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winch: drc barriers - #14081

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macovedj wants to merge 9 commits into
bytecodealliance:mainfrom
macovedj:winch-drc-barriers
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winch: drc barriers#14081
macovedj wants to merge 9 commits into
bytecodealliance:mainfrom
macovedj:winch-drc-barriers

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@macovedj

@macovedj macovedj commented Aug 5, 2026

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Continues the work tracked in #14057 by adding DRC barriers for GC references in Winch. This unblocks exception-handling support work in Winch.
Closes #14057.

macovedj added 9 commits August 5, 2026 12:29
@macovedj
winch: gc barrier predicate; un-gate DRC for development
fde76b8
@macovedj
winch: straight-line DRC write barrier in global.set
5a78fd5 
Step 2 of the barrier plan: all of the spec's memory traffic, none of
its branches. Knowingly wrong for null until the guards land; the disas
test is the oracle.
@macovedj
winch: null guard around the write barrier increment
e90a8ab 
First of the two step-3 guards; the decrement guard is next.
@macovedj
winch: null guard around the write barrier decrement
bb4020a 
Completes the write barrier's null guards; a global.set of ref.null
touches no refcount on either side.
@macovedj
winch: drop the old value when its refcount reaches zero
ab03d14 
Completes the write barrier: the decremented count is only written back
on the nonzero path, matching the cranelift DRC barrier; at zero the
old value goes to the drop_gc_ref builtin in a cold block. The call
gets a stack map like any other.
@macovedj
winch: observe the write barrier dropping the last reference
e5e8721 
Proven load-bearing: with the barrier arm disabled the object is freed
while the global still holds it (premature free), failing the first
assertion.
@macovedj
environ: expose DRC header constants without the gc-drc feature
c6d3d95 
Winch's barriers reference these constants unconditionally, so move
them to the always-compiled gc module and make TRAP_GC_HEAP_CORRUPT
public for the barrier bounds checks.
@macovedj
winch: DRC read barrier and write-barrier bounds checks
a2770fe 
Adds the read barrier for global.get: references loaded onto the stack
are enrolled in the over-approximated stack-roots list so a collection
cannot free them out from under the frame that loaded them, forcing a
GC when the list outgrows its threshold. The loaded reference is homed
into its stack slot first so the force-gc call's stack map covers it.

Both barriers now trap GC_HEAP_CORRUPT if an object's DRC header would
extend past the GC heap's current length before any header access,
matching Cranelift's prepare_gc_ref_access. Read-barrier test proven
load-bearing under gc_zeal.
@macovedj
winch: complete DRC global barrier coverage
ee3a463
@macovedj
macovedj requested review from a team as code owners August 5, 2026 17:39
@macovedj
macovedj requested review from alexcrichton and removed request for a team August 5, 2026 17:39
@saulecabrera

saulecabrera commented Aug 5, 2026

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Happy to take this one as well @alexcrichton.

@saulecabrera
saulecabrera requested review from saulecabrera and removed request for a team and alexcrichton August 5, 2026 19:00
@github-actions github-actions Bot added wasmtime:api Related to the API of the `wasmtime` crate itself wasmtime:config Issues related to the configuration of Wasmtime wasmtime:docs Issues related to Wasmtime's documentation winch Winch issues or pull requests labels Aug 5, 2026
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cc @saulecabrera

Details This issue or pull request has been labeled: "wasmtime:api", "wasmtime:config", "wasmtime:docs", "winch"

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Label Messager: wasmtime:config

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wasmtime:api Related to the API of the `wasmtime` crate itself wasmtime:config Issues related to the configuration of Wasmtime wasmtime:docs Issues related to Wasmtime's documentation winch Winch issues or pull requests

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winch: gc infra for exception handling support and future gc work

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