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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#include "jit/WasmBCE.h"
#include "jit/MIRGenerator.h"
#include "jit/MIRGraph.h"
using namespace js;
using namespace js::jit;
using namespace mozilla;
typedef js::HashMap<uint32_t, MDefinition*, DefaultHasher<uint32_t>, SystemAllocPolicy>
LastSeenMap;
// The Wasm Bounds Check Elimination (BCE) pass looks for bounds checks
// on SSA values that have already been checked. (in the same block or in a
// dominating block). These bounds checks are redundant and thus eliminated.
//
// Note: This is safe in the presense of dynamic memory sizes as long as they
// can ONLY GROW. If we allow SHRINKING the heap, this pass should be
// RECONSIDERED.
//
// TODO (dbounov): Are there a lot of cases where there is no single dominating
// check, but a set of checks that together dominate a redundant check?
//
// TODO (dbounov): Generalize to constant additions relative to one base
bool
jit::EliminateBoundsChecks(MIRGenerator* mir, MIRGraph& graph)
{
// Map for dominating block where a given definition was checked
LastSeenMap lastSeen;
if (!lastSeen.init())
return false;
for (ReversePostorderIterator bIter(graph.rpoBegin()); bIter != graph.rpoEnd(); bIter++) {
MBasicBlock* block = *bIter;
for (MDefinitionIterator dIter(block); dIter;) {
MDefinition* def = *dIter++;
switch (def->op()) {
case MDefinition::Op_WasmBoundsCheck: {
MWasmBoundsCheck* bc = def->toWasmBoundsCheck();
MDefinition* addr = def->getOperand(0);
LastSeenMap::AddPtr ptr = lastSeen.lookupForAdd(addr->id());
if (ptr) {
if (ptr->value()->block()->dominates(block))
bc->setRedundant(true);
} else {
if (!lastSeen.add(ptr, addr->id(), def))
return false;
}
break;
}
case MDefinition::Op_Phi: {
MPhi* phi = def->toPhi();
bool phiChecked = true;
MOZ_ASSERT(phi->numOperands() > 0);
// If all incoming values to a phi node are safe (i.e. have a
// check that dominates this block) then we can consider this
// phi node checked.
//
// Note that any phi that is part of a cycle
// will not be "safe" since the value coming on the backedge
// cannot be in lastSeen because its block hasn't been traversed yet.
for (int i = 0, nOps = phi->numOperands(); i < nOps; i++) {
MDefinition* src = phi->getOperand(i);
LastSeenMap::Ptr checkPtr = lastSeen.lookup(src->id());
if (!checkPtr || !checkPtr->value()->block()->dominates(block)) {
phiChecked = false;
break;
}
}
if (phiChecked) {
if (!lastSeen.put(def->id(), def))
return false;
}
break;
}
default:
break;
}
}
}
return true;
}
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