Revision: 13505
Author: [email protected]
Date: Fri Jan 25 03:51:48 2013
Log: Avoid excessive memory usage during redundant phi elimination.
Basically, the work list for the fixed point iteration has been removed.
BUG=v8:2510
Review URL: https://codereview.chromium.org/12052075
http://code.google.com/p/v8/source/detail?r=13505
Modified:
/branches/bleeding_edge/src/hydrogen.cc
=======================================
--- /branches/bleeding_edge/src/hydrogen.cc Thu Jan 24 09:54:30 2013
+++ /branches/bleeding_edge/src/hydrogen.cc Fri Jan 25 03:51:48 2013
@@ -1329,37 +1329,50 @@
}
+// Replace all phis consisting of a single non-loop operand plus any
number of
+// loop operands by that single non-loop operand.
void HGraph::EliminateRedundantPhis() {
HPhase phase("H_Redundant phi elimination", this);
- // Worklist of phis that can potentially be eliminated. Initialized with
- // all phi nodes. When elimination of a phi node modifies another phi
node
- // the modified phi node is added to the worklist.
- ZoneList<HPhi*> worklist(blocks_.length(), zone());
- for (int i = 0; i < blocks_.length(); ++i) {
- worklist.AddAll(*blocks_[i]->phis(), zone());
- }
-
- while (!worklist.is_empty()) {
- HPhi* phi = worklist.RemoveLast();
- HBasicBlock* block = phi->block();
-
- // Skip phi node if it was already replaced.
- if (block == NULL) continue;
-
- // Get replacement value if phi is redundant.
- HValue* replacement = phi->GetRedundantReplacement();
-
- if (replacement != NULL) {
- // Iterate through the uses and replace them all.
- for (HUseIterator it(phi->uses()); !it.Done(); it.Advance()) {
- HValue* value = it.value();
- value->SetOperandAt(it.index(), replacement);
- if (value->IsPhi()) worklist.Add(HPhi::cast(value), zone());
+ // We do a simple fixed point iteration without any work list, because
+ // machine-generated JavaScript can lead to a very dense Hydrogen graph
with
+ // an enormous work list and will consequently result in OOM. Experiments
+ // showed that this simple algorithm is good enough, and even e.g.
tracking
+ // the set or range of blocks to consider is not a real improvement.
+ bool need_another_iteration;
+ ZoneList<HPhi*> redundant_phis(blocks_.length(), zone());
+ do {
+ need_another_iteration = false;
+ for (int i = 0; i < blocks_.length(); ++i) {
+ HBasicBlock* block = blocks_[i];
+ for (int j = 0; j < block->phis()->length(); j++) {
+ HPhi* phi = block->phis()->at(j);
+ HValue* replacement = phi->GetRedundantReplacement();
+ if (replacement != NULL) {
+ // Remember phi to avoid concurrent modification of the block's
phis.
+ redundant_phis.Add(phi, zone());
+ for (HUseIterator it(phi->uses()); !it.Done(); it.Advance()) {
+ HValue* value = it.value();
+ value->SetOperandAt(it.index(), replacement);
+ need_another_iteration |= value->IsPhi();
+ }
+ }
}
- block->RemovePhi(phi);
+ for (int i = 0; i < redundant_phis.length(); i++) {
+ block->RemovePhi(redundant_phis[i]);
+ }
+ redundant_phis.Clear();
+ }
+ } while (need_another_iteration);
+
+#if DEBUG
+ // Make sure that we *really* removed all redundant phis.
+ for (int i = 0; i < blocks_.length(); ++i) {
+ for (int j = 0; j < blocks_[i]->phis()->length(); j++) {
+ ASSERT(blocks_[i]->phis()->at(j)->GetRedundantReplacement() == NULL);
}
}
+#endif
}
--
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