Previously, various doc comments heavily disagreed with the
implementation on both what lives where on the filesystem at what time,
and how that was represented in code. Notably, the combination of emit
paths outside the cache and `disable_lld_caching` created a kind of
ad-hoc "cache disable" mechanism -- which didn't actually *work* very
well, 'most everything still ended up in this cache. There was also a
long-standing issue where building using the LLVM backend would put a
random object file in your cwd.
This commit reworks how emit paths are specified in
`Compilation.CreateOptions`, how they are represented internally, and
how the cache usage is specified.
There are now 3 options for `Compilation.CacheMode`:
* `.none`: do not use the cache. The paths we have to emit to are
relative to the compiler cwd (they're either user-specified, or
defaults inferred from the root name). If we create any temporary
files (e.g. the ZCU object when using the LLVM backend) they are
emitted to a directory in `local_cache/tmp/`, which is deleted once
the update finishes.
* `.whole`: cache the compilation based on all inputs, including file
contents. All emit paths are computed by the compiler (and will be
stored as relative to the local cache directory); it is a CLI error to
specify an explicit emit path. Artifacts (including temporary files)
are written to a directory under `local_cache/tmp/`, which is later
renamed to an appropriate `local_cache/o/`. The caller (who is using
`--listen`; e.g. the build system) learns the name of this directory,
and can get the artifacts from it.
* `.incremental`: similar to `.whole`, but Zig source file contents, and
anything else which incremental compilation can handle changes for, is
not included in the cache manifest. We don't need to do the dance
where the output directory is initially in `tmp/`, because our digest
is computed entirely from CLI inputs.
To be clear, the difference between `CacheMode.whole` and
`CacheMode.incremental` is unchanged. `CacheMode.none` is new
(previously it was sort of poorly imitated with `CacheMode.whole`). The
defined behavior for temporary/intermediate files is new.
`.none` is used for direct CLI invocations like `zig build-exe foo.zig`.
The other cache modes are reserved for `--listen`, and the cache mode in
use is currently just based on the presence of the `-fincremental` flag.
There are two cases in which `CacheMode.whole` is used despite there
being no `--listen` flag: `zig test` and `zig run`. Unless an explicit
`-femit-bin=xxx` argument is passed on the CLI, these subcommands will
use `CacheMode.whole`, so that they can put the output somewhere without
polluting the cwd (plus, caching is potentially more useful for direct
usage of these subcommands).
Users of `--listen` (such as the build system) can now use
`std.zig.EmitArtifact.cacheName` to find out what an output will be
named. This avoids having to synchronize logic between the compiler and
all users of `--listen`.
As of this commit, every backend other than self-hosted Wasm and
self-hosted SPIR-V compiles and (at least somewhat) functions again.
Those two backends are currently disabled with panics.
Note that `Zcu.Feature.separate_thread` is *not* enabled for the fixed
backends. Avoiding linker references from codegen is a non-trivial task,
and can be done after this branch.
Similar to the previous commit, this commit untangles LLD integration
from the self-hosted linkers. Despite the big network of functions which
were involved, it turns out what was going on here is quite simple. The
LLD linking logic is actually very self-contained; it requires a few
flags from the `link.File.OpenOptions`, but that's really about it. We
don't need any of the mutable state on `Elf`/`Coff`/`Wasm`, for
instance. There was some legacy code trying to handle support for using
self-hosted codegen with LLD, but that's not a supported use case, so
I've just stripped it out.
For now, I've just pasted the logic for linking the 3 targets we
currently support using LLD for into this new linker implementation,
`link.Lld`; however, it's almost certainly possible to combine some of
the logic and simplify this file a bit. But to be honest, it's not
actually that bad right now.
This commit ends up eliminating the distinction between `flush` and
`flushZcu` (formerly `flushModule`) in linkers, where the latter
previously meant something along the lines of "flush, but if you're
going to be linking with LLD, just flush the ZCU object file, don't
actually link"?. The distinction here doesn't seem like it was properly
defined, and most linkers seem to treat them as essentially identical
anyway. Regardless, all calls to `flushZcu` are gone now, so it's
deleted -- one `flush` to rule them all!
The end result of this commit and the preceding one is that LLVM and LLD
fit into the pipeline much more sanely:
* If we're using LLVM for the ZCU, that state is on `zcu.llvm_object`
* If we're using LLD to link, then the `link.File` is a `link.Lld`
* Calls to "ZCU link functions" (e.g. `updateNav`) lower to calls to the
LLVM object if it's available, or otherwise to the `link.File` if it's
available (neither is available under `-fno-emit-bin`)
* After everything is done, linking is finalized by calling `flush` on
the `link.File`; for `link.Lld` this invokes LLD, for other linkers it
flushes self-hosted linker state
There's one messy thing remaining, and that's how self-hosted function
codegen in a ZCU works; right now, we process AIR with a call sequence
something like this:
* `link.doTask`
* `Zcu.PerThread.linkerUpdateFunc`
* `link.File.updateFunc`
* `link.Elf.updateFunc`
* `link.Elf.ZigObject.updateFunc`
* `codegen.generateFunction`
* `arch.x86_64.CodeGen.generate`
So, we start in the linker, take a scenic detour through `Zcu`, go back
to the linker, into its implementation, and then... right back out, into
code which is generic over the linker implementation, and then dispatch
on the *backend* instead! Of course, within `arch.x86_64.CodeGen`, there
are some more places which switch on the `link` implementation being
used. This is all pretty silly... so it shall be my next target.
The main goal of this commit is to make it easier to decouple codegen
from the linkers by being able to do LLVM codegen without going through
the `link.File`; however, this ended up being a nice refactor anyway.
Previously, every linker stored an optional `llvm.Object`, which was
populated when using LLVM for the ZCU *and* linking an output binary;
and `Zcu` also stored an optional `llvm.Object`, which was used only
when we needed LLVM for the ZCU (e.g. for `-femit-llvm-bc`) but were not
emitting a binary.
This situation was incredibly silly. It meant there were N+1 places the
LLVM object might be instead of just 1, and it meant that every linker
had to start a bunch of methods by checking for an LLVM object, and just
dispatching to the corresponding method on *it* instead if it was not
`null`.
Instead, we now always store the LLVM object on the `Zcu` -- which makes
sense, because it corresponds to the object emitted by, well, the Zig
Compilation Unit! The linkers now mostly don't make reference to LLVM.
`Compilation` makes sure to emit the LLVM object if necessary before
calling `flush`, so it is ready for the linker. Also, all of the
`link.File` methods which act on the ZCU -- like `updateNav` -- now
check for the LLVM object in `link.zig` instead of in every single
individual linker implementation. Notably, the change to LLVM emit
improves this rather ludicrous call chain in the `-fllvm -flld` case:
* Compilation.flush
* link.File.flush
* link.Elf.flush
* link.Elf.linkWithLLD
* link.Elf.flushModule
* link.emitLlvmObject
* Compilation.emitLlvmObject
* llvm.Object.emit
Replacing it with this one:
* Compilation.flush
* llvm.Object.emit
...although we do currently still end up in `link.Elf.linkWithLLD` to do
the actual linking. The logic for invoking LLD should probably also be
unified at least somewhat; I haven't done that in this commit.
Each target can opt into different sets of legalize features.
By performing these transformations before liveness, instructions
that become unreferenced will have up-to-date liveness information.
This allows emitting object files for s390x-zos (GOFF) and powerpc(64)-aix
(XCOFF).
Note that GOFF emission in LLVM is still being worked on upstream for LLVM 21;
the resulting object files are useless right now. Also, -fstrip is required, or
LLVM will SIGSEGV during DWARF emission.
