Make clearing and restoring the may_leave flag more amenable to opt…#13713
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cfallin merged 1 commit intoJun 23, 2026
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…imization
The clear-and-restore is now structured so that if translation doesn't actually
call any functions (e.g. due to inlining) then a future dead-store elimination
pass in Cranelift can remove all the flag juggling entirely (other than trapping
when `!may_leave`):
may_leave = load vmctx+MAY_LEAVE_OFFSET ;; (0)
trapz may_leave
...
zero = iconst 0
store zero, vmctx+MAY_LEAVE_OFFSET ;; (1)
...
store may_leave, vmctx+MAY_LEAVE_OFFSET ;; (2)
First, the dead-store elimination pass will see that the the store at
(1): is dead and remove it. Then, the idempotent-store eliminator will recognize
that the store at (2) is storing the same value that the memory location already
contains and it will also be removed. The more we can reuse locals to make this
idempotency obvious, rather than force Cranelift's optimizer to rediscover this
information, the better.
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Great idea -- the separate may-leave flag global is of course a cost but it seems like that should be tiny compared to the advantages we get being able to reason about it separately. LGTM!
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…imization
The clear-and-restore is now structured so that if translation doesn't actually call any functions (e.g. due to inlining) then a future dead-store elimination pass in Cranelift can remove all the flag juggling entirely (other than trapping when
!may_leave):First, the dead-store elimination pass will see that the the store at (1): is dead and remove it. Then, the idempotent-store eliminator will recognize that the store at (2) is storing the same value that the memory location already contains and it will also be removed. The more we can reuse locals to make this idempotency obvious, rather than force Cranelift's optimizer to rediscover this information, the better.