irinterp: Don't introduce invalid CFGs #49797
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This is yet another followup to #49692 and #49750. With the introduced change, we kill the CFG edge from the basic block with the discovered error to its successors. However, we have an invariant in the verifier that the CFG should always match the IR. Turns out this is for good reason, as we assume in a number of places (including, ironically in the irinterp) that a GotoNode/GotoIfNot terminator means that the BB has the corresponding number of successors in the IR. Fix all this by killing the rest of the basic block when we discover that it is unreachable and if possible introducing an unreachable node at the end. However, of course if the erroring statement is the fallthrough terminator itself, there is no space for an unreachable node. We fix this by tweaking the verification to allow this case, as its really no worse than the other problems with fall-through terminators (#41476), but of course it would be good to address that as part of a more general IR refactor.
Keno
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May 14, 2023
If a fall-through terminator was already Bottom, we should not attempt to rekill the successor edge, because it was already deleted. Yet another fix in the #49692, #49750, #49797 series, which is turning out to be quite a rabit hole. Also fix a typo in the verifer tweak where we were looking at the BB idx rather than the terminator idx.
Keno
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May 15, 2023
If a fall-through terminator was already Bottom, we should not attempt to rekill the successor edge, because it was already deleted. Yet another fix in the #49692, #49750, #49797 series, which is turning out to be quite a rabit hole. Also fix a typo in the verifer tweak where we were looking at the BB idx rather than the terminator idx.
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This is yet another followup to #49692 and #49750.
With the introduced change, we kill the CFG edge from the basic block with the discovered error to its successors.
However, we have an invariant in the verifier that the CFG should always match the IR.
Turns out this is for good reason, as we assume in a number of places (including, ironically in the irinterp) that a GotoNode/GotoIfNot terminator means that the BB has the corresponding number of successors in the IR. Fix all this by killing the rest of the basic block when we discover that it is unreachable and if possible introducing an unreachable node at the end.
However, of course if the erroring statement is the fallthrough terminator itself, there is no space for an unreachable node. We fix this by tweaking the verification to allow this case, as its really no worse than the other problems with fall-through terminators (#41476), but of course it would be good to address that as part of a more general IR refactor.