ExpressionPtr OrExpression::getDNFImpl() const
{
UnaryExpression * leftLeaf = dynamic_cast<UnaryExpression*>(left());
UnaryExpression * rightLeaf = dynamic_cast<UnaryExpression*>(right());
// A or B
if (leftLeaf && rightLeaf)
{
return clone();
}
// A or (B and C) => (A and B) or (A and C)
BinaryExpression * rightAnd = dynamic_cast<AndExpression*>(right());
if (leftLeaf && rightAnd)
{
return Or(And(leftLeaf, rightAnd->left()),
And(leftLeaf, rightAnd->right()));
}
// (A and B) or C => (A and C) or (B and C)
BinaryExpression * leftAnd = dynamic_cast<AndExpression*>(left());
if (leftAnd && rightLeaf)
{
return Or(And(leftAnd->left(), rightLeaf),
And(leftAnd->right(), rightLeaf));
}
return Or(left()->getDNF(), right()->getDNF());
}
// visitForTest helps optimize statements with expressions like:
// if (x && y || z)
//
// Where, with a naive conversion we would reduce (x && y) to a boolean, and
// then reduce (that || z) to another boolean, and another test - we can simply
// test each individual component and combine the jump paths with the paths
// the if-test needs to take.
//
// Unfortunately, this logic comes at a complexity price: it is illegal to
// emit non-statements from SemA, since the following situation could occur:
// (1) SemA builds HIR chain #1 for statement X.
// (2) SemA builds HIR chain #2 for statement X.
// (3) SemA forces HIR chain #2 to be emitted to the code stream.
// (4) SemA emits statement X, with HIR chain #1.
//
// In this situation, the HIR chains have been emitted out-of-order. So,
// visitForTest takes in a HIRList which it populates with instructions, which
// may include internal statements (like jumps and binds).
//
// The trueBranch and falseBranch parameters are self-explanatory. The
// fallthrough case describes which branch comes immediately after the
// test. This is used to optimize cases like:
// if (x) {
// ...
// }
// do {
// ...
// } while (y);
//
// After testing |x|, if it evaluated to true, there is no need to jump to the
// true branch, because we'll fallthrough (as long as the expression does not
// have logical ands/ors which could short-circuit). Similarly for |y|, the
// false condition does not need an actual jump target.
//
void
SemanticAnalysis::visitForTest(HIRList *output,
Expression *expr,
Label *trueBranch,
Label *falseBranch,
Label *fallthrough)
{
Type *boolType = cc_.types()->getPrimitive(PrimitiveType::Bool);
// Handle logical and/or.
BinaryExpression *bin = expr->asBinaryExpression();
if (bin && (bin->token() == TOK_AND || bin->token() == TOK_OR)) {
HLabel *next = new (pool_) HLabel();
if (bin->token() == TOK_AND)
visitForTest(output, bin->left(), next, falseBranch, next);
else
visitForTest(output, bin->left(), trueBranch, next, next);
output->append(new (pool_) HBind(expr, next));
visitForTest(output, bin->right(), trueBranch, falseBranch, fallthrough);
return;
}
// Handle equality and relational operators.
if (bin && (bin->token() >= TOK_EQUALS && bin->token() <= TOK_GE)) {
HIR *left = rvalue(bin->left());
HIR *right = rvalue(bin->right());
if (!left || !right)
return;
Type *coercion = coercionType(bin->token(), left, right);
if (!coercion ||
((left = coerce(left, coercion, Coerce_Arg)) == nullptr) ||
((right = coerce(right, coercion, Coerce_Arg)) == nullptr))
{
return;
}
Label *target = (fallthrough == trueBranch) ? falseBranch : trueBranch;
TokenKind token = (fallthrough == trueBranch)
? InvertTest(bin->token())
: bin->token();
output->append(new (pool_) HCompareAndJump(bin, token, left, right, target));
return;
}
// Handle unary not (!)
UnaryExpression *unary = expr->asUnaryExpression();
if (unary && unary->token() == TOK_NOT) {
// Re-invoke visitForTest but invert the branches. Note that we don't touch
// |fallthrough|, since it's used to determine whether to jump on success
// or failure, so inverting it as well would do nothing.
visitForTest(output, unary->expression(), falseBranch, trueBranch, fallthrough);
return;
}
// We couldn't match anything easy, so just coerce the input to a boolean.
HIR *hir = rvalue(expr);
if (!hir || ((hir = coerce(hir, boolType, Coerce_Assign)) == nullptr))
return;
if (fallthrough == falseBranch)
output->append(new (pool_) HJump(expr, hir, true, trueBranch));
else
output->append(new (pool_) HJump(expr, hir, false, falseBranch));
}