Memory, exceptions, CSS, HMR

The NodeDef graph is App-lifetime. Transient work inside @connect, list put, and Lodash bodies is not. Those two sentences are the memory model. Everything else on this page is the machinery that makes them true on wasm32-unknown-wasi.

WasmAllocator and the pool stack

_start calls alloc_init(heap_base), which is WasmAllocator.init. That allocator is a bump pointer over wasm linear memory. wasm_malloc, _d_allocclass, _d_newclass, _d_newarrayT / _d_newarrayU, and _d_arrayappendcTX all go through it. Language new therefore bumps the heap and is never recycled. A page that news a tree of items on every click will grow until the tab dies.

PoolStack.initialize() creates an empty stack. A ManagedPool(64 * 1024) (what App.construct and URLRouter allocate) or a ScopedPool(m_pool) pushes a pool. While that pool is current, alloc / _d_allocmemory / allocString come from it. When the ScopedPool is destroyed, that memory is gone. Graph lifetime ≠ pool lifetime. @prop fields, item structs, and Array fields must not alias a popped pool. Copy or freeze before put / assign.

Heavy printed methods wrap ScopedPool(m_pool) and copy survivors onto the NodeDef graph. That is the idiom. A lang=d script that does the same is following the engine, not inventing a GC.

libwasm.intrinsics exposes the wasm memory builtins the allocator sits on:

auto wasmMemorySize();   // number of 64 KiB pages (mem 0)
auto wasmMemoryGrow(int delta);

You almost never call these from a page. They are how WasmAllocator asks the host for more linear memory. They are listed here because code-d will complete them under import libwasm; and they are not a second heap.

There is no GC. GC symbols exist as stubs and types so Phobos snippets compile; they do not reclaim. -fno-moduleinfo is required because this runtime does not implement LDC’s __minfo registry. Stock -Iimport on a wasm triple must stay empty so druntime-wasm / runtime-v1.43.0 object.d wins.

Exceptions: wasm-eh, not asyncify

The default engine configuration is --wasm-enable-eh, -mattr=+exception-handling, --foptimize-nothrow=false. libwasm.rt.eh is the personality. _d_throw_exception is a real wasm throw of tag 0 so try_table / catch_ref can catch it. ExceptionHeader storage is a single __gshared slot; a second in-flight exception overwrites it. Nested throw during catch is rare in the SPA and is not a heap-allocated header yet.

Printed structs are nothrow:. LDC 1.43’s default --foptimize-nothrow deletes landing pads inside nothrow functions. Without --foptimize-nothrow=false a D catch vanishes and you will debug a “working” binary that never enters the handler. The engine’s dub.sdl already passes the flag.

svelte-d’s throwBoundary is same-function try / throw / catch on the same struct. Personality is first-catch-type only. An exception that crosses a function boundary is an abort, not a boundary. std.conv.to and friends belong inside that try. An uncaught throw is _d_print_throwable → console.error, then abort.

Binaryen ≥123 parses this try_table module and is the official -Oz post-link. Do not compose it with Flatten --asyncify. Do not print .await on this cell. See JsPromise and wasm-eh.

CSS

GetCss!(Application, Theme) walks @style / @styleset at compile time and produces a string. _start calls addCss on it before first render. Instance <style> blocks in a .svelte file become addCss plus @style!".box" on the matching NodeDef. :global(.wide) still yields .wide — the :global wrapper is an official-compiler concern this printer does not reimplement, but the class name must survive or DaisyUI / Tailwind utilities in the engine would vanish.

changeClass updates a live handle. Dynamic class lists are @style fields you update, not a second className runtime.

Diet templates under src-d-views/*.dt are wasm-cell stringImportPaths only. They are not the SSR engine and they are not vibe.0 Diet (which this fork deleted).

HMR

version (hmr) on mixin Spa exports:

extern(C) export string dumpApp();   // application.dumpState()
extern(C) export void loadApp(string state);

JS reload instantiates the new module, calls _start, then loadApp. Lists serialise as :l:N:[{item}…]. After load, dumped items win over construct() seeds. Still skipped: pointers, EventEmitter, NamedNode, ManagedPool, enums. The engine’s dub.sdl sets versions "hmr" on the wasm cell. A dest you built without that version has no dump/load exports.

HMR is not a reason to store the only copy of a value in a skipped field. It is also not a reason to edit dest D and expect the next compile to keep it. The .svelte file is the source; the dest is the IR.