[llvm-dev] Objects of MemoryLocation class are created for ‘llvm.memset.*‘ intrinsics

[Roman Gareev via llvm-dev]

2015-12-12 12:36 GMT+05:00 Roman Gareev <gareevroman at gmail.com>:
> 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!

Hi Hal,

I'm sorry for disturbing you. Could you please advise me how to skip
checks, which are performed in AliasSet::aliasesUnknownInst, of
unknown instructions from different alias sets of an alias set tracker
that is a parameter of ‘AliasSetTracker::add(const AliasSetTracker
&AST)’?

If this wasn’t available at the moment and you could review me, I
would try to implement it. A temporary patch can be found attached. It
passes the LLVM regression tests and helps to reduce the runtime of
'opt -basicaa -licm out.opt.ll’ from 130ms to 67ms and the runtime of
'opt -basicaa -licm out.opt2.ll’ from 117ms to 62ms (out.opt.ll and
out.opt2.ll can be found on the following link
https://llvm.org/bugs/show_bug.cgi?id=23077).

P.S.: Thank you again for the explanation. I think that, in the case
of the Bug 23077, getModRefInfo works correctly. If we modify, for
example, out.opt.ll and change the third parameter of ‘llvm.memset.*’
intrinsics from i64 256 to, for example, i64 512, there is aliasing.
Furthermore, I’ve manually created objects of the MemoryLocation
class, which are corresponds to ‘llvm.memset.*’ intrinsics, and
checked whether they alias each other. The result corresponds to a
result of getModRefInfo.

-- 
                                    Cheers, Roman Gareev.
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