JS: Move BasicBlocks.qll -> internal/BasicBlocksInternal.qll

Author: asgerf

javascript/ql/lib/semmle/javascript/BasicBlocks.qll

@@ -1,368 +1 @@
-/**
- * Provides classes for working with basic blocks, and predicates for computing
- * liveness information for local variables.
- */
-
-import javascript
-private import internal.StmtContainers
-private import semmle.javascript.internal.CachedStages
-
-module BasicBlockInternal {
-  // TODO: Expose these as public predicate in a private module instead of this hack.
-  predicate getImmediateBasicBlockDominator = immediateDominator/1;
-}
-
-/**
- * Holds if `nd` starts a new basic block.
- */
-private predicate startsBB(ControlFlowNode nd) {
-  not exists(nd.getAPredecessor()) and exists(nd.getASuccessor())
-  or
-  nd.isJoin()
-  or
-  nd.getAPredecessor().isBranch()
-}
-
-/**
- * Holds if the first node of basic block `succ` is a control flow
- * successor of the last node of basic block `bb`.
- */
-private predicate succBB(BasicBlock bb, BasicBlock succ) { succ = bb.getLastNode().getASuccessor() }
-
-/**
- * Holds if the first node of basic block `bb` is a control flow
- * successor of the last node of basic block `pre`.
- */
-private predicate predBB(BasicBlock bb, BasicBlock pre) { succBB(pre, bb) }
-
-/** Holds if `bb` is an entry basic block. */
-private predicate entryBB(BasicBlock bb) { bb.getFirstNode() instanceof ControlFlowEntryNode }
-
-/** Holds if `bb` is an exit basic block. */
-private predicate exitBB(BasicBlock bb) { bb.getLastNode() instanceof ControlFlowExitNode }
-
-cached
-private module Internal {
-  /**
-   * Holds if `succ` is a control flow successor of `nd` within the same basic block.
-   */
-  private predicate intraBBSucc(ControlFlowNode nd, ControlFlowNode succ) {
-    succ = nd.getASuccessor() and
-    not succ instanceof BasicBlock
-  }
-
-  /**
-   * Holds if `nd` is the `i`th node in basic block `bb`.
-   *
-   * In other words, `i` is the shortest distance from a node `bb`
-   * that starts a basic block to `nd` along the `intraBBSucc` relation.
-   */
-  cached
-  predicate bbIndex(BasicBlock bb, ControlFlowNode nd, int i) =
-    shortestDistances(startsBB/1, intraBBSucc/2)(bb, nd, i)
-
-  cached
-  int bbLength(BasicBlock bb) { result = strictcount(ControlFlowNode nd | bbIndex(bb, nd, _)) }
-
-  cached
-  predicate useAt(BasicBlock bb, int i, Variable v, VarUse u) {
-    Stages::BasicBlocks::ref() and
-    v = u.getVariable() and
-    bbIndex(bb, u, i)
-  }
-
-  cached
-  predicate defAt(BasicBlock bb, int i, Variable v, VarDef d) {
-    exists(VarRef lhs |
-      lhs = d.getTarget().(BindingPattern).getABindingVarRef() and
-      v = lhs.getVariable()
-    |
-      lhs = d.getTarget() and
-      bbIndex(bb, d, i)
-      or
-      exists(PropertyPattern pp |
-        lhs = pp.getValuePattern() and
-        bbIndex(bb, pp, i)
-      )
-      or
-      exists(ObjectPattern op |
-        lhs = op.getRest() and
-        bbIndex(bb, lhs, i)
-      )
-      or
-      exists(ArrayPattern ap |
-        lhs = ap.getAnElement() and
-        bbIndex(bb, lhs, i)
-      )
-    )
-  }
-
-  cached
-  predicate reachableBB(BasicBlock bb) {
-    entryBB(bb)
-    or
-    exists(BasicBlock predBB | succBB(predBB, bb) | reachableBB(predBB))
-  }
-}
-
-private import Internal
-
-/** Gets the immediate dominator of `bb`. */
-cached
-private BasicBlock immediateDominator(BasicBlock bb) =
-  idominance(entryBB/1, succBB/2)(_, result, bb)
-
-/** Gets the immediate post-dominator of `bb`. */
-cached
-private BasicBlock immediatePostDominator(BasicBlock bb) =
-  idominance(exitBB/1, predBB/2)(_, result, bb)
-
-/**
- * A basic block, that is, a maximal straight-line sequence of control flow nodes
- * without branches or joins.
- *
- * At the database level, a basic block is represented by its first control flow node.
- */
-class BasicBlock extends @cfg_node, NodeInStmtContainer {
-  cached
-  BasicBlock() { Stages::BasicBlocks::ref() and startsBB(this) }
-
-  /** Gets a basic block succeeding this one. */
-  BasicBlock getASuccessor() { succBB(this, result) }
-
-  /** Gets a basic block preceding this one. */
-  BasicBlock getAPredecessor() { result.getASuccessor() = this }
-
-  /** Gets a node in this block. */
-  ControlFlowNode getANode() { result = this.getNode(_) }
-
-  /** Gets the node at the given position in this block. */
-  ControlFlowNode getNode(int pos) { bbIndex(this, result, pos) }
-
-  /** Gets the first node in this block. */
-  ControlFlowNode getFirstNode() { result = this }
-
-  /** Gets the last node in this block. */
-  ControlFlowNode getLastNode() { result = this.getNode(this.length() - 1) }
-
-  /** Gets the length of this block. */
-  int length() { result = bbLength(this) }
-
-  /** Holds if this basic block uses variable `v` in its `i`th node `u`. */
-  predicate useAt(int i, Variable v, VarUse u) { useAt(this, i, v, u) }
-
-  /** Holds if this basic block defines variable `v` in its `i`th node `d`. */
-  predicate defAt(int i, Variable v, VarDef d) { defAt(this, i, v, d) }
-
-  /**
-   * Holds if `v` is live at entry to this basic block and `u` is a use of `v`
-   * witnessing the liveness.
-   *
-   * In other words, `u` is a use of `v` that is reachable from the
-   * entry node of this basic block without going through a redefinition
-   * of `v`. The use `u` may either be in this basic block, or in another
-   * basic block reachable from this one.
-   */
-  predicate isLiveAtEntry(Variable v, VarUse u) {
-    // restrict `u` to be reachable from this basic block
-    u = this.getASuccessor*().getANode() and
-    (
-      // shortcut: if `v` is never defined, then it must be live
-      this.isDefinedInSameContainer(v)
-      implies
-      // otherwise, do full liveness computation
-      this.isLiveAtEntryImpl(v, u)
-    )
-  }
-
-  /**
-   * Holds if `v` is live at entry to this basic block and `u` is a use of `v`
-   * witnessing the liveness, where `v` is defined at least once in the enclosing
-   * function or script.
-   */
-  private predicate isLiveAtEntryImpl(Variable v, VarUse u) {
-    this.isLocallyLiveAtEntry(v, u)
-    or
-    this.isDefinedInSameContainer(v) and
-    not this.defAt(_, v, _) and
-    this.getASuccessor().isLiveAtEntryImpl(v, u)
-  }
-
-  /**
-   * Holds if `v` is defined at least once in the function or script to which
-   * this basic block belongs.
-   */
-  private predicate isDefinedInSameContainer(Variable v) {
-    exists(VarDef def | def.getAVariable() = v and def.getContainer() = this.getContainer())
-  }
-
-  /**
-   * Holds if `v` is a variable that is live at entry to this basic block.
-   *
-   * Note that this is equivalent to `bb.isLiveAtEntry(v, _)`, but may
-   * be more efficient on large databases.
-   */
-  predicate isLiveAtEntry(Variable v) {
-    this.isLocallyLiveAtEntry(v, _)
-    or
-    not this.defAt(_, v, _) and this.getASuccessor().isLiveAtEntry(v)
-  }
-
-  /**
-   * Holds if local variable `v` is live at entry to this basic block and
-   * `u` is a use of `v` witnessing the liveness.
-   */
-  predicate localIsLiveAtEntry(LocalVariable v, VarUse u) {
-    this.isLocallyLiveAtEntry(v, u)
-    or
-    not this.defAt(_, v, _) and this.getASuccessor().localIsLiveAtEntry(v, u)
-  }
-
-  /**
-   * Holds if local variable `v` is live at entry to this basic block.
-   */
-  predicate localIsLiveAtEntry(LocalVariable v) {
-    this.isLocallyLiveAtEntry(v, _)
-    or
-    not this.defAt(_, v, _) and this.getASuccessor().localIsLiveAtEntry(v)
-  }
-
-  /**
-   * Holds if `d` is a definition of `v` that is reachable from the beginning of
-   * this basic block without going through a redefinition of `v`.
-   */
-  predicate localMayBeOverwritten(LocalVariable v, VarDef d) {
-    this.isLocallyOverwritten(v, d)
-    or
-    not this.defAt(_, v, _) and this.getASuccessor().localMayBeOverwritten(v, d)
-  }
-
-  /**
-   * Gets the next index after `i` in this basic block at which `v` is
-   * defined or used, provided that `d` is a definition of `v` at index `i`.
-   * If there are no further uses or definitions of `v` after `i`, the
-   * result is the length of this basic block.
-   */
-  private int nextDefOrUseAfter(PurelyLocalVariable v, int i, VarDef d) {
-    this.defAt(i, v, d) and
-    result =
-      min(int j |
-        (this.defAt(j, v, _) or this.useAt(j, v, _) or j = this.length()) and
-        j > i
-      )
-  }
-
-  /**
-   * Holds if `d` defines variable `v` at the `i`th node of this basic block, and
-   * the definition is live, that is, the variable may be read after this
-   * definition and before a re-definition.
-   */
-  predicate localLiveDefAt(PurelyLocalVariable v, int i, VarDef d) {
-    exists(int j | j = this.nextDefOrUseAfter(v, i, d) |
-      this.useAt(j, v, _)
-      or
-      j = this.length() and this.getASuccessor().localIsLiveAtEntry(v)
-    )
-  }
-
-  /**
-   * Holds if `u` is a use of `v` in this basic block, and there are
-   * no definitions of `v` before it.
-   */
-  private predicate isLocallyLiveAtEntry(Variable v, VarUse u) {
-    exists(int n | this.useAt(n, v, u) | not exists(int m | m < n | this.defAt(m, v, _)))
-  }
-
-  /**
-   * Holds if `d` is a definition of `v` in this basic block, and there are
-   * no other definitions of `v` before it.
-   */
-  private predicate isLocallyOverwritten(Variable v, VarDef d) {
-    exists(int n | this.defAt(n, v, d) | not exists(int m | m < n | this.defAt(m, v, _)))
-  }
-
-  /**
-   * Gets the basic block that immediately dominates this basic block.
-   */
-  ReachableBasicBlock getImmediateDominator() { result = immediateDominator(this) }
-
-  /**
-   * Holds if this if a basic block whose last node is an exit node.
-   */
-  predicate isExitBlock() { exitBB(this) }
-}
-
-/**
- * An unreachable basic block, that is, a basic block
- * whose first node is unreachable.
- */
-class UnreachableBlock extends BasicBlock {
-  UnreachableBlock() { this.getFirstNode().isUnreachable() }
-}
-
-/**
- * An entry basic block, that is, a basic block
- * whose first node is the entry node of a statement container.
- */
-class EntryBasicBlock extends BasicBlock {
-  EntryBasicBlock() { entryBB(this) }
-}
-
-/**
- * A basic block that is reachable from an entry basic block.
- */
-class ReachableBasicBlock extends BasicBlock {
-  ReachableBasicBlock() { reachableBB(this) }
-
-  /**
-   * Holds if this basic block strictly dominates `bb`.
-   */
-  pragma[inline]
-  predicate strictlyDominates(ReachableBasicBlock bb) { this = immediateDominator+(bb) }
-
-  /**
-   * Holds if this basic block dominates `bb`.
-   *
-   * This predicate is reflexive: each reachable basic block dominates itself.
-   */
-  pragma[inline]
-  predicate dominates(ReachableBasicBlock bb) { this = immediateDominator*(bb) }
-
-  /**
-   * Holds if this basic block strictly post-dominates `bb`.
-   */
-  pragma[inline]
-  predicate strictlyPostDominates(ReachableBasicBlock bb) { this = immediatePostDominator+(bb) }
-
-  /**
-   * Holds if this basic block post-dominates `bb`.
-   *
-   * This predicate is reflexive: each reachable basic block post-dominates itself.
-   */
-  pragma[inline]
-  predicate postDominates(ReachableBasicBlock bb) { this = immediatePostDominator*(bb) }
-}
-
-/**
- * A reachable basic block with more than one predecessor.
- */
-class ReachableJoinBlock extends ReachableBasicBlock {
-  ReachableJoinBlock() { this.getFirstNode().isJoin() }
-
-  /**
-   * Holds if this basic block belongs to the dominance frontier of `b`, that is
-   * `b` dominates a predecessor of this block, but not this block itself.
-   *
-   * Algorithm from Cooper et al., "A Simple, Fast Dominance Algorithm" (Figure 5),
-   * who in turn attribute it to Ferrante et al., "The program dependence graph and
-   * its use in optimization".
-   */
-  predicate inDominanceFrontierOf(ReachableBasicBlock b) {
-    b = this.getAPredecessor() and not b = this.getImmediateDominator()
-    or
-    exists(ReachableBasicBlock prev | this.inDominanceFrontierOf(prev) |
-      b = prev.getImmediateDominator() and
-      not b = this.getImmediateDominator()
-    )
-  }
-}
+import internal.BasicBlockInternal