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type-updating.h
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type-updating.h
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/*
* Copyright 2017 WebAssembly Community Group participants
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef wasm_ir_type_updating_h
#define wasm_ir_type_updating_h
#include "wasm-traversal.h"
namespace wasm {
// a class that tracks type dependencies between nodes, letting you
// update types efficiently when removing and altering code.
// altering code can alter types in the following way:
// * removing a break can make a block unreachable, if nothing else
// reaches it
// * altering the type of a child to unreachable can make the parent
// unreachable
struct TypeUpdater
: public ExpressionStackWalker<TypeUpdater,
UnifiedExpressionVisitor<TypeUpdater>> {
// Part 1: Scanning
// track names to their blocks, so that when we remove a break to
// a block, we know how to find it if we need to update it
struct BlockInfo {
Block* block = nullptr;
int numBreaks = 0;
};
std::map<Name, BlockInfo> blockInfos;
// track the parent of each node, as child type changes may lead to
// unreachability
std::map<Expression*, Expression*> parents;
void visitExpression(Expression* curr) {
if (expressionStack.size() > 1) {
parents[curr] = expressionStack[expressionStack.size() - 2];
} else {
parents[curr] = nullptr; // this is the top level
}
// discover block/break relationships
if (auto* block = curr->dynCast<Block>()) {
if (block->name.is()) {
blockInfos[block->name].block = block;
}
} else if (auto* br = curr->dynCast<Break>()) {
// ensure info exists, discoverBreaks can then fill it
blockInfos[br->name];
} else if (auto* sw = curr->dynCast<Switch>()) {
// ensure info exists, discoverBreaks can then fill it
for (auto target : sw->targets) {
blockInfos[target];
}
blockInfos[sw->default_];
} else if (auto* br = curr->dynCast<BrOnExn>()) {
blockInfos[br->name];
}
// add a break to the info, for break and switch
discoverBreaks(curr, +1);
}
// Part 2: Updating
// Node replacements, additions, removals and type changes should be noted. An
// exception is nodes you know will never be looked at again.
// note the replacement of one node with another. this should be called
// after performing the replacement.
// this does *not* look into the node by default. see
// noteReplacementWithRecursiveRemoval (we don't support recursive addition
// because in practice we do not create new trees in the passes that use this,
// they just move around children)
void noteReplacement(Expression* from,
Expression* to,
bool recursivelyRemove = false) {
auto parent = parents[from];
if (recursivelyRemove) {
noteRecursiveRemoval(from);
} else {
noteRemoval(from);
}
// if we are replacing with a child, i.e. a node that was already present
// in the ast, then we just have a type and parent to update
if (parents.find(to) != parents.end()) {
parents[to] = parent;
if (from->type != to->type) {
propagateTypesUp(to);
}
} else {
noteAddition(to, parent, from);
}
}
void noteReplacementWithRecursiveRemoval(Expression* from, Expression* to) {
noteReplacement(from, to, true);
}
// note the removal of a node
void noteRemoval(Expression* curr) {
noteRemovalOrAddition(curr, nullptr);
parents.erase(curr);
}
// note the removal of a node and all its children
void noteRecursiveRemoval(Expression* curr) {
struct Recurser
: public PostWalker<Recurser, UnifiedExpressionVisitor<Recurser>> {
TypeUpdater& parent;
Recurser(TypeUpdater& parent, Expression* root) : parent(parent) {
walk(root);
}
void visitExpression(Expression* curr) { parent.noteRemoval(curr); }
};
Recurser(*this, curr);
}
void noteAddition(Expression* curr,
Expression* parent,
Expression* previous = nullptr) {
assert(parents.find(curr) == parents.end()); // must not already exist
noteRemovalOrAddition(curr, parent);
// if we didn't replace with the exact same type, propagate types up
if (!(previous && previous->type == curr->type)) {
propagateTypesUp(curr);
}
}
// if parent is nullptr, this is a removal
void noteRemovalOrAddition(Expression* curr, Expression* parent) {
parents[curr] = parent;
discoverBreaks(curr, parent ? +1 : -1);
}
// adds (or removes) breaks depending on break/switch contents
void discoverBreaks(Expression* curr, int change) {
if (auto* br = curr->dynCast<Break>()) {
noteBreakChange(br->name, change, br->value);
} else if (auto* sw = curr->dynCast<Switch>()) {
applySwitchChanges(sw, change);
} else if (auto* br = curr->dynCast<BrOnExn>()) {
noteBreakChange(br->name, change, br->sent);
}
}
void applySwitchChanges(Switch* sw, int change) {
std::set<Name> seen;
for (auto target : sw->targets) {
if (seen.insert(target).second) {
noteBreakChange(target, change, sw->value);
}
}
if (seen.insert(sw->default_).second) {
noteBreakChange(sw->default_, change, sw->value);
}
}
// note the addition of a node
void noteBreakChange(Name name, int change, Expression* value) {
noteBreakChange(name, change, value ? value->type : Type::none);
}
void noteBreakChange(Name name, int change, Type type) {
auto iter = blockInfos.find(name);
if (iter == blockInfos.end()) {
return; // we can ignore breaks to loops
}
auto& info = iter->second;
info.numBreaks += change;
assert(info.numBreaks >= 0);
auto* block = info.block;
if (block) { // if to a loop, can ignore
if (info.numBreaks == 0) {
// dropped to 0! the block may now be unreachable. that
// requires that it doesn't have a fallthrough
makeBlockUnreachableIfNoFallThrough(block);
} else if (change == 1 && info.numBreaks == 1) {
// bumped to 1! the block may now be reachable
if (block->type != Type::unreachable) {
return; // was already reachable, had a fallthrough
}
changeTypeTo(block, type);
}
}
}
// alters the type of a node to a new type.
// this propagates the type change through all the parents.
void changeTypeTo(Expression* curr, Type newType) {
if (curr->type == newType) {
return; // nothing to do
}
curr->type = newType;
propagateTypesUp(curr);
}
// given a node that has a new type, or is a new node, update
// all the parents accordingly. the existence of the node and
// any changes to it already occurred, this just updates the
// parents following that. i.e., nothing is done to the
// node we start on, it's done.
// the one thing we need to do here is propagate unreachability,
// no other change is possible
void propagateTypesUp(Expression* curr) {
if (curr->type != Type::unreachable) {
return;
}
while (1) {
auto* child = curr;
curr = parents[child];
if (!curr) {
return;
}
// get ready to apply unreachability to this node
if (curr->type == Type::unreachable) {
return; // already unreachable, stop here
}
// most nodes become unreachable if a child is unreachable,
// but exceptions exist
if (auto* block = curr->dynCast<Block>()) {
// if the block has a fallthrough, it can keep its type
if (block->list.back()->type.isConcrete()) {
return; // did not turn
}
// if the block has breaks, it can keep its type
if (!block->name.is() || blockInfos[block->name].numBreaks == 0) {
curr->type = Type::unreachable;
} else {
return; // did not turn
}
} else if (auto* iff = curr->dynCast<If>()) {
// may not be unreachable if just one side is
iff->finalize();
if (curr->type != Type::unreachable) {
return; // did not turn
}
} else if (auto* tryy = curr->dynCast<Try>()) {
tryy->finalize();
if (curr->type != Type::unreachable) {
return; // did not turn
}
} else {
curr->type = Type::unreachable;
}
}
}
// efficiently update the type of a block, given the data we know. this
// can remove a concrete type and turn the block unreachable when it is
// unreachable, and it does this efficiently, without scanning the full
// contents
void maybeUpdateTypeToUnreachable(Block* curr) {
if (!curr->type.isConcrete()) {
return; // nothing concrete to change to unreachable
}
if (curr->name.is() && blockInfos[curr->name].numBreaks > 0) {
return; // has a break, not unreachable
}
// look for a fallthrough
makeBlockUnreachableIfNoFallThrough(curr);
}
void makeBlockUnreachableIfNoFallThrough(Block* curr) {
if (curr->type == Type::unreachable) {
return; // no change possible
}
if (!curr->list.empty() && curr->list.back()->type.isConcrete()) {
// should keep type due to fallthrough, even if has an unreachable child
return;
}
for (auto* child : curr->list) {
if (child->type == Type::unreachable) {
// no fallthrough, and an unreachable, => this block is now unreachable
changeTypeTo(curr, Type::unreachable);
return;
}
}
}
// efficiently update the type of an if, given the data we know. this
// can remove a concrete type and turn the if unreachable when it is
// unreachable
void maybeUpdateTypeToUnreachable(If* curr) {
if (!curr->type.isConcrete()) {
return; // nothing concrete to change to unreachable
}
curr->finalize();
if (curr->type == Type::unreachable) {
propagateTypesUp(curr);
}
}
void maybeUpdateTypeToUnreachable(Try* curr) {
if (!curr->type.isConcrete()) {
return; // nothing concrete to change to unreachable
}
curr->finalize();
if (curr->type == Type::unreachable) {
propagateTypesUp(curr);
}
}
};
} // namespace wasm
#endif // wasm_ir_type_updating_h