[LLVMdev] Alias analysis issue with structs on PPC

[Daniel Berlin]

On Sun, Mar 15, 2015 at 4:34 PM Olivier Sallenave <ol.sall at gmail.com> wrote:

> Hi Daniel,
>
> Thanks for your feedback. I would prefer not to write a new AA. Can't we
> directly implement that traversal in BasicAA?
>
Can I ask why?
Outside of the "well, it's another pass", i mean?

BasicAA is stateless, so you can't cache, and you really don't want to redo
these walks repeatedly (especially when the answer doesn't change unless AA
is invalidated).  It would be really expensive.
Doing this in a separate analysis pass, caching the answer, and producing
AA results, seems to me exactly the right thing.


> Otherwise, I'll investigate why this i64 was generated in the first place
> (but like you, I don't really want to know why :-)
>

Reid walked over to my desk and told me all the gory details - the clang
ABI lowering of  structs like these for anything but x86 basically says
"oh, this  really should be 2 8 byte GPR's, and the way it makes this
happen is by using i64's :)

If you want to do it at a clang level, the right thing to do is to fixup
the ABI lowerings for pointers to keep them pointers in this case.

For something simpler, if you wanted, you could do the *exact* opposite of
happens now and you'd get better results.

This is, pass everything that needs to be 2 8 byte regs as 8 byte pointers,
and cast it back to something if it's not really a pointer, using ptrtoint.

if it's not really a pointer, we don't care what AA says about it.
If it is a pointer, now we don't get bad AA answers, because there is no
inttoptr being used like there is now.

Of course, i have no idea if this strategy is going to produce correct ABI
results on all platforms, it depends on whether they treat pointers as
special.

Olivier
>
> 2015-03-13 18:51 GMT-04:00 Daniel Berlin <dberlin at dberlin.org>:
>
>>
>>
>> On Fri, Mar 13, 2015 at 2:54 PM Daniel Berlin <dberlin at dberlin.org>
>> wrote:
>>
>>> On Fri, Mar 13, 2015 at 2:39 PM Olivier H Sallenave <ohsallen at us.ibm.com>
>>> wrote:
>>>
>>>> Hi,
>>>>
>>>> I have the following C loop to vectorize:
>>>>
>>>> struct box {
>>>>     double* source;
>>>> };
>>>>
>>>> void test(double* restrict result, struct box my_struct, int len)
>>>> {
>>>>     for (int i=0 ; i<len; i++) {
>>>>         result[i] = my_struct.source[i] * my_struct.source[i];
>>>>     }
>>>> }
>>>>
>>>> There are two references in the loop, result[i] (restrict) and
>>>> my_struct.source[i] (readonly). The compiler should easily figure out that
>>>> they do not alias.
>>>>
>>>> Compiling for x86, the loop alias analysis works just fine:
>>>>   AST: Alias Set Tracker: 2 alias sets for 2 pointer values.
>>>>   AliasSet[0x7fd8e2f32290, 1] must alias, No access Pointers: (double*
>>>> %arrayidx5, 18446744073709551615)
>>>>   AliasSet[0x7fd8e2f322e0, 1] must alias, No access Pointers: (double*
>>>> %arrayidx, 18446744073709551615)
>>>>
>>>> Compiling for PPC with -target powerpc64le-ibm-linux-gnu, the two
>>>> addresses now alias:
>>>>   AST: Alias Set Tracker: 1 alias sets for 2 pointer values.
>>>>   AliasSet[0x7f931bd5bdc0, 2] may alias, No access Pointers: (double*
>>>> %arrayidx5, 18446744073709551615), (double* %arrayidx, 18446744073709551615)
>>>>
>>>> BasicAA is used for both targets by default. The difference is that in
>>>> PPC, the IR obtained from Clang takes an i64 as parameter instead of a
>>>> double* for my_struct.
>>>>
>>>
>>> I don't even want to know why this would be the case :)
>>>
>>>
>>>> This parameter is then coerced into double* using an inttoptr
>>>> instruction. The code in BasicAliasAnalysis.cpp which is triggered for x86
>>>> is the following:
>>>>
>>>>     // Function arguments can't alias with things that are known to be
>>>>     // unambigously identified at the function level.
>>>>     if ((isa<Argument>(O1) && isIdentifiedFunctionLocal(O2)) ||
>>>>         (isa<Argument>(O2) && isIdentifiedFunctionLocal(O1)))
>>>>       return NoAlias;
>>>>
>>>> isIdentifiedFunctionLocal(V) returns true for a noalias argument (such
>>>> as result), but the other address (my_struct) must be a function argument
>>>> in order to return NoAlias, which is not the case anymore for PPC (since
>>>> my_struct is now the result from an inttoptr instruction). If I understand,
>>>> the problem is that we cannot trust the fact that locals do not alias with
>>>> restrict parameters (because the compiler could generate some locals which
>>>> alias)?
>>>>
>>> Yes, because pointers *based on* the noalias'd argument are legal
>>> aliases.
>>>
>>> So if you don't know it's an argument or an identified local, it could
>>> be based on the restricted pointer, and thus, alias it.
>>>
>>>
>>> If someone has suggestions about this, that would help a lot.
>>>>
>>>
>>> The only way you could prove something in this case would be to walk the
>>> chain and prove the value comes directly from an argument with no
>>> modification.
>>>
>>
>> Actually, you could do the opposite, too, pretty cheaply.
>>
>> You could write a new pass or AA.
>> It traverses chains in the reverse direction (IE it goes from the
>> arguments, and walks down the immediate use chain, marking things as based
>> on arguments or not), and makes a lookup table of things it can prove are
>> also  unmolested identified objects.
>> (which would be the result of inttoptr in your case).
>>
>> You can then use this simple lookup table to answer the
>> isIdentifiedObject question better.
>> (You'd have to make isIdentifiedObject part of the AA interface, or take
>> an optional table, blah blah blah)
>>
>>
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