate::loop_analysis::{Loop, LoopAnalysis, LoopLevel};
use crate::loop_analysis::{Loop, LoopAnalysis};
use crate::scoped_hash_map::ScopedHashMap;
use crate::trace;
use crate::unionfind::UnionFind;
Expand Down Expand Up @@ -211,26 +211,24 @@ impl<'a> Elaborator<'a> {
// at this point, only the side-effecting skeleton),
// then it must be computed and thus we give it zero
// cost.
ValueDef::Result(inst, _) if self.func.layout.inst_block(inst).is_some() => {
best[value] = (Cost::zero(), value);
}
ValueDef::Result(inst, _) => {
trace!(" -> value {}: result, computing cost", value);
let inst_data = &self.func.dfg.insts[inst];
let loop_level = self
.func
.layout
.inst_block(inst)
.map(|block| self.loop_analysis.loop_level(block))
.unwrap_or(LoopLevel::root());
// N.B.: at this point we know that the opcode is
// pure, so `pure_op_cost`'s precondition is
// satisfied.
let cost = self.func.dfg.inst_values(inst).fold(
pure_op_cost(inst_data.opcode()).at_level(loop_level.level()),
|cost, value| cost + best[value].0,
);
best[value] = (cost, value);
if let Some(_) = self.func.layout.inst_block(inst) {
best[value] = (Cost::zero(), value);
} else {
trace!(" -> value {}: result, computing cost", value);
let inst_data = &self.func.dfg.insts[inst];
// N.B.: at this point we know that the opcode is
// pure, so `pure_op_cost`'s precondition is
// satisfied.
let cost = self
.func
.dfg
.inst_values(inst)
.fold(pure_op_cost(inst_data.opcode()), |cost, value| {
cost + best[value].0
});
best[value] = (cost, value);
}
}
};
debug_assert_ne!(best[value].0, Cost::infinity());
Expand Down Expand Up @@ -259,12 +257,9 @@ impl<'a> Elaborator<'a> {
}

fn process_elab_stack(&mut self) {
while let Some(entry) = self.elab_stack.last() {
while let Some(entry) = self.elab_stack.pop() {
match entry {
&ElabStackEntry::Start { value, before } => {
// We always replace the Start entry, so pop it now.
self.elab_stack.pop();

ElabStackEntry::Start { value, before } => {
debug_assert_ne!(value, Value::reserved_value());
let value = self.func.dfg.resolve_aliases(value);

Expand Down Expand Up @@ -376,15 +371,13 @@ impl<'a> Elaborator<'a> {
}
}

&ElabStackEntry::PendingInst {
ElabStackEntry::PendingInst {
inst,
result_idx,
num_args,
remat,
before,
} => {
self.elab_stack.pop();

trace!(
"PendingInst: {} result {} args {} remat {} before {}",
inst,
Expand Down