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This iteration already has significantly better incremental support.
Closes #24110
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If an error occured which prevented a prelink task from being queued,
then `pending_prelink_tasks` would never be decremented, which could
cause deadlocks in some cases. So, instead of calculating ahead of time
the number of prelink tasks to expect, we use a simpler strategy which
is much like a wait group: we add 1 to a value when we spawn a worker,
and in the worker function, `defer` decrementing the value. The initial
value is 1, and there's a decrement after all of the workers are
spawned, so once it hits 0, prelink is done (be it with a failure or a
success).
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Did you know that allocators reuse addresses? If not, then don't feel
bad, because apparently I don't either! This dumb mistake was probably
responsible for the CI failures on `master` yesterday.
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I messed up atomic orderings on this variable because they changed in a
local refactor at some point. We need to always release on the store and
acquire on the loads so that a linker thread observing `.ready` sees the
stored MIR.
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Without this cap, unlucky scheduling and/or details of what pipeline
stages perform best on the host machine could cause many gigabytes of
MIR to be stuck in the queue. At a certain point, pause the main thread
until some of the functions in flight have been processed.
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* "Flush" nodes ("LLVM Emit Object", "ELF Flush") appear under "Linking"
* "Code Generation" disappears when all analysis and codegen is done
* We only show one node under "Semantic Analysis" to accurately convey
that analysis isn't happening in parallel, but rather that we're
pausing one task to do another
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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.
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The idea here is that instead of the linker calling into codegen,
instead codegen should run before we touch the linker, and after MIR is
produced, it is sent to the linker. Aside from simplifying the call
graph (by preventing N linkers from each calling into M codegen
backends!), this has the huge benefit that it is possible to
parallellize codegen separately from linking. The threading model can
look like this:
* 1 semantic analysis thread, which generates AIR
* N codegen threads, which process AIR into MIR
* 1 linker thread, which emits MIR to the binary
The codegen threads are also responsible for `Air.Legalize` and
`Air.Liveness`; it's more efficient to do this work here instead of
blocking the main thread for this trivially parallel task.
I have repurposed the `Zcu.Feature.separate_thread` backend feature to
indicate support for this 1:N:1 threading pattern. This commit makes the
C backend support this feature, since it was relatively easy to divorce
from `link.C`: it just required eliminating some shared buffers. Other
backends don't currently support this feature. In fact, they don't even
compile -- the next few commits will fix them back up.
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