Method GetCodegendComputeFnImpl

be/src/exprs/case-expr.cc:180–280

Sample IR output when there is a case expression and else expression define i16 @CaseExpr(%"class.impala::ScalarExprEvaluator"* %context, %"class.impala::TupleRow"* %row) #20 { eval_case_expr: %case_val = call i64 @GetSlotRef(%"class.impala::ScalarExprEvaluator"* %context, %"class.impala::TupleRow"* %row) %is_null = trunc i64 %case_val to i1 br i1 %is_null, label %return_else_expr, label %eval_fir

//   ret i16 %else_val
// }
Status CaseExpr::GetCodegendComputeFnImpl(LlvmCodeGen* codegen, llvm::Function** fn) {
  const int num_children = GetNumChildren();
  vector<llvm::Function*> child_fns(num_children, nullptr);
  for (int i = 0; i < num_children; ++i) {
    RETURN_IF_ERROR(GetChild(i)->GetCodegendComputeFn(codegen, false, &child_fns[i]));
  }

  llvm::LLVMContext& context = codegen->context();
  LlvmBuilder builder(context);

  llvm::Value* args[2];
  llvm::Function* function = CreateIrFunctionPrototype("CaseExpr", codegen, &args);
  llvm::BasicBlock* eval_case_expr_block = nullptr;

  // This is the block immediately after the when/then exprs. It will either point to a
  // block which returns the else expr, or returns NULL if no else expr is specified.
  llvm::BasicBlock* default_value_block = llvm::BasicBlock::Create(
      context, has_else_expr() ? "return_else_expr" : "return_null", function);

  // If there is a case expression, create a block to evaluate it.
  CodegenAnyVal case_val;
  llvm::BasicBlock* eval_first_when_expr_block = llvm::BasicBlock::Create(
      context, "eval_first_when_expr", function, default_value_block);
  llvm::BasicBlock* current_when_expr_block = eval_first_when_expr_block;
  if (has_case_expr()) {
    // Need at least case, when and then expr, and optionally an else expr
    DCHECK_GE(num_children, has_else_expr() ? 4 : 3);
    // If there is a case expr, create block eval_case_expr to evaluate the
    // case expr. Place this block before eval_first_when_expr_block
    eval_case_expr_block = llvm::BasicBlock::Create(
        context, "eval_case_expr", function, eval_first_when_expr_block);
    builder.SetInsertPoint(eval_case_expr_block);
    case_val = CodegenAnyVal::CreateCallWrapped(
        codegen, &builder, children()[0]->type(), child_fns[0], args, "case_val");
    builder.CreateCondBr(
        case_val.GetIsNull(), default_value_block, eval_first_when_expr_block);
  } else {
    DCHECK_GE(num_children, has_else_expr() ? 3 : 2);
  }

  const int loop_end = has_else_expr() ? num_children - 1 : num_children;
  const int last_loop_iter = loop_end - 2;
  // The loop increments by two each time, because each iteration handles one when/then
  // pair. Both when and then subexpressions are single children. If there is a case expr
  // start loop at index 1. (case expr is GetChild(0) and has already be evaluated.
  for (int i = has_case_expr() ? 1 : 0; i < loop_end; i += 2) {
    llvm::BasicBlock* check_when_expr_block = llvm::BasicBlock::Create(
        context, "check_when_expr_block", function, default_value_block);
    llvm::BasicBlock* return_then_expr_block = llvm::BasicBlock::Create(
        context, "return_then_expr", function, default_value_block);

    // continue_or_exit_block either points to the next eval_next_when_expr block,
    // or points to the defaut_value_block if there are no more when/then expressions.
    llvm::BasicBlock* continue_or_exit_block = nullptr;
    if (i == last_loop_iter) {
      continue_or_exit_block = default_value_block;
    } else {
      continue_or_exit_block = llvm::BasicBlock::Create(