Handle alias values in SafepointSpiller::rewrite_use#13228
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cfallin merged 1 commit intobytecodealliance:mainfrom Apr 29, 2026
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The `rewrite_use` method of the safepoint spiller was not checking for value aliases, and therefore some uses of needs-stack-map values would not be reloaded from their associated stack slot. Note that, because the liveness analysis *does* correctly analyze alias values and will always correctly spill them at safepoints, this could not result in any bug with non-moving collectors (where reloading after safepoints is unnecessary), like those that Wasmtime has today. However, with the introduction of a moving collector in bytecodealliance#13107, this lack-of-reload bug in the safepoint spiller does trigger bugs in Wasmtime (and, reassuringly, our testing and fuzzing infrastructure finds it ~immediately). Uses of a non-reloaded GC reference are stale because the object they previously pointed to moved but the non-reloaded GC reference was not updated to point to the object's new location, resulting in use-after-move bugs. The fix for the safepoint spiller is simple: resolve aliases before rewriting uses. This commit additionally sprinkles some debug assertions throughout all the safepoint spiller code to double check that we have already resolved aliases and are no longer dealing with alias values in, e.g., our current set of live values in the liveness analysis.
fitzgen
requested review from
a team,
cfallin and
uweigand
and removed request for
a team and
uweigand
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bytecodealliance:main
with commit Apr 29, 2026
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The
rewrite_usemethod of the safepoint spiller was not checking for value aliases, and therefore some uses of needs-stack-map values would not be reloaded from their associated stack slot. Note that, because the liveness analysis does correctly analyze alias values and will always correctly spill them at safepoints, this could not result in any bug with non-moving collectors (where reloading after safepoints is unnecessary), like those that Wasmtime has today.However, with the introduction of a moving collector in #13107, this lack-of-reload bug in the safepoint spiller does trigger bugs in Wasmtime (and, reassuringly, our testing and fuzzing infrastructure finds it ~immediately). Uses of a non-reloaded GC reference are stale because the object they previously pointed to moved but the non-reloaded GC reference was not updated to point to the object's new location, resulting in use-after-move bugs.
The fix for the safepoint spiller is simple: resolve aliases before rewriting uses. This commit additionally sprinkles some debug assertions throughout all the safepoint spiller code to double check that we have already resolved aliases and are no longer dealing with alias values in, e.g., our current set of live values in the liveness analysis.