[PATCH] D132512: [SmallVector] Reallocate if assigned memory is right after the current vector, created with capacity 0

[Alina Sbirlea via Phabricator via llvm-commits]

asbirlea added a comment.

In D132512#3758117 <https://reviews.llvm.org/D132512#3758117>, @fhahn wrote:

> Can this be limited to instantiations where the original original size is 0, as IIUC this only applies to `llvm::SmallVector<T, 0>`?

I don't know how to properly check for that.

Say the original instantiation is `llvm::SmallVector<T, 0>`, so initially capacity is set to 0, but after the vector is used, the size and capacity have increased, and the heap allocated memory is in a completely different region (safe). Now say that we're at a point where we want to further increase capacity, and it just so happens that the memory right after (at FirstEl) is available and, this time, *that's* what `safe_realloc` returns. This is the case at line:153.
The case at line :145, we're already in a case where the vector has no heap allocated since BeginX == FirstEl  (if we entered this condition and there's heap allocated, we failed to fix the problem this patch is trying to fix...). So the only scenario in which `NewElts` can be equal to `FirstEl` is when the vector was created with capacity 0. The check would be redundant here.
The third case, when `mallocForGrow` is called (line :132), it's also possible the capacity was already increased and this is called to increase it further. So checking for capacity 0 here wouldn't stop allocating right at FirstEl.

Please let me know if I missed something in the above reasoning.


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