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Copy pathB3HoistLoopInvariantValues.cpp
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251 lines (214 loc) · 9.72 KB
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/*
* Copyright (C) 2017 Apple Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include "config.h"
#include "B3HoistLoopInvariantValues.h"
#if ENABLE(B3_JIT)
#include "B3BackwardsDominators.h"
#include "B3Dominators.h"
#include "B3EnsureLoopPreHeaders.h"
#include "B3NaturalLoops.h"
#include "B3PhaseScope.h"
#include "B3ProcedureInlines.h"
#include "B3ValueInlines.h"
#include <wtf/RangeSet.h>
namespace JSC { namespace B3 {
// A load that can trap reports exitsSideways, which normally disqualifies it from hoisting. The
// fault is its only escaping effect, so it can still move to a point where it is certain to run and
// where nothing observable precedes it. WasmGC field and length loads depend on this: their null
// check is a trap rather than a branch, which would otherwise be all that blocks hoisting them.
static bool isTrappingLoad(const Value* value)
{
if (!value->traps())
return false;
switch (value->opcode()) {
case Load8Z:
case Load8S:
case Load16Z:
case Load16S:
case Load:
// A fenced load also orders memory, which is an effect that cannot move.
return !value->as<MemoryValue>()->hasFence();
case WasmStructGet:
case WasmArrayLength:
return true;
default:
return false;
}
}
static bool isReachedWithoutTrapBarrier(const NaturalLoops& loops, const NaturalLoop& loop, BasicBlock* block, const IndexSet<BasicBlock*>& blockBarriers)
{
if (block == loop.header())
return true;
// Tracing ancestors from the block. If any blocks belonging to this loop has a trap,
// then we cannot hoist since that trap needs to be executed before the hoisted value causes
// a trap at a pre-header block.
Vector<BasicBlock*, 8> worklist;
IndexSet<BasicBlock*> seen;
auto push = [&](BasicBlock* predecessor) {
if (loops.belongsTo(predecessor, loop)) {
if (seen.add(predecessor)) {
// Every value in these blocks runs before the one being hoisted.
if (blockBarriers.contains(predecessor))
return false;
// Stopping at the header is enough even though earlier iterations run other blocks:
// callers only reach here for a value whose block backwards-dominates the pre-header,
// and backwards dominance counts a back edge as a way for the procedure to end, so
// such a value always runs before any iteration completes.
if (predecessor == loop.header())
return true;
worklist.append(predecessor);
}
}
return true;
};
for (BasicBlock* predecessor : block->predecessors()) {
if (!push(predecessor))
return false;
}
while (!worklist.isEmpty()) {
BasicBlock* current = worklist.takeLast();
for (BasicBlock* predecessor : current->predecessors()) {
if (!push(predecessor))
return false;
}
}
return true;
}
bool hoistLoopInvariantValues(Procedure& proc)
{
PhaseScope phaseScope(proc, "hoistLoopInvariantValues"_s);
ensureLoopPreHeaders(proc);
NaturalLoops& loops = proc.naturalLoops();
if (!loops.numLoops())
return false;
proc.resetValueOwners();
Dominators& dominators = proc.dominators();
BackwardsDominators& backwardsDominators = proc.backwardsDominators();
// FIXME: We should have a reusable B3::EffectsSet data structure.
// https://bugs.webkit.org/show_bug.cgi?id=174762
struct LoopData {
RangeSet<HeapRange> writes;
bool writesLocalState { false };
bool writesPinned { false };
bool exitsSideways { false };
BasicBlock* preHeader { nullptr };
};
IndexMap<NaturalLoop, LoopData> data(loops.numLoops());
IndexSet<BasicBlock*> blockBarriers;
for (unsigned loopIndex = loops.numLoops(); loopIndex--;) {
const NaturalLoop& loop = loops.loop(loopIndex);
for (BasicBlock* predecessor : loop.header()->predecessors()) {
if (loops.belongsTo(predecessor, loop))
continue;
RELEASE_ASSERT(!data[loop].preHeader);
data[loop].preHeader = predecessor;
}
}
for (BasicBlock* block : proc) {
const NaturalLoop* loop = loops.innerMostLoopOf(block);
if (!loop)
continue;
for (Value* value : *block) {
Effects effects = value->effects();
data[*loop].writes.add(effects.writes);
data[*loop].writesLocalState |= effects.writesLocalState;
data[*loop].writesPinned |= effects.writesPinned;
data[*loop].exitsSideways |= effects.exitsSideways;
if (effects.isTrapBarrier())
blockBarriers.add(block);
}
}
for (unsigned loopIndex = loops.numLoops(); loopIndex--;) {
const NaturalLoop& loop = loops.loop(loopIndex);
for (const NaturalLoop* current = loops.innerMostOuterLoop(loop); current; current = loops.innerMostOuterLoop(*current)) {
data[*current].writes.addAll(data[loop].writes);
data[*current].writesLocalState |= data[loop].writesLocalState;
data[*current].writesPinned |= data[loop].writesPinned;
data[*current].exitsSideways |= data[loop].exitsSideways;
}
}
bool changed = false;
// Pre-order ensures that we visit our dominators before we visit ourselves. Otherwise we'd miss some
// hoisting opportunities in complex CFGs.
for (BasicBlock* block : proc.blocksInPreOrder()) {
Vector<const NaturalLoop*> blockLoops = loops.loopsOf(block);
if (blockLoops.isEmpty())
continue;
// Values that stay in this block run before everything later in it, so once one of them is a
// trap barrier nothing after it can be hoisted out on the strength of trapping first.
bool trapBarrierInBlock = false;
for (Value*& value : *block) {
Effects effects = value->effects();
bool trappingLoad = isTrappingLoad(value);
bool hoisted = false;
// We never hoist write effects or control constructs. A trapping load is allowed
// through. The fault it reports as a side exit is its own, and the checks below decide
// whether it may move.
if (!effects.mustExecute() || trappingLoad) {
// Try outermost loop first.
for (unsigned i = blockLoops.size(); i--;) {
const NaturalLoop& loop = *blockLoops[i];
bool ok = true;
for (Value* child : value->children()) {
if (!dominators.dominates(child->owner, data[loop].preHeader)) {
ok = false;
break;
}
}
if (!ok)
continue;
if (effects.controlDependent || trappingLoad) {
// The block is backward-dominating the pre-header.
// The value needs to be guaranteed to run on every iteration.
if (!backwardsDominators.dominates(block, data[loop].preHeader))
continue;
if (data[loop].exitsSideways) {
// Even if the loop can exit (after this value for example), if we do not have
// any exit between pre-header to this value, we can still hoist a value because
// this can be a first value causing a trap.
if (trapBarrierInBlock || !isReachedWithoutTrapBarrier(loops, loop, block, blockBarriers))
continue;
}
}
if (effects.readsPinned && data[loop].writesPinned)
continue;
if (effects.readsLocalState && data[loop].writesLocalState)
continue;
if (data[loop].writes.overlaps(effects.reads))
continue;
data[loop].preHeader->appendNonTerminal(value);
value = proc.add<Value>(Nop, Void, value->origin());
changed = true;
hoisted = true;
}
}
if (!hoisted)
trapBarrierInBlock |= effects.isTrapBarrier();
}
}
return changed;
}
} } // namespace JSC::B3
#endif // ENABLE(B3_JIT)