[PATCH] D117095: [BasicAA] Add support for memmove intrinsic

[Evgeniy via Phabricator via llvm-commits]

ebrevnov added a comment.

Thank you all for your input. The reason why generic argmemonly case (pointed by @nikic) doesn't handle my case is presence of deopt bundle on the call. Currently, "BasicAAResult::getModRefBehavior(const CallBase *Call)" simply ignores function attributes in presence of deopt bundles. By specification: "From the compiler’s perspective, deoptimization operand bundles make the call sites they’re attached to at least readonly. They read through all of their pointer typed operands (even if they’re not otherwise escaped) and the entire visible heap". That has two consequences:

1. Current code which special case AnyMemCpyInst (the one I was trying to extend to any memory transfer) is incorrect because it may return NoModRef if memcpy intrinsic has operand bundles. Should it be fixed or just removed (can remove only once the next item is resolved though)?
2. It's impossible to describe memcpy/memmove modref behavior with existing encoding defined by FunctionModRefBehavior. In particular, for memmove/memcopy with deopt bundle we need to encode "reads everything, writes inaccessible or argmemonly". Current scheme doesn't support setting different modref for individual type of memory. The best solution which came to me so far is to split available space (currently it's 32-bits given by "enum FunctionModRefBehavior") to 4 regions (4 regions is enough to represent 8 variants defined by "enum class ModRefInfo") and associate each region with specific modref value from ModRefInfo enum.  With this approach we can represent any variation for 8 independent memory types in 32-bits which seems to be enough (assuming modref info won't change). In case that turns out to be insufficient can easily double number of supported attributes by switching to 64-bits.

To make it more clear I can give an example. Say we need to encode "reads everything, writes inaccessible or argmemonly". Assume we split our 32-bits by regions in the following way "MustMod | MustRef | Mod | Ref" (each region is 4-bits).  Then the value would be "FMRL_Nowhere | FMRL_Nowhere | FMRL_ArgumentPointees_OR_FMRL_InaccessibleMem | FMRL_Anywhere".
Do you think this is reasonable? Anything better?
Anyway I assume it should be discussed on dev list...

Thanks
Evgeniy


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