src/passes/ConstraintAnalysis.cpp

/*
 * Copyright 2026 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.
 */

//
// Use mathematical constraint solving to optimize. For example:
//
//  if (x == 10) {
//    assert(x != 0); // redundant and can be removed.
//  }
//

#include "cfg/cfg-traversal.h"
#include "ir/constraint.h"
#include "ir/drop.h"
#include "ir/literal-utils.h"
#include "ir/local-graph.h"
#include "ir/properties.h"
#include "pass.h"
#include "support/unique_deferring_queue.h"
#include "support/utilities.h"
#include "wasm-builder.h"
#include "wasm.h"

namespace wasm {

using namespace wasm::constraint;

namespace {

// In each basic block we will store the relevant operations, which are all
// local gets and sets, branches, and uses of them.
struct Info {
  std::vector<Expression**> actions;

  // For each local index, we track the constraints we know about it. We only do
  // so at the start of each block, which is enough for the analysis below.
  LocalConstraintMap startConstraints;
};

struct ConstraintAnalysis
  : public WalkerPass<
      CFGWalker<ConstraintAnalysis, Visitor<ConstraintAnalysis>, Info>> {
  bool isFunctionParallel() override { return true; }

  // Locals are not modified here.
  bool requiresNonNullableLocalFixups() override { return false; }

  std::unique_ptr<Pass> create() override {
    return std::make_unique<ConstraintAnalysis>();
  }

  // Branches outside of the function can be ignored, as we only look at local
  // state in the function.
  bool ignoreBranchesOutsideOfFunc = true;

  // Store the actions we care about.
  void addAction() {
    if (currBasicBlock) {
      currBasicBlock->contents.actions.push_back(getCurrentPointer());
    }
  }

  void visitLocalSet(LocalSet* curr) { addAction(); }
  void visitUnary(Unary* curr) { addAction(); }
  void visitBinary(Binary* curr) { addAction(); }
  void visitRefEq(RefEq* curr) { addAction(); }
  void visitRefIsNull(RefIsNull* curr) { addAction(); }

  void visitFunction(Function* curr) {
    if (!entry) {
      // Body is unreachable, no entry block.
      return;
    }
    // TODO: optimize for speed, find relevant locals etc.
    flow();
    optimize();
  }

  // Flow infos around until we have inferred all we can about the constraints
  // in each location.
  void flow() {
    // Start from the entry. That block has incoming values - defaults - for
    // each var.
    auto& entryConstraints = entry->contents.startConstraints;
    auto* func = getFunction();
    auto numLocals = func->getNumLocals();
    for (Index i = 0; i < numLocals; i++) {
      auto type = func->getLocalType(i);
      // TODO: support tuples
      if (func->isParam(i) || type.size() != 1) {
        // We can't do anything in this case.
        entryConstraints[i].setProvesNothing();
      } else if (LiteralUtils::canMakeZero(type)) {
        // We have a default value.
        auto value = Literal::makeZero(type);
        entryConstraints[i].set(Constraint{Abstract::Eq, {value}});
      } else {
        // Otherwise, this is non-nullable, and it is unreachable until set.
        assert(type.isNonNullable());
        entryConstraints[i].setProvesEverything();
      }
    }

    // Starting from the entry, keep going while we find something new.
    UniqueDeferredQueue<BasicBlock*> work;
    work.push(entry);
    while (!work.empty()) {
      auto* block = work.pop();

      // Start at the top of the block, then go through, applying things.
      LocalConstraintMap constraints = block->contents.startConstraints;
      for (auto** currp : block->contents.actions) {
        applyToConstraints(*currp, constraints);
      }

      // We now know the values at the end of the block. Flow it onward, and
      // where it causes changes, queue more work.
      for (auto* out : block->out) {
        auto& outStartConstraints = out->contents.startConstraints;
        auto old = outStartConstraints;
        outStartConstraints.approximateOr(constraints);
        if (outStartConstraints != old) {
          work.push(out);
        }
      }
    }
  }

  // After inferring all we can, apply it to optimize the code.
  void optimize() {
    for (auto& block : basicBlocks) {
      // Follow the general shape of flow(): we need to see what the state is
      // at each intermediate point inside the block. (Flowing between blocks is
      // of course not needed at this stage.)
      auto& constraints = block->contents.startConstraints;
      for (auto** currp : block->contents.actions) {
        applyToConstraints(*currp, constraints);
        optimizeExpression(currp, constraints);
      }
    }
  }

  // Given an expression and the constraints on it, optimize it.
  void optimizeExpression(Expression** currp,
                          const LocalConstraintMap& constraints) {
    auto* curr = *currp;
    auto parsed = LocalConstraint::parse(curr);
    if (!parsed) {
      return;
    }

    auto iter = constraints.find(parsed->local);
    if (iter == constraints.end()) {
      return;
    }
    auto& localConstraints = iter->second;
    Result result = localConstraints.proves(parsed->constraint);
    if (result == Unknown) {
      // If we parsed something using two locals, like x != y, we can also look
      // for the flipped condition among y's constraints TODO
      return;
    }

    // We know the result!
    auto& wasm = *getModule();
    auto value =
      LiteralUtils::makeFromInt32(result == True ? 1 : 0, curr->type, wasm);
    *currp = getDroppedChildrenAndAppend(
      curr, wasm, getPassOptions(), value, DropMode::IgnoreParentEffects);
  }

  // Given an expression, apply it to the constraints. For example, a local.set
  // sets the value for that local.
  void applyToConstraints(Expression* curr, LocalConstraintMap& constraints) {
    if (auto* set = curr->dynCast<LocalSet>()) {
      auto& localConstraints = constraints[set->index];
      if (Properties::isSingleConstantExpression(set->value)) {
        // We know this one constraint.
        auto value = Properties::getLiteral(set->value);
        localConstraints.set(Constraint{Abstract::Eq, {value}});
      } else {
        // We know and can prove nothing.
        localConstraints.setProvesNothing();
      }
    }
  }
};

} // anonymous namespace

Pass* createConstraintAnalysisPass() { return new ConstraintAnalysis(); }

} // namespace wasm