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egraphs: disable GVN of effectful idempotent ops (temporarily). (#5808)

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This is a short-term fix to the same bug that #5800 is addressing
(#5796), but with less risk: it simply turns off GVN'ing of effectful
but idempotent ops. Because we have an upcoming release, and this is a
miscompile (albeit to do with trapping behavior), we would like to make
the simplest possible fix that avoids the bug, and backport it. I will
then rebase #5800 on top of a revert of this followed by the more
complete fix.
This commit is contained in:
Chris Fallin
2023-02-16 13:29:03 -08:00
committed by GitHub
parent 3ce439ce57
commit c7e2571866
3 changed files with 5 additions and 147 deletions
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41
cranelift/codegen/src/egraph.rs
View File

@@ -7,7 +7,7 @@ use crate::dominator_tree::DominatorTree;
use crate::egraph::domtree::DomTreeWithChildren;
use crate::egraph::elaborate::Elaborator;
use crate::fx::FxHashSet;
use crate::inst_predicates::{is_mergeable_for_egraph, is_pure_for_egraph};
use crate::inst_predicates::is_pure_for_egraph;
use crate::ir::{
DataFlowGraph, Function, Inst, InstructionData, Type, Value, ValueDef, ValueListPool,
};
@@ -285,41 +285,10 @@ where
fn optimize_skeleton_inst(&mut self, inst: Inst) -> bool {
self.stats.skeleton_inst += 1;
// First, can we try to deduplicate? We need to keep some copy
// of the instruction around because it's side-effecting, but
// we may be able to reuse an earlier instance of it.
if is_mergeable_for_egraph(self.func, inst) {
let result = self.func.dfg.inst_results(inst)[0];
trace!(" -> mergeable side-effecting op {}", inst);
let inst = NewOrExistingInst::Existing(inst);
let gvn_context = GVNContext {
union_find: self.eclasses,
value_lists: &self.func.dfg.value_lists,
};
// Does this instruction already exist? If so, add entries to
// the value-map to rewrite uses of its results to the results
// of the original (existing) instruction. If not, optimize
// the new instruction.
let key = inst.get_inst_key(&self.func.dfg);
if let Some(&orig_result) = self.gvn_map.get(&key, &gvn_context) {
// Hit in GVN map -- reuse value.
self.value_to_opt_value[result] = orig_result;
self.eclasses.union(orig_result, result);
trace!(" -> merges result {} to {}", result, orig_result);
true
} else {
// Otherwise, insert it into the value-map.
self.value_to_opt_value[result] = result;
self.gvn_map.insert(key, result, &gvn_context);
trace!(" -> inserts as new (no GVN)");
false
}
}
// Otherwise, if a load or store, process it with the alias
// analysis to see if we can optimize it (rewrite in terms of
// an earlier load or stored value).
else if let Some(new_result) =
// If a load or store, process it with the alias analysis to see
// if we can optimize it (rewrite in terms of an earlier load or
// stored value).
if let Some(new_result) =
self.alias_analysis
.process_inst(self.func, self.alias_analysis_state, inst)
{