[analyzer][Liveness][NFC] Remove an unneeded pass to collect variables that appear in an assignment

[Szelethus]

Summary

Suppose you stumble across a DeclRefExpr in the AST, that references a VarDecl. How would you know that that variable is written in the containing statement, or not? One trick would be to ascend the AST through Stmt::getParent, and see whether the variable appears on the left hand side of the assignment.

Liveness does something similar, but instead of ascending the AST, it descends into it with a StmtVisitor, and after finding an assignment, it notes that the LHS appears in the context of an assignemnt. However, as [1] demonstrates, the analysis isn't ran on the AST of an entire function, but rather on CFG, where the order of the statements, visited in order, would make it impossible to know this information by descending.

void f() {
  int i;

  i = 5;
}

`-FunctionDecl 0x55a6e1b070b8 <test.cpp:1:1, line:5:1> line:1:6 f 'void ()'
  `-CompoundStmt 0x55a6e1b07298 <col:10, line:5:1>
    |-DeclStmt 0x55a6e1b07220 <line:2:3, col:8>
    | `-VarDecl 0x55a6e1b071b8 <col:3, col:7> col:7 used i 'int'
    `-BinaryOperator 0x55a6e1b07278 <line:4:3, col:7> 'int' lvalue '='
      |-DeclRefExpr 0x55a6e1b07238 <col:3> 'int' lvalue Var 0x55a6e1b071b8 'i' 'int'
      `-IntegerLiteral 0x55a6e1b07258 <col:7> 'int' 5

void f()
 [B2 (ENTRY)]
   Succs (1): B1

 [B1]
   1: int i;
   2: 5
   3: i
   4: [B1.3] = [B1.2]
   Preds (1): B2
   Succs (1): B0

 [B0 (EXIT)]
   Preds (1): B1

You can see that the arguments (rightfully so, they need to be evaluated first) precede the assignment operator. For this reason, Liveness implemented a pass to scan the CFG and note which variables appear in an assignment.

BUT.

This problem only exists if we traverse a CFGBlock in order. And Liveness in fact does it reverse order. So a distinct pass is indeed unnecessary, we can note the appearance of the assignment by the time we reach the variable.

[1] http://lists.llvm.org/pipermail/cfe-dev/2020-July/066330.html

[xazax.hun]

clang/lib/Analysis/LiveVariables.cpp

326

Do we actually need to specially handle BO_Assign? What about += and other assignment operators? I know that the original code did not consider them, but I do not have a good intuition why.

[xazax.hun]

clang/lib/Analysis/LiveVariables.cpp

326

Never mind, I those operators are reads too :)

[xazax.hun]

LGTM! Looks a lot cleaner this way.

[xazax.hun]

clang/lib/Analysis/LiveVariables.cpp

332

Nit: Maybe moving this to the front of the function would simplify the code a bit.

[martong]

This problem only exists if we traverse a CFGBlock in order. And Liveness in fact does it reverse order. So a distinct pass is indeed unnecessary, we can note the appearance of the assignment by the time we reach the variable.

Should this patch depend on https://reviews.llvm.org/D87519?

clang/include/clang/Analysis/CFG.h

1311

Do we convert this to CFGBlock *Entry here? If yes, please comment it there in the code, so others should not be forced to dig up the first data member. (If no then this code is hard to understand.)

Even better, why not return *Entry?

[martong]

clang/include/clang/Analysis/CFG.h

1311

Uuups, okay, so we use begin and end here, my bad. But then perhaps it is better to explicitly write this down. E.g. return {begin(), end()}, would be much cleaner IMHO.

clang/include/clang/Analysis/CFG.h

public:
 
   iterator                  begin()                { return Blocks.begin(); }
   iterator                  end()                  { return Blocks.end(); }
   const_iterator            begin()       const    { return Blocks.begin(); }
   const_iterator            end()         const    { return Blocks.end(); }
 
   iterator nodes_begin() { return iterator(Blocks.begin()); }
   iterator nodes_end() { return iterator(Blocks.end()); }
+
+  llvm::iterator_range<iterator> nodes() { return *this; }
+  llvm::iterator_range<const_iterator> const_nodes() const { return *this; }
+
   const_iterator nodes_begin() const { return const_iterator(Blocks.begin()); }
   const_iterator nodes_end() const { return const_iterator(Blocks.end()); }
 
   reverse_iterator          rbegin()               { return Blocks.rbegin(); }
   reverse_iterator          rend()                 { return Blocks.rend(); }
   const_reverse_iterator    rbegin()      const    { return Blocks.rbegin(); }
   const_reverse_iterator    rend()        const    { return Blocks.rend(); }
 
+  llvm::iterator_range<reverse_iterator> reverse_nodes() {
+    return {rbegin(), rend()};
+  }
+  llvm::iterator_range<const_reverse_iterator> const_reverse_nodes() const {
+    return {rbegin(), rend()};
+  }
+
   CFGBlock &                getEntry()             { return *Entry; }
   const CFGBlock &          getEntry()    const    { return *Entry; }
   CFGBlock &                getExit()              { return *Exit; }
   const CFGBlock &          getExit()     const    { return *Exit; }
 
   CFGBlock *       getIndirectGotoBlock() { return IndirectGotoBlock; }
   const CFGBlock * getIndirectGotoBlock() const { return IndirectGotoBlock; }
 

clang/lib/Analysis/LiveVariables.cpp

 }
 
 static bool writeShouldKill(const VarDecl *VD) {
   return VD && !VD->getType()->isReferenceType() &&
     !isAlwaysAlive(VD);
 }
 
 void TransferFunctions::VisitBinaryOperator(BinaryOperator *B) {
+  if (LV.killAtAssign && B->getOpcode() == BO_Assign) {
+    if (const auto *DR = dyn_cast<DeclRefExpr>(B->getLHS()->IgnoreParens())) {
+      LV.inAssignment[DR] = 1;
+    }
+  }
   if (B->isAssignmentOp()) {
     if (!LV.killAtAssign)
       return;
 
     // Assigning to a variable?
     Expr *LHS = B->getLHS()->IgnoreParens();
 
     if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(LHS)) {
       const Decl* D = DR->getDecl();
       bool Killed = false;
LiveVariables::computeLiveness(AnalysisDeclContext &AC, bool killAtAssign) {
   LiveVariablesImpl *LV = new LiveVariablesImpl(AC, killAtAssign);
 
   // Construct the dataflow worklist.  Enqueue the exit block as the
   // start of the analysis.
   BackwardDataflowWorklist worklist(*cfg, AC);
   llvm::BitVector everAnalyzedBlock(cfg->getNumBlockIDs());
 
   // FIXME: we should enqueue using post order.
-  for (CFG::const_iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it) {
-    const CFGBlock *block = *it;
-    worklist.enqueueBlock(block);
-
-    // FIXME: Scan for DeclRefExprs using in the LHS of an assignment.
-    // We need to do this because we lack context in the reverse analysis
-    // to determine if a DeclRefExpr appears in such a context, and thus
-    // doesn't constitute a "use".
-    if (killAtAssign)
-      for (CFGBlock::const_iterator bi = block->begin(), be = block->end();
-           bi != be; ++bi) {
-        if (Optional<CFGStmt> cs = bi->getAs<CFGStmt>()) {
-          const Stmt* stmt = cs->getStmt();
-          if (const auto *BO = dyn_cast<BinaryOperator>(stmt)) {
-            if (BO->getOpcode() == BO_Assign) {
-              if (const auto *DR =
-                    dyn_cast<DeclRefExpr>(BO->getLHS()->IgnoreParens())) {
-                LV->inAssignment[DR] = 1;
-              }
-            }
-          }
-        }
-      }
+  for (const CFGBlock *B : cfg->nodes()) {
+    worklist.enqueueBlock(B);
   }
 
   while (const CFGBlock *block = worklist.dequeue()) {
     // Determine if the block's end value has changed.  If not, we
     // have nothing left to do for this block.
     LivenessValues &prevVal = LV->blocksEndToLiveness[block];
 
     // Merge the values of all successor blocks.