Table of Contents

5.4.2.6 Do Statement

Note

This section describes the implementation of MS-VBAL §5.4.2.6 Do Statement.

Syntax

Do...Loop has five header shapes, each represented by its own node type.

AST node Instruction kind(s) Notes
DoLoopStatementNode Jump Bare Do … Loop. The back-edge Jump carries the loop's own node; its Target is the body's first instruction.
DoWhileLoopStatementNode ConditionalBranch, Jump Pre-test Do While … Loop. The header's Else is right past the loop; a synthesized back-edge Jump (Node = null) targets the header.
DoUntilLoopStatementNode ConditionalBranch, Jump Pre-test Do Until … Loop; the same shape as Do While.
DoLoopWhileStatementNode LoopBack Post-test Do … Loop While. The LoopBack closer carries the loop's own node; its Target is the body's first instruction.
DoLoopUntilStatementNode LoopBack Post-test Do … Loop Until; the same shape as Loop While.

The …Until half of each pre-test and post-test pair shares its node shape with the …While half. Loop has no AST node of its own (RD-VBAL §3.4.1 Block Statements; RD-VBAL §3.5.2 Instruction).

Static Semantics

MS-VBAL models …While and …Until loops as the same construct with opposite exit polarity.

Exit Do resolves against the innermost enclosing loop of any of the five forms (RD-VBAL §5.4.2.7 Exit Do Statement).

Runtime Semantics

Form Dispatch
Pre-test: Do While, Do Until A ConditionalBranch, dispatched in the same way as an If header (RD-VBAL §5.4.2.8 If Statement). When the loop continues, execution falls through into the body; when it ends, it goes to the header's Else offset, right past the loop.
Post-test: Do…Loop While, Do…Loop Until LoopBack: evaluate the condition; branch back to the body's first instruction (Instruction.Target) when the loop continues, and fall through when it ends.
Bare: Do…Loop An unconditional Jump back to its own body. It needs no dispatch of its own.

The condition of a pre-test or post-test loop is a Boolean condition, evaluated by the same ConditionEvaluator an If uses, and forced to Boolean (MS-VBAL §5.5.1.2.2 Let-coercion to and from Boolean).

Loop Exits when the condition is
…While False
…Until True

For an …Until loop, the evaluated Boolean is inverted before branching.

Pre-test, post-test and bare Do loops need no per-activation state.

Implementation

  • ProcedureExecutor dispatches the pre-test header's ConditionalBranch, the post-test closer's LoopBack, and the bare loop's Jump.
  • A post-test loop's closer evaluates the condition and decides whether to branch back. The LoopBack instruction carries the loop's own node: it is the loop's only instruction (RD-VBAL §3.5.3 Lowering Block Statements).
  • GetCondition returns a (Condition, Negate) pair instead of a bare expression; Negate is true for an …Until loop. The evaluated Boolean loop condition is inverted before branching when Negate is true. The pair avoids duplicating the branch logic for a second polarity.
  • RDCore.Runtime.Execution.ConditionEvaluator forces the condition to Boolean (RD-VBAL §3.5.4 Execution).

⏮️ RD-VBAL §5.4.2.5 Exit For Statement | ⏭️ RD-VBAL §5.4.2.7 Exit Do Statement

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