Joseph Ferano
290d322b44
Two bugs, both in lib/x86.ml, both reached by signalling a condition from inside the value being assigned. The backend has no register wide enough to hold a struct, so it built every aggregate in its destination, element by element as they were computed. A transfer out of the middle left the destination in the middle: a global of four numbers read part old and part new, and so did a local, a struct field, an array element, a place behind a pointer. A union case was worse than the rest, since its destination is zeroed before the fields are written. The second one only looks like the first. An aggregate result is written through a hidden pointer the caller supplies, and the transfer exit zeroed that result on its way out, on the correct reasoning that the caller never reads it. At (set g (f)) the pointer is g, so it zeroed the caller's variable. It zeroes scalars only now — and with the first half in place nothing reaches it, so that one is defence in depth and FIX.org says so rather than claiming a test it does not have. Assignment builds into a frame temporary and copies, which is the shape emit.ml always had. The copy is paid where it buys something: [settles] says whether lowering an expression is bound to reach the end of it, and a right-hand side that settles is still built in place. That includes a conversion, except the one direction that is checked — a float narrowed to an integer — because an array of this language's literals is written [(u32 1) (u32 2)] and refusing every cast would have taxed the commonest aggregate there is. A let pays only inside a loop, because a slot reached once per frame is recorded as bound after the value lands. half-write.flan is every destination crossed with every right-hand side, run on both backends, with a scalar row so a regression in one is not read as the other. dev-halfwrite.flan asks the break loop the half a running program cannot ask itself: the local, which is invisible to the program and plain to an editor reading the frame. The def-edit paragraph in docs/BUILT.md described this as open and is now the claim it was waiting for. FIX.org records the two lanes the review of this one turned up and this one does not take: an aggregate built in place can read its own destination, which is an aliasing question rather than a transfer one; and the temporary is a frame buffer no root table names, which matters the day a struct may hold a dyn field.
What is in here, and what is still true
These are the documents that are written once and read occasionally: the reasons behind the code, the
reports from finished investigations, and the record of individual work sessions. The documents that are
edited every day stay at the repository root, because source comments cite them by bare filename from
dozens of places and a path that moves is a path that rots. So plan.org, NEXT.md, spec-memory.md
and spec-conditions.md are one directory up, and everything here points back at them.
A note on how to read the citations below: a document in this directory that says plan.org means the
one at the repository root. Nothing in docs/ is named for a file at the root, so there is no ambiguity,
and rewriting a hundred prose mentions into ../plan.org would have cost more in readability than it
bought in precision.
If you have thirty seconds
Read BUILT.md. It is by far the largest document here and it is the one that pays: it holds the
reason behind every part of the compiler that exists, from why nothing is ever dlclosed to why the
printer is a compile-time walk over a type rather than a runtime function. It is current, it is
maintained, and deleting it would mean deriving all of it again. If you want to know why a thing is the
shape it is, the answer is in here.
After that, ../NEXT.md at the root for what is actually in flight, and REFERENCES.md here for
where the evidence comes from — the reference clones on this machine, what each one answers, and the
standing rule that a claim in these notes was read out of a clone rather than recalled.
The current documents
| File | What it is |
|---|---|
BUILT.md |
Why the parts that exist are shaped the way they are. The longest and the most load-bearing document in the repository. Current. |
DISCUSS.md |
Open questions, raised and deliberately not answered. Nothing in it is a decision or a task; entries that have since been answered say so and point at where the answer landed. Current, though it is half archive by now. |
PORTING.md |
What the author's game, siam-farmer, needs from Flan that Flan does not have yet, measured against the code that exists rather than against a plan. It is the requirements document the language is actually steering by. Current. |
REFERENCES.md |
The reference clones under ~/Repositories, what each one is consulted for, and the specific files and lines already cited from them. Current. |
The reports
| File | What it is |
|---|---|
SPIKE-GENERICS.md |
Milestone 5's parametric polymorphism, run early and out of order as a spike. The spike succeeded and generics landed, so this is the report of finished work rather than a live plan — but its findings about where the cost falls are still the reason the implementation looks the way it does. Historical, findings still stand. |
overview.md |
The first brainstorm, from before the language had S-expressions. It says in its own first line that it is superseded and no longer accurate. Kept for history only; its table of what changed is the only part worth reading. Superseded. |
handoffs/
One report per work session, each written by whoever held the branch at the time. They are the
operational record: what was attempted, what was measured, what was decided without being able to ask,
and what was left open for the next lane. They are all historical the moment they are written — a handoff
describes a session that has ended — but they are cited by name from source comments and from NEXT.md,
because the reasoning behind a guard or a calling convention is often only written down once and this is
where it was written.
Six of them are the hand-written x86-64 backend, read in the order the work happened:
handoffs/HANDOFF-x86-rt.md set the remaining-items list that the four after it close, handoffs/HANDOFF-x86-redef.md
built the redefinition emitter, handoffs/HANDOFF-x86-aggregates.md took that across the struct boundary,
handoffs/HANDOFF-x86-guards.md settled the two guards nothing reaches, handoffs/HANDOFF-x86-debug.md added debug
information, and handoffs/HANDOFF-x86-cost.md measured what the backend costs and set the survey running on its
own so a refusal cannot sit unnoticed again.
The other six sessions are unrelated to each other. handoffs/HANDOFF-arith.md is why a divide by zero is a condition
rather than a SIGFPE. handoffs/HANDOFF-raylib-ports.md is the running record of the last two raylib example
ports, whose lasting findings were folded into PORTING.md, and handoffs/HANDOFF-cimport-ptr.md is the arm
cimport.ml's header check had been promising in a comment and not implementing — which is what those two ports
found, worked around and wrote up. handoffs/HANDOFF-devtest-noise.md is the linker
error dune test used to print on every run. handoffs/HANDOFF-emacs-flake.md is the test_emacs flake that
turned out to be SIGPIPE killing the daemon mid-reply, and it is worth reading for the two mechanisms it
rules out as much as for the one it found. handoffs/HANDOFF-tidy.md is this reorganisation.