Introduce the no_preserve_tags_attribute #582
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arichardson
commented
Oct 15, 2021
This is the opposite of the existing must_preserve_cheri_tags and can be
used to indicated that a memcpy/memmove will never copy capabilities and
can therefore be inlined even when larger than the capability size and
aligned to less than the capability size.
The motivating case for this is that we currently call memcpy when
performing structure assignment for struct { long a; long b;} rather
than using two loads+stores.
Also add a global helper function that can be used to get the attribute from calls to library functions such as memcpy (since those CallInsts are not subclasses of MemTransferInst).
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Slightly less churn in this version by adding a default argument. |
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Longer-term I am not sure we need the must_preserve_tags attribute anymore since not having the attribute means we have to assume tags are preserved. |
This refactoring is required to allow inlining of non-tag-preserving memcpy instructions with size > sizeof(cap) && alignment < alignof(cap).
…ends This will allow setting the PreserveCheriTags flag in Clang and in optimization passes such as SimplifyLibcalls, etc. This commit only adds the argument with a default value to avoid merge conflicts for passes where we would just pass ::Unknown. Eventually the default argument should be removed but that means we have to audit all current call sites. A follow-up commit will use this flag in the individual backends to ensure that structures without capabilities can be copied efficiently.
This avoids churn in upstream tests due to additional attributes. To check whether we CHERI capabilities are supported, we check whether the DataLayout has any address spaces tagged as fat pointers.
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Split out from #506 to make that a bit easier to review.