llvm dev - Dec 2015 - Objects of MemoryLocation class are created for ‘llvm.memset.*‘ intrinsics

Dear llvm contributors,

Could you please advise me where objects of MemoryLocation class are
created for ‘llvm.memset.*‘ intrinsics?

In the Bug 23077 (https://llvm.org/bugs/show_bug.cgi?id=23077) the
AliasSetTracker constructs 128 alias sets for 0 pointer values, which
contain only unknown instructions. In this case, all unknown
instructions, which are added to new alias sets in the
AliasSetTracker::addUnknown, have the following form:

call void @llvm.memset.p0i8.i64(i8* %scevgep..., i8 0, i64 256, i32 8, i1 false)

Furthermore, in this case, there aren’t any unknown instructions,
which are added by AliasSetTracker::addUnknown to alias sets that are
found by findAliasSetForUnknownInst. That’s why I would like to check
objects of MemoryLocation class that are created  for ‘llvm.memset.*‘
intrinsics. If I’m not mistaken, objects of MemoryLocation class are
involved in alias analysis and, in particular, in work of
findAliasSetForUnknownInst.

Thank you for the attention!

-- 
                                    Cheers, Roman Gareev.

Hi Roman,

The MemoryLocation objects are involved in findAliasSetForUnknownInst, but none
are created there for the memset intrinsic. MemoryLocation objects are only
involved for the regular memory accesses being compared to the unknown
instruction. See AliasSet::aliasesUnknownInst in
lib/Analysis/AliasSetTracker.cpp.

We do know how to create MemoryLocation objects for memsets, see
MemoryLocation::getForDest, but we don't do so in the AliasSetTracker.
We're taking advantage here of the fast that AA's getModRefInfo
understands (or should understand) enough about memset to accurately answer the
queries.

 -Hal

----- Original Message -----
> From: "Roman Gareev via llvm-dev" <llvm-dev at
lists.llvm.org>
> To: llvm-dev at lists.llvm.org
> Cc: "Tobias Grosser" <tobias at grosser.es>
> Sent: Sunday, December 6, 2015 4:24:06 AM
> Subject: [llvm-dev] Objects of MemoryLocation class are created for
‘llvm.memset.*‘ intrinsics
> 
> Dear llvm contributors,
> 
> Could you please advise me where objects of MemoryLocation class are
> created for ‘llvm.memset.*‘ intrinsics?
> 
> In the Bug 23077 (https://llvm.org/bugs/show_bug.cgi?id=23077) the
> AliasSetTracker constructs 128 alias sets for 0 pointer values, which
> contain only unknown instructions. In this case, all unknown
> instructions, which are added to new alias sets in the
> AliasSetTracker::addUnknown, have the following form:
> 
> call void @llvm.memset.p0i8.i64(i8* %scevgep..., i8 0, i64 256, i32
> 8, i1 false)
> 
> Furthermore, in this case, there aren’t any unknown instructions,
> which are added by AliasSetTracker::addUnknown to alias sets that are
> found by findAliasSetForUnknownInst. That’s why I would like to check
> objects of MemoryLocation class that are created  for ‘llvm.memset.*‘
> intrinsics. If I’m not mistaken, objects of MemoryLocation class are
> involved in alias analysis and, in particular, in work of
> findAliasSetForUnknownInst.
> 
> Thank you for the attention!
> 
> --
>                                     Cheers, Roman Gareev.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

2015-12-11 23:44 GMT+05:00 Hal Finkel <hfinkel at
anl.gov>:
> Hi Roman,
>
> The MemoryLocation objects are involved in findAliasSetForUnknownInst, but
none are created there for the memset intrinsic. MemoryLocation objects are only
involved for the regular memory accesses being compared to the unknown
instruction. See AliasSet::aliasesUnknownInst in
lib/Analysis/AliasSetTracker.cpp.
>
> We do know how to create MemoryLocation objects for memsets, see
MemoryLocation::getForDest, but we don't do so in the AliasSetTracker.
We're taking advantage here of the fast that AA's getModRefInfo
understands (or should understand) enough about memset to accurately answer the
queries.
Hi Hal,

Thank you for the explanation!


-- 
                                    Cheers, Roman Gareev.