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dart2js: Insert HTypeKnown refinement nodes on dominated edges of HPhi nodes.

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Consider:

    value ??= "";

We fail to infer in SSA type progagation that value is non-null.

At CFG level this looks like

    if (value == null) {
      value = "";
    } else {
      // There is always an empty else-block.
    }
   value1 = phi("", value);

Previously we considered the phi use of 'value' to be outside the
region dominated by the condition 'value != null', leaving nowhere to
attach the type refinement. Now we consider it inside the region,
allowing:

    if (value == null) {
      value = "";
    } else {
      value1 = HTypeKnown(not-null, value);
    }
    value2 = phi("", value1);

Type propagation now determines value2 is non-null.

R=efortuna@google.com

Review-Url: https://codereview.chromium.org/2755353003 .
This commit is contained in:
Stephen Adams 2017-04-06 10:44:03 -07:00
parent b65cd81b45
commit 49e78d7e32
3 changed files with 164 additions and 63 deletions
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182
pkg/compiler/lib/src/ssa/nodes.dart
View file

@ -866,6 +866,8 @@ class HBasicBlock extends HInstructionList {
} while (other != null && other.id >= id);
return dominatesCache[other] = false;
}
toString() => 'HBasicBlock($id)';
}
abstract class HInstruction implements Spannable {
@ -1277,60 +1279,9 @@ abstract class HInstruction implements Spannable {
removeFromList(oldInput.usedBy, this);
}
// Compute the set of users of this instruction that is dominated by
// [other]. If [other] is a user of [this], it is included in the
// returned set.
Setlet<HInstruction> dominatedUsers(HInstruction other) {
// Keep track of all instructions that we have to deal with later
// and count the number of them that are in the current block.
Setlet<HInstruction> users = new Setlet<HInstruction>();
int usersInCurrentBlock = 0;
// Run through all the users and see if they are dominated or
// potentially dominated by [other].
HBasicBlock otherBlock = other.block;
for (int i = 0, length = usedBy.length; i < length; i++) {
HInstruction current = usedBy[i];
HBasicBlock currentBlock = current.block;
if (otherBlock.dominates(currentBlock)) {
if (identical(currentBlock, otherBlock)) usersInCurrentBlock++;
users.add(current);
}
}
// Run through all the phis in the same block as [other] and remove them
// from the users set.
if (usersInCurrentBlock > 0) {
for (HPhi phi = otherBlock.phis.first; phi != null; phi = phi.next) {
if (users.contains(phi)) {
users.remove(phi);
if (--usersInCurrentBlock == 0) break;
}
}
}
// Run through all the instructions before [other] and remove them
// from the users set.
if (usersInCurrentBlock > 0) {
HInstruction current = otherBlock.first;
while (!identical(current, other)) {
if (users.contains(current)) {
users.remove(current);
if (--usersInCurrentBlock == 0) break;
}
current = current.next;
}
}
return users;
}
void replaceAllUsersDominatedBy(
HInstruction cursor, HInstruction newInstruction) {
Setlet<HInstruction> users = dominatedUsers(cursor);
for (HInstruction user in users) {
user.changeUse(this, newInstruction);
}
DominatedUses.of(this, cursor).replaceWith(newInstruction);
}
void moveBefore(HInstruction other) {
@ -1417,6 +1368,133 @@ abstract class HInstruction implements Spannable {
}
}
/// The set of uses of [source] that are dominated by [dominator].
class DominatedUses {
final HInstruction _source;
final HInstruction _dominator;
// Two list of matching length holding (instruction, input-index) pairs for
// the dominated uses.
final List<HInstruction> _instructions = <HInstruction>[];
final List<int> _indexes = <int>[];
DominatedUses._(this._source, this._dominator);
/// The uses of [source] that are dominated by [dominator].
///
/// The uses by [dominator] are included in the result, unless
/// [excludeDominator] is `true`, so `true` selects uses following
/// [dominator].
///
/// The uses include the in-edges of a HPhi node that corresponds to a
/// dominated block. (There can be many such edges on a single phi at the exit
/// of a loop with many break statements). If [excludePhiOutEdges] is `true`
/// then these edge uses are not included.
static of(HInstruction source, HInstruction dominator,
{bool excludeDominator: false, bool excludePhiOutEdges: false}) {
return new DominatedUses._(source, dominator)
.._compute(source, dominator, excludeDominator, excludePhiOutEdges);
}
bool get isEmpty => _instructions.isEmpty;
bool get isNotEmpty => !isEmpty;
/// Changes all the uses in the set to [newInstruction].
void replaceWith(HInstruction newInstruction) {
assert(!identical(newInstruction, _source));
if (isEmpty) return;
for (int i = 0; i < _instructions.length; i++) {
HInstruction user = _instructions[i];
int index = _indexes[i];
HInstruction oldInstruction = user.inputs[index];
assert(
identical(oldInstruction, _source),
'Input ${index} of ${user} changed.'
'\n Found: ${oldInstruction}\n Expected: ${_source}');
user.inputs[index] = newInstruction;
oldInstruction.usedBy.remove(user);
newInstruction.usedBy.add(user);
}
}
void _addUse(HInstruction user, int inputIndex) {
_instructions.add(user);
_indexes.add(inputIndex);
}
void _compute(HInstruction source, HInstruction dominator,
bool excludeDominator, bool excludePhiOutEdges) {
// Keep track of all instructions that we have to deal with later and count
// the number of them that are in the current block.
Set<HInstruction> users = new Setlet<HInstruction>();
Set<HInstruction> seen = new Setlet<HInstruction>();
int usersInCurrentBlock = 0;
HBasicBlock dominatorBlock = dominator.block;
// Run through all the users and see if they are dominated, or potentially
// dominated, or partially dominated by [dominator]. It is easier to
// de-duplicate [usedBy] and process all inputs of an instruction than to
// track the repeated elements of usedBy and match them up by index.
for (HInstruction current in source.usedBy) {
if (!seen.add(current)) continue;
HBasicBlock currentBlock = current.block;
if (dominatorBlock.dominates(currentBlock)) {
users.add(current);
if (identical(currentBlock, dominatorBlock)) usersInCurrentBlock++;
} else if (!excludePhiOutEdges && current is HPhi) {
// A non-dominated HPhi.
// See if there a dominated edge into the phi. The input must be
// [source] and the position must correspond to a dominated block.
List<HBasicBlock> predecessors = currentBlock.predecessors;
for (int i = 0; i < predecessors.length; i++) {
if (current.inputs[i] != source) continue;
HBasicBlock predecessor = predecessors[i];
if (dominatorBlock.dominates(predecessor)) {
_addUse(current, i);
}
}
}
}
// Run through all the phis in the same block as [dominator] and remove them
// from the users set. These come before [dominator].
// TODO(sra): Could we simply not add them in the first place?
if (usersInCurrentBlock > 0) {
for (HPhi phi = dominatorBlock.phis.first; phi != null; phi = phi.next) {
if (users.remove(phi)) {
if (--usersInCurrentBlock == 0) break;
}
}
}
// Run through all the instructions before [dominator] and remove them from
// the users set.
if (usersInCurrentBlock > 0) {
HInstruction current = dominatorBlock.first;
while (!identical(current, dominator)) {
if (users.remove(current)) {
if (--usersInCurrentBlock == 0) break;
}
current = current.next;
}
if (excludeDominator) {
users.remove(dominator);
}
}
// Convert users into a list of (user, input-index) uses.
for (HInstruction user in users) {
var inputs = user.inputs;
for (int i = 0; i < inputs.length; i++) {
if (inputs[i] == source) {
_addUse(user, i);
}
}
}
}
}
/// A reference to a [HInstruction] that can hold its own source information.
///
/// This used for attaching source information to reads of locals.

41
pkg/compiler/lib/src/ssa/optimize.dart
View file

@ -63,7 +63,7 @@ class SsaOptimizerTask extends CompilerTask {
void runPhase(OptimizationPhase phase) {
measureSubtask(phase.name, () => phase.visitGraph(graph));
_backend.tracer.traceGraph(phase.name, graph);
assert(graph.isValid());
assert(graph.isValid(), 'Graph not valid after ${phase.name}');
}
bool trustPrimitives = _options.trustPrimitives;
@ -733,10 +733,29 @@ class SsaInstructionSimplifier extends HBaseVisitor
void simplifyCondition(
HBasicBlock block, HInstruction condition, bool value) {
condition.dominatedUsers(block.first).forEach((user) {
HInstruction newCondition = _graph.addConstantBool(value, _closedWorld);
user.changeUse(condition, newCondition);
});
// `excludePhiOutEdges: true` prevents replacing a partially dominated phi
// node input with a constant. This tends to add unnecessary assignments, by
// transforming the following, which has phi(false, x),
//
// if (x) { init(); x = false; }
//
// into this, which has phi(false, false)
//
// if (x) { init(); x = false; } else { x = false; }
//
// which is further simplifed to:
//
// if (x) { init(); }
// ...
// x = false;
//
// This is mostly harmless (if a little confusing) but does cause a lot of
// `x = false;` copies to be inserted when a loop body has many continue
// statements or ends with a switch.
var uses =
DominatedUses.of(condition, block.first, excludePhiOutEdges: true);
if (uses.isEmpty) return;
uses.replaceWith(_graph.addConstantBool(value, _closedWorld));
}
HInstruction visitIf(HIf node) {
@ -1622,6 +1641,10 @@ class SsaDeadCodeEliminator extends HGraphVisitor implements OptimizationPhase {
}
// Run through the phis of the block and replace them with their input
// that comes from the only live predecessor if that dominates the phi.
//
// TODO(sra): If the input is directly in the only live predecessor, it
// might be possible to move it into [block] (e.g. all its inputs are
// dominating.)
block.forEachPhi((HPhi phi) {
HInstruction replacement =
(indexOfLive >= 0) ? phi.inputs[indexOfLive] : zapInstruction;
@ -2170,14 +2193,12 @@ class SsaTypeConversionInserter extends HBaseVisitor
// non-movable.
void insertTypePropagationForDominatedUsers(
HBasicBlock dominator, HInstruction input, TypeMask convertedType) {
Setlet<HInstruction> dominatedUsers = input.dominatedUsers(dominator.first);
if (dominatedUsers.isEmpty) return;
DominatedUses dominatedUses = DominatedUses.of(input, dominator.first);
if (dominatedUses.isEmpty) return;
HTypeKnown newInput = new HTypeKnown.pinned(convertedType, input);
dominator.addBefore(dominator.first, newInput);
dominatedUsers.forEach((HInstruction user) {
user.changeUse(input, newInput);
});
dominatedUses.replaceWith(newInput);
}
void visitIs(HIs instruction) {

4
pkg/compiler/lib/src/ssa/types_propagation.dart
View file

@ -397,7 +397,9 @@ class SsaTypePropagator extends HBaseVisitor implements OptimizationPhase {
addDependentInstructionsToWorkList(next);
}
} else {
bool hasCandidates() => receiver.dominatedUsers(instruction).length > 1;
bool hasCandidates() => DominatedUses
.of(receiver, instruction, excludeDominator: true)
.isNotEmpty;
if ((receiver.usedBy.length <= _MAX_QUICK_USERS)
? (hasCandidates() && computeNewType() != receiverType)