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ci: the per-PR repsel matrix arms cannot reproduce the relocating-minor defect class (evac base is push-only) #6993
Description
The hole
scripts/gc_repsel_matrix.sh runs on pull requests with --arms pr:
PR_ARMS="default,verify_evac,cons_scan_off,shipped_default"
and on push with --arms all (.github/workflows/test.yml, gc-stress job).
But the evacuating base that #6981 established as the discriminator needs both halves together:
PERRY_GC_INCREMENTAL=0 PERRY_CONSERVATIVE_STACK_SCAN=off
and that combination appears only in --arms all (evac_minor / cons_scan_off_force). cons_scan_off on its own is pressure + precise roots with incremental mode still on, which does not take the relocating path.
Measurement
Built the five #6982 crashers from aa1c15028 (i.e. unfixed) and ran each PR-gated arm. All green:
default verify_evac cons_scan_off shipped_default
repsel_canonical_i32 0 0 0 0
typedarray_param_read 0 0 0 0
The same binaries crash immediately under PERRY_GC_HEAP_LIMIT=8 PERRY_GC_INCREMENTAL=0 PERRY_CONSERVATIVE_STACK_SCAN=off (SIGBUS/SIGSEGV, copied_objects 4 579 / 5 965).
So the entire "raw reference held across a relocating collection" class — #6951, #6972, #6982, #6991, #6992 — is invisible to the per-PR gate. It can only go red after merge, on push.
Why I did not just fix it
Adding evac_minor to PR_ARMS today would fail every PR: 13 of 21 corpus cells are still red on that arm (2 crashes + 11 mismatches after #6982's fix). The gate has to become green-able first.
Suggested shape
- Land the outstanding rooting fixes (fix(gc): operand temporaries are precise roots — collection methods, constructor args, string-method receiver (#6970, #6969, #6971) #6983, gc: compiled constructor receiver goes stale across the globalThis-population collection (unmasked by #6982) #6991, gc: proven-
thismethod value is lost across a relocating minor (TypeError: bump is not a function) #6992, gc: scalar-replaced object/array locals holding heap values are not precise roots #6968) until theevac_minorcolumn is green or has a documented, enumerated skip list. - Then add
evac_minortoPR_ARMS, with the remaining known-red cells in an explicit allowlist (same provenance discipline astest-parity/known_failures.json, cf. test-parity: audit known_failures.json — every entry needs an issue # and date #797) so new breakage is caught but known breakage does not block. - Until then, consider a non-blocking
evac_minorPR run so the signal is at least visible on the PR page rather than only after merge.
Activity
proggeramlug commentedon Jul 30, 2026
Datapoint for this issue's thesis, from a GC measurement run (#7056): the relocating-minor defect class reproduces in the shipped default configuration, with no special arm at all.
#7055 is an ordinary event-loop TypeScript program that produces a deterministic wrong checksum under plain
perry file.ts -o bin && ./bin. The discriminator is exactlycopied_objects > 0:PERRY_GC_SCAVENGE_NURSERY_MB=128PERRY_GC_MOVING_LOOP_POLLS=0PERRY_GC_MOVING_LOOP_POLLS=0 PERRY_GC_SCAVENGE=1Note the second row: three minor cycles, right answer, because none of them relocated.
This bears on the "suggested shape" here in two ways.
The gate hole may be wider than
evac_minor. This issue frames the missing coverage as thePERRY_GC_INCREMENTAL=0 PERRY_CONSERVATIVE_STACK_SCAN=offcombination that only--arms allreaches. But the case above needs neither half — it is the default.PERRY_GC_TRACE=1on the shipped arm showsroot_sources.native_stack_fallback.decision = "skip_disabled"on 62 of 63 cycles, so the shipped moving path is already running with precise roots and no conservative scan. #6981's reasoning thatgc_check_trigger's forcedforce_full_scan()keeps this class out of production does not cover the deferred/moving path.What a green-able discriminator might key on. Since the discriminator here is
copied_objects > 0rather than any particular arm, a cheaper gate than addingevac_minortoPR_ARMSmay be available: run the corpus in the default configuration but fail if a test that relocated disagrees with acopied_objects == 0run of the same binary. That is a self-checking property (no oracle needed beyond Perry-vs-Perry), it does not require the 13-of-21 red cells to be fixed first, and it would have caught #7055 on the PR that introduced it.Full measurement context, including the per-cycle trace figures quoted above: #7056.
fix(gc): measure the moving-defer allowance from the deferral point s…