Cx Dev Log - 2026-03-17

#cx #programming

Bringing control flow to a compiler and shipping a more feature-complete frontend branch in one day changes the landscape for the Cx language project.

Backend Phase 5: Control Flow Takes Shape

We nailed the largest lump commit in backend history on codex/backend-ir-from-main. If you looked under the hood, you'd see 2,559 insertions across three files. Most notable are the 1,656 new lines within lower.rs and the creation of validate.rs with 1,052 lines.

Prior to this, IR was pretty much linear—a straight shot from start to finish. Now we’ve integrated true control flow graphs. This means we've got conditional branches, explicit blocks for then/else/merge, and chaining for else-if clauses. Critical here is how SSA (Static Single Assignment) environments are split and merged, ditching phi nodes in favor of block parameters especially for merge points. This shift wasn't new—it harks back to decisions made in Phase 1, but today marked its real-world test and validation. It's cleaner, lighter on bookkeeping, and passed through branching logic without a hitch.

We’ve also dealt with dead branches elegantly, ensured branch-specific temporaries work seamlessly, and the validator is up to scratch for multi-block functions and a synthetic main routine.

Let's not overlook the validator overhaul itself. This beast of a file includes over a thousand lines of checks, making sure nothing slips through unnoticed. Mismatching IR or malformed structures are caught immediately—a necessity as we expand capabilities without trialing features in the wild before they're tested.

Submain: A Frontend on the Brink of Change

Over in the submain branch, three distinct commits moved the needle further than ever, elevating submain above the main for now.

Starting with structs, the first two development phases landed in one go, stepping through 19 files across the pipeline. Struct definitions, instancing, reading/writing fields—it's all there. Add in method dispatch via impl blocks, including mutations propagated back to callers, and you're looking at robust support for compound assignments like p.health -= amount, facilitated by a new AssignTarget enum helping differentiate variable versus field targets. Three verification matrix tests ensure these changes don't break anything: impl_basic, impl_return, and compound_assign_dot.

A smaller change, but significant, added != and unary ! operators while also tidying up process exit codes to properly signal errors. Negative testing got a boost with .expected_fail markers, and run_matrix.sh entered as the definitive test runner.

Last up, we tackled Semantic Parity Phase 1. This translates frontend structures into the semantic layers fully, covering constructs from StructDef to MethodCall. Important note—while some paths still rely on the raw AST, this phase preps everything for the full semantic shift later.

Every change passed the verification matrix with greens across all tests—nothing broke.

Why It Matters

Submain is standing as the proving ground for frontend consolidation. It’s about gestating feature changes together and deploying them as coherent units. Today’s push exemplified this as structs, semantic improvements, and operator handling merged into submain.

Incremental semantic parity is the name of the game. We pinpoint types and analysis, leaving the interpreter with the raw AST for now, but eventually, that dependency will dissolve post-parity verification.

The Road Ahead

The endgame is merging these branches, both carrying critical updates. Submain’s structural and semantic parity features need to meet main, clearing the way for further semantic work. But there’s backend momentum too, heading into Phase 6 which involves function call lowering, allowing these lowered functions to communicate fully—arguments, validations, the works. Keep an eye on Semantic Parity Phase 2, as it transitions from the raw AST to a SemanticProgram for interpretation. These advancements will run parallel until convergence in main, pushing Cx to new heights.

Follow the Cx language project: