Fix bug where a ref assignment is moved ouside a conditional #7176
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Fixes #7170 Assignment of a ref, or other record with a single mutable field, is compiled to simple variable assignment. This triggers an incorrect optimisation that moves identical assignments in the true and false branches of a conditional above if they are identical and the guard has no side effects. Added a check that if the assigments to be moved are to variable, that variable must not occur free in the guard of the conditional.
cristianoc
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The optimisation is still incorrect. In fact it would be only sound under very unlikely circumstances. |
cristianoc
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Nov 26, 2024
| if (x.TAG === "A") { | ||
| v = 1; |
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This is the only case where the optimisation was kicking.
It's pretty unlikely to fire to begin with, and when it fires the cases where one can legitimately move the common instruction up are pretty rare.
In this case, it would be OK because assigning to v does not change the value of x.TAG, because the two values cannot be aliases.
zth
approved these changes
Nov 26, 2024
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Fixes #7170
There is an optimisation of this form:
is transformed to:
The current check is that
guarddoes not have side effects. (Also, this same optimisation does not apply toguard ? {c; c1} : {c; c2}which is what the vast majority of cases of this form are compiled to -- that's why the original issue is not so easy to repro.However, the check for side effects is not enough. What you want to know is: executing
cdoes not change the truth value ofguard, which is a form of non-interference, much harder to establish.