[llvm-dev] Can someone give me some pointer on alias analysis ?

[Amaury SECHET via llvm-dev]

After a bit more investigation, it turns out that because %0 is stored into
%1 (after bitcast) and so %3 may have access to it and clobber it.

After a bit of thought, it is correct in the general case, but definitively
something stricter is needed here. Looking at inaccessiblememonly I'm not
sure this is what is needed. What if the memory allocator is defined is the
current module ?

This leads me to conclude this is way more linked to the memory allocation
pass than I expected it to be in the first place. Can I ask what you plan
to use inaccessiblememonly for ? Should the semantic be refined to fit the
bill better ?

2016-01-04 12:29 GMT+01:00 Amaury SECHET <deadalnix at gmail.com>:

>
>
> 2015-12-26 18:32 GMT+01:00 Philip Reames <listmail at philipreames.com>:
>
>> On 12/26/2015 02:17 AM, Amaury SECHET via llvm-dev wrote:
>>
>> I'm trying to fix that bug: https://llvm.org/bugs/show_bug.cgi?id=20049
>>
>> It turns out this is the kind of optimization that I really need, as when
>> it isn't done, all kind of other optimizations opportunities down the road
>> are not realized as they are not exposed.
>>
>> I have no idea where to start digging for this. I assume there is some
>> kind of interaction between memory dependency and alias analysis that is
>> overly conservative. Can someone gives me some infos on how the 2 interact
>> together ? What is the code that is supposed to remove these kind of loads ?
>>
>> First, it looks like you're using an older version of LLVM (the load
>> syntax has changed).  That's definitely not recommended since it will
>> greatly limit your choices when encountering an issue like this.
>>
>>
> I just reused an old code sample. The problem still exists in recent
> version of LLVM.
>
>
>> Second, the aliasing problem has to do with the effects of the
>> "allocmemory" callsite on a memory location associated with an earlier
>> alloca.  There's nothing that prevents your allocmemory function from
>> writing to global state.  You have to teach the alias analysis that an
>> unescaped noalias pointer can't alias the global state allocmemory might
>> access.  Slightly surprised we don't get this today, but oh well.  Take a
>> look at the isNonEscapingLocalObject predicate in BasicAA.  Then look at
>> "getModRefInfo(ImmutableCallSite CS, const MemoryLocation &Loc)".  Double
>> check to make sure this is the one that MDA actually calls.
>>
>>
> I don't think this is the problem. When there is only 2 calls to
> allocmemory, loads are optimized away as expected. But it seems that the
> analysis is confused with 3+ calls.
>
>
>> Third, you might want to take a look the new inaccessiblememonly
>> attribute. Depending on how you're modelling your allocation, this might
>> help resolve the aliasing problem in different way.  However, be aware that
>> this is *very* new.  In particular, I triggered an optimizer bug by adding
>> the new attribute to your particular example.  :(
>>
>> Fourth, have you considered implementing a simple escape analysis pass?
>> I notice that the store-load forwarding would just fall out once you
>> removed the last allocation.  I believe the fixed point would then become a
>> function which returns the constant answer and does nothing else.
>>
>>
> Yes I did. More specifically, there is a pass that try to recognize memory
> allocation like calls and optimize them, which right now have libc and some
> overloads of operator new hardcoded in it but not much more. This could be
> greatly improved IMO, to be language agnostic (and better support C++ in
> the process), but really, I do think relying on this to get the load
> eliminated, when noalias already provide this information to the optimizer,
> is overkill.
>
>
>> Fifth, some pointers on debugging this yourself.  GVN (which is the one
>> which does complicated *-load forwarding) calls MDA
>> (MemoryDepedenceAnalysis).  Using the appropriate -debug-only options will
>> likely get you to the right area.  You can also consider using the
>> MemDepPrinter pass to bypass GVN.  I don't know of a way to issue raw AA
>> queries for testing.  That would be useful, but I don't know that we have
>> it.
>>
>> Hope that helps.
>>
>>
> Yes, thanks. I'll probably come back with more question once I've played
> with these options a bit. Sorry for the late answer, I was mostly off the
> grid the past 2 weeks.
>
>
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