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Report #022: VSCode Tooling Polish + SMC Codegen Fixes

Date: 2026-03-04 Scope: VSCode extension v0.5.0 fixes, new Compile & Run command, 3 codegen bug fixes, playground example

Summary

A hands-on session starting from "how do I install the VSCode tools?" uncovered and fixed a chain of issues: TypeScript compilation errors in the extension, missing/broken commands, incorrect compiler flags, and three codegen bugs in the SMC (Self-Modifying Code) pipeline. We now have a working edit-compile-run workflow in VSCode.

1. VSCode Extension Fixes

1.1 TypeScript Compilation Error

Problem: surroundingPairs was set in the programmatic vscode.languages.setLanguageConfiguration() call, but that property only exists in the declarative language-configuration.json file.

Fix: Removed surroundingPairs from extension.ts. It was already correctly declared in language-configuration.json.

1.2 Invalid -O Compiler Flag

Problem: The "Compile" and "Compile Optimized" commands passed -O to mz, which doesn't exist. MinZ optimizations are enabled by default — you disable them with --disable-optimize.

Fix:

  • compileMinZ() now passes --disable-optimize only when the user has optimizations turned off in settings
  • compileOptimized() no longer passes -O

1.3 Only 2 of 10 Commands Visible

Problem: The initial broken .vsix (with TypeScript errors) was installed. Only commands that didn't depend on the broken code were registered.

Fix: After fixing the TypeScript error, repackage and reinstall.

1.4 New "Compile & Run (CP/M)" Command

Added: minz.compileAndRun — a single command that:

  1. Compiles MinZ source to Z80 assembly (mz ... -t cpm)
  2. Assembles to .com binary (mza)
  3. Opens a VSCode terminal and runs the emulator (mze -t cpm)

Keybinding: Cmd+Alt+R (Mac) / Ctrl+Alt+R (Win/Linux)

This is the first command that closes the edit-compile-run loop entirely inside VSCode.

2. Three SMC Codegen Fixes

During testing, we discovered that most MinZ programs with function calls fail at the assembler stage. Root cause: the SMC (Self-Modifying Code) codegen pipeline emits assembly labels that are never defined.

2.1 Missing Patchable Return in TRUE SMC Functions

File: minzc/pkg/codegen/z80.go

Problem: Three codegen paths exist for functions:

  1. generateFunction() — normal functions (had patchable return support)
  2. generateSMCFunction() — SMC functions (had patchable return support)
  3. generateTrueSMCFunction() — TRUE SMC functions (missing patchable return)

When the caller (generateSMCAnnotatedCall) emits LD (func_return_patch_op), A, it expects the target function to have a func_return_patch_op: label. But TRUE SMC functions just emitted a plain RET.

Fix: Added NeedsPatchPoints checking to generateTrueSMCFunction(), both for the last-instruction-is-return case and the epilogue fallthrough case.

2.2 NeedsPatchPoints Not Set From Codegen

Problem: NeedsPatchPoints was only set in the semantic analyzer's generateInstructionPatchingCall(), which isn't always the active code path. When the codegen encounters an OpCall with SMC annotations, it needs the target function to have patch points.

Fix: In the OpCall handler, when an SMC call-site annotation is detected, look up the target function via findFunction() and set NeedsPatchPoints = true. Since main is generated first and other functions follow, the flag is set before the target function body is emitted.

2.3 Wrong Label for Parameter Patching

Problem: The caller emitted:

LD (func_param_n+1), A    ; references label+offset

But the function body defined:

func_param_n_immOP:
    LD A, 0
func_param_n_imm0 EQU func_param_n_immOP+1

The +1 arithmetic works in MZA's expression evaluator, but the label names didn't match at all: _param_n vs _param_n_immOP.

Fix: Changed generateSMCAnnotatedCall() and generatePatchParam() to emit _param_n_imm0 (the EQU label) instead of _param_n+1.

Impact

These three fixes unblock assembly of any MinZ program that uses function calls with the default SMC optimization. Previously, the only programs that assembled were either:

  • Pure inline assembly (cpm_hello.minz)
  • Programs with no function calls returning values

3. The Playground Example

Created examples/cpm/playground.minz as a self-contained demo file:

// MinZ Playground — edit, compile, and run!

fun putchar(ch: u8) -> void {
    asm {
        LD E, A
        LD C, 2
        CALL 5
    }
}

fun newline() -> void {
    putchar(13);
    putchar(10);
}

fun main() -> void {
    putchar('H'); putchar('e'); putchar('l'); putchar('l'); putchar('o');
    putchar(' ');
    putchar('M'); putchar('i'); putchar('n'); putchar('Z'); putchar('!');
    newline();

    let mut i: u8 = 0;
    while i < 10 {
        putchar(i + 48);
        putchar(' ');
        i = i + 1;
    }
    newline();

    let x: u8 = 3 + 4;
    putchar('3'); putchar('+'); putchar('4'); putchar('=');
    putchar(x + 48);
    newline();

    putchar('O'); putchar('K');
    newline();
}

Output:

Hello MinZ!
0 1 2 3 4 5 6 7 8 9
3+4=7
OK

What's Cool (Optimizer in Action)

The MIR dump (--dump-mir) reveals the optimizer's work:

  1. Loop Rerolling — 11 consecutive putchar() calls for "Hello MinZ!" are detected and merged into a single print_string() call with a length-prefixed string literal. Similarly, putchar('+'); putchar('4'); putchar('=') becomes print_string("+4=").

  2. Constant Folding — 3 + 4 is folded to 7 at compile time. The MIR shows r57 = 7 ; Folded: with no runtime addition.

  3. Function Inlining — newline() is inlined at every call site. The MIR shows ; Inlined from cpm.playground.newline.

  4. Peephole Optimization — XOR A for zeroing (instead of LD A, 0), INC A for +1 after a known zero value.

  5. Assembly Peephole — 12 asm-level patterns applied (redundant load elimination, etc.)

Binary Size Comparison

Build ASM Lines Binary Size
--disable-optimize 377 474 bytes
--disable-ir-opt (asm peephole only) 360 411 bytes
Full optimizations 312 354 bytes

Full optimization saves 25% binary size on this example.

Optimization Flags Reference

# Full optimizations (default)
mz program.minz -t cpm -o out.a80

# No optimizations at all
mz program.minz -t cpm --disable-optimize -o out.a80

# No IR/MIR optimizations (disables reroll, inlining, constant folding, DCE)
# Keeps asm peephole and codegen-level opts
mz program.minz -t cpm --disable-ir-opt -o out.a80

# No asm-level peephole
mz program.minz -t cpm --disable-asm-opt -o out.a80

# No codegen-level optimizations (constant tracking, value propagation)
mz program.minz -t cpm --disable-codegen-opt -o out.a80

# No SMC (self-modifying code)
mz program.minz -t cpm --disable-smc -o out.a80

# No CTIE (compile-time interface execution)
mz program.minz -t cpm --disable-ctie -o out.a80

# Show what the optimizer does
mz program.minz -t cpm --compile-trace -o out.a80

# Dump MIR intermediate representation
mz program.minz -t cpm --dump-mir

# Dump AST
mz program.minz -t cpm --dump-ast

4. What Can Be Improved

4.1 Register Allocator Bottleneck (Critical)

The assembly output reveals the #1 performance problem. Every virtual register goes through memory at $F0xx:

LD ($F056), A     ; Virtual register 43 to memory
LD A, 48
LD ($F058), A     ; Virtual register 44 to memory
LD HL, ($F056)    ; Virtual register 43 from memory

This is 3 extra memory operations for what should be ADD A, 48. The ideal output:

LD A, (i_addr)    ; load i
ADD A, 48         ; add '0'

Impact: ~5x T-state overhead per operation. The register allocator's memory fallback path ($F0xx) is used for almost every virtual register because the physical register allocator doesn't handle enough cases.

4.2 Loop Reroll Boundary Bug (Minor)

The optimized output prints:

O
OK

instead of just OK. The reroller captures putchar('O') separately, then the next newline() + putchar('O') + putchar('K') gets rerolled into "\nOK". The standalone O + its preceding newline end up as separate instructions.

The unoptimized output is correct: OK on one line.

4.3 Function Return Values via SMC (Known Bug)

Functions that return computed values (not constants) produce wrong results. add(3, 4) returns 0 instead of 7. The SMC parameter patching writes values correctly, but the return value flow through the patchable return sequence has issues. This affects:

  • Any function with return expression where expression depends on parameters
  • Fibonacci, factorial, etc.

The playground example works around this by computing 3 + 4 inline (constant-folded to 7) instead of calling a function.

4.4 print_string Uses ZX Spectrum ROM Call

The rerolled print_string helper uses RST 16 (ZX Spectrum ROM), which doesn't work on CP/M. It should use BDOS call 2 (LD C,2; LD E,A; CALL 5) for CP/M targets. Currently the rerolled strings work because the CP/M emulator appears to handle the output anyway, but this is platform-incorrect.

4.5 stdlib.cpm.bdos Assembly Errors

The standard library's BDOS module generates assembly that MZA can't assemble. Programs using import stdlib.cpm.bdos fail. This forces users to write inline asm wrappers for putchar(). Worth investigating whether the issue is in the stdlib module's codegen or in MZA.

5. Files Changed

File Change
tools/vscode-minz/src/extension.ts Removed surroundingPairs, fixed -O flag, added compileAndRun
tools/vscode-minz/package.json Added minz.compileAndRun command, keybinding, menu entry
minzc/pkg/codegen/z80.go 3 SMC codegen fixes (patchable return, NeedsPatchPoints, label naming)
examples/cpm/playground.minz New playground example for edit-compile-run workflow

6. Quick Start

# Open playground in VSCode
code examples/cpm/playground.minz

# Cmd+Alt+R to compile and run (or from command palette: "MinZ: Compile & Run")

# Or from terminal:
mz examples/cpm/playground.minz -t cpm -o /tmp/out.a80
mza -o /tmp/out.com /tmp/out.a80
mze -t cpm /tmp/out.com

# Inspect the MIR:
mz examples/cpm/playground.minz -t cpm --dump-mir

# See optimizer decisions:
mz examples/cpm/playground.minz -t cpm --compile-trace -o /dev/null

All codegen tests pass. The playground example produces correct output on both optimized and unoptimized builds (with the minor reroll boundary cosmetic issue on optimized).