[llvm-dev] adding 2 new functions to the AliasAnalysis interface

[Atheel Massalha via llvm-dev]

thanks for your reply,
1) what is Im trying to analyze is the 2 memory instructions are using the
same "Bank" of memory, Bank is a column in the physical memory rows of a
memory block, for that I need to known if 2 instructions are using same
block (usually, 2 different arrays are mapped to different blocks, but for
[int* a=b+1], 'a' and 'b' will be in the same block, do oyu mean that
"unkownSize" will always return "mayAlias"/"noAlias" for same/different
blocks? sounds good idea, I will try it.

2) true. because it needs the size and the pointerInfo

another use fir this 2 interfaces is for "merging" load instructions
registers, as for today, LLVM generates different register names for ld
instructions with same base pointer but different offset, this can be
optimized using the knowledge of the distance between them.
*any suggestions for improving this?

sorry for the late response,

thanks,
Atheel

On 20 June 2018 at 22:02, Friedman, Eli <efriedma at codeaurora.org> wrote:

> On 6/20/2018 2:31 AM, Atheel Massalha via llvm-dev wrote:
>
> Hi,
> RFC
>
> Im adding 2 new functions to the AliasAnalysis interface:
>
> 1) ModRefSameBufferCheck:
>
> this function will receive 2 memoryLocation as parameters, and returns
> true if the parameters are modifying or refering the same buffer or
> same memory block, false otherwise.
>
>
> I'm not completely sure what "same buffer" means; is it just that given
> MemoryLocations with addresses A and B, an offset N exists such that
> gep(A,N) is mustalias with B?  What happens if alias can't prove whether
> the two locations point to the "same buffer"?
>
> Is that actually the property you care about?  DependenceAnalysis would
> probably be more appropriate if you're trying to analyze cross-iteration
> dependencies in loops.
>
> Could you implement this by just calling alias() with the sizes set to
> UnknownSize?
>
> 2) getAddressesDistance:
>
> the function will recieve 2 memoryLocation, if they are mod/ref-ing
> the same buffer/memory block, the function will return the distance
> between those locations at the memory. None otherwise
> (Optional<int64_t>)
>
>
> If I'm following correctly, given two values A and B, if a small constant
> offset N is known such that A+N==B, N is returned; if the distance is
> unknown or not a constant, None is returned.  Not sure why the API takes
> memoryLocations, instead of just two Value*.  Not sure this really belongs
> in AliasAnalysis.
>
> -Eli
>
> --
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>
>
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