Editing a defvar or a defn is a symbol the host exports. Adding one is not:
there is nothing to bind to and ELF cannot grow a symbol. runtime/flan_dev.c is
the two lookups that cover it - flan_dev_cell for a new function's cell,
flan_dev_global for a new global's storage - both idempotent, so the second
module to mention a name gets what the first one got. That is the whole point:
two modules with their own copy of a new function would each call their own,
and redefining it would update one of them.
The compiler picks per name. A name the host has is a symbol and costs one load
at a call site; a name it lacks is a registry lookup cached at install time in
a module-local slot, and costs two. The common case pays nothing for the
general one.
The redefinition unit is now a list of top-level forms rather than one
function. It has to be: v3 of the fixture adds a var and uses it from a
redefined bump, and splitting that into two loads leaves a module referring to
storage that does not exist yet. C-c C-c passes one name, C-c C-k passes a
file's worth, one path either way.
Four rules, each silent if broken. Every lookup resolves before any body is
published, or a caller reaches a function whose slots are still null - asserted
on the emitted flan_reload_install, since it cannot be race-tested.
flan_dev_global refuses a size change, which is the layout-drift rule's first
enforcement point rather than another exception to it. Nothing is ever
dlclosed, because a cell holds an address inside a module's text. And the table
is fixed capacity, because a module holds a cell's address for as long as it is
loaded and a realloc would strand it.
The test that separates this from a plausible wrong version is v4, which
redefines a name v3 introduced at run time. v3's bump is already installed and
is not rebuilt, so it picks v4 up only if its call goes through a cell both
modules found by the same name. Had v3 cached the function's address instead,
every other assertion would still pass and the transcript would read 246
instead of 432.
Sizes are spelled LLVM's way, ptrtoint getelementptr null 1, rather than by a
layout calculator in OCaml that would have to agree with LLVM's on every
target.
Two things, and either alone is useless, so they are one commit.
Emit.redefinition compiles one function into its own module against a host
that is already running. What it does *not* define is the design: a global is
external, so state survives a reload and sand's grid is not reset by editing
the code; every other function is a declare, so a redefined settle calls the
host's move-grain rather than a frozen copy; there is no main. Build.shared
puts that text through llc + ld -shared. ld, not clang, because a shared object
is allowed undefined symbols and that is the whole mechanism - and because the
driver is 50ms of a 20ms job. Measured here: llc 16ms, ld 3ms, dlopen 0.04ms.
Loading a body is not installing it, though. A call bound at link time cannot
notice a new one, so a dev build routes every Flan-to-Flan call through a cell
- a mutable global holding the address of the function that is current - and a
module publishes itself with one store. The cell load is emitted after the
arguments, so a redefinition between two calls cannot land inside one.
Three details that are not free choices. flan_reload_install is a named
function rather than an ELF constructor, because the agent has to choose when
the store happens and a constructor would do it during dlopen, mid-frame, on
whatever thread called it. A redefinition's own body is hidden, because default
visibility in a shared object is interposable and that applies to taking the
address too: plain @"flan.bump" inside the module resolves to the host's copy,
so the installer would publish the function it was replacing and the reload
would silently do nothing. And -rdynamic is what exports the cells at all, so
it and cells are one flag: Build.opts.dev, flan build --dev, the first time
opts means something semantic rather than an optimisation level.
The test is one process, because two runs would prove nothing about a swap,
and two .so paths, because dlopen caches by path and would hand back the first
handle. Every call in it goes through outer, compiled once into the host and
never rebuilt, so a changed answer can only mean its call site followed. v2
recurses through its own cell, which is the interposition case; it would print
the old body's text if it did not. helper differs between the fixtures purely
as a tripwire for a module that grew its own copy.
LLVM cannot fold the indirection - the cell is an external mutable global - and
a --dev calc-me keeps 46 indirect calls at -O2. values, machine and
sand-headless now run as dev builds in the acceptance table too; the sand hash
is the one result that would notice a call reaching the wrong function.
