Program Dependence Graph (PDG) — Engineering Design

Program dependence graph — data and control dependencies between statements within a function. Foundation for slicing, taint, and dataflow visualization.

Dataflow tab — PDG graph (gbuilder)

Figure 1: Dataflow tab in PDG mode — variable selector, control/data edge toggles, and Sigma PDG layout.

1. Goals

Goal	How
Statement-level deps	Data edges (def-use) + control edges
Slice foundation	Backward/forward slice = graph reachability
Interactive exploration	Dashboard variable filter + statement list
CLI export	`inspect SYMBOL pdg` with `--edge-layer data

2. Architecture overview

flowchart TB
  CFG[ControlFlowGraph]
  PDG[ProgramDependenceGraph]
  CFG --> PDG

  subgraph edges["PDG edges"]
    DATA[Data dependence]
    CTRL[Control dependence]
  end

  PDG --> DATA
  PDG --> CTRL

  subgraph consumers["Consumers"]
    SLICE[Program slice]
    TAINT[Taint flows]
    DF[DataflowView]
  end

  PDG --> SLICE
  PDG --> TAINT
  PDG --> DF

PDG is built after CFG dominator analysis (crates/rgctl-analysis/src/pdg.rs).

Opt-in fidelity (hybrid CPG P3)

Flag / option	Effect
`discover --with-dfg-loops`	Sets `PdgBuildOptions::classify_loop_carried` — tags `DataDependency.loop_carried` when the use block can reach the def block on the CFG (loop-carried)
`cpg flows --with-alias`	On-demand may-alias expansion (`alias::may_alias_names`) for copies + field bases; does not rewrite the PDG archive

Default discover --with-cfg leaves loop_carried = false on all edges.

3. Dashboard dataflow mode

Control	Effect
Variable dropdown	Highlight def-use neighborhood for one name
Include control	Add control-dependence edges
Include CFG overlay	Superimpose block structure
Statement list	Click row ↔ graph node

Exported via dataflow_index.json + shared slice/{id}.json PDG payloads.

4. Rust implementation map

Component	Path
PDG construction	`crates/rgctl-analysis/src/pdg.rs`
Dominators (input)	`crates/rgctl-analysis/src/dominance.rs`
Slicing	`crates/rgctl-analysis/src/slicing.rs`
CLI	`src/cli/inspect.rs`
Dashboard engine	`dashboard/src/dataflowEngine.ts`

5. Dashboard implementation

Piece	Path
Tab	`dashboard/src/DataflowView.tsx`
View mode	`dataflow` (PDG) vs `dominator` (see dominance design)
Legends	`PDG_NODE_LEGEND`, `PDG_EDGE_LEGEND` in `viewLegendData.ts`
Worker	`loadCfgDetail` + bundle PDG from `slice/`

6. CLI usage

rgctl discover . --cfg
rgctl inspect process pdg --edge-layer data
rgctl inspect process pdg --def-use
rgctl -f mermaid inspect process pdg
rgctl slice src/Foo.java --line 42 --variable x --view pdg

7. Testing

Layer	Location
PDG unit tests	`crates/rgctl-analysis/src/pdg.rs`
Dataflow export	`tests/dashboard_harness.rs` (`dataflow_index.json`)

Screenshots: capture-design-screenshots.mjs → docs/images/design/pdg/.