[llvm-dev] Field sensitive alias analysis?

[Daniel Berlin via llvm-dev]

TBAA is about types, not accesses.
TBAA "struct path data" is about access paths and types that are the end of
access paths, for the most part.

It has no notion of access size, etc, only offset.

It is possible to extend it and handle *very basic* cases in the frontend
(IE structs not containing unions anywhere, with constant accesses, etc)
But it would degrade quite quickly (you would likely need a sane way to say
"this is an access to an unknown offset into this type")

Generally, rather than just try to produce metadata in the frontend, most
compilers perform field-sensitive points-to or something similar for
fields, and then rely on data dependence for differentiating array
subscripts.

(This is what GCC does, LLVM has CFL-AA, but it's not field sensitive yet)

So handling .size vs .a[] is probably possible in the frontend.

Doing array subscript analysis in general, probably not something you want
in the frontend.
Handling tricky cases of what pointers to structs point to, probably the
domain of field-sensitive points-to

On Mon, Dec 7, 2015 at 7:13 AM, Dmitry Polukhin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> BTW, I have found why it doesn't work for arrays. TBAA information
> propagation is not implemented in CodeGenFunction::EmitArraySubscriptExpr
> with "TODO: Preserve/extend path TBAA metadata?".
>
> On Fri, Dec 4, 2015 at 1:38 PM, Dmitry Polukhin <dmitry.polukhin at gmail.com
> > wrote:
>
>> As far as I can see it is specifics of arrays inside structs. Current
>> TBAA does distinguish non-array members with path sensitive TBAA (see !1
>> and !6 in my example below, TBAA has reference to struct !2 and offset). As
>> for arrays information that it was member of some struct get lost
>> completely (!7 has nothing about struct !2).
>>
>> struct S {
>>   int a;
>>   int b;
>>   int c[3];
>> };
>>
>> void foo(struct S* p) {
>>   p->a = 1;
>>   p->b = 2;
>>   p->c[0] = 3;
>>   p->c[1] = 4;
>> }
>>
>> define void @foo(%struct.S* nocapture %p) #0 {
>> entry:
>>   %a = getelementptr inbounds %struct.S, %struct.S* %p, i64 0, i32 0
>>   store i32 1, i32* %a, align 4, !tbaa !1
>>   %b = getelementptr inbounds %struct.S, %struct.S* %p, i64 0, i32 1
>>   store i32 2, i32* %b, align 4, !tbaa !6
>>   %arrayidx = getelementptr inbounds %struct.S, %struct.S* %p, i64 0, i32
>> 2, i64 0
>>   store i32 3, i32* %arrayidx, align 4, !tbaa !7
>>   %arrayidx2 = getelementptr inbounds %struct.S, %struct.S* %p, i64 0,
>> i32 2, i64 1
>>   store i32 4, i32* %arrayidx2, align 4, !tbaa !7
>>   ret void
>> }
>>
>> !0 = !{!"clang version 3.8.0 "}
>> !1 = !{!2, !3, i64 0}
>> !2 = !{!"S", !3, i64 0, !3, i64 4, !4, i64 8}
>> !3 = !{!"int", !4, i64 0}
>> !4 = !{!"omnipotent char", !5, i64 0}
>> !5 = !{!"Simple C/C++ TBAA"}
>> !6 = !{!2, !3, i64 4}
>> !7 = !{!3, !3, i64 0}
>>
>> I'm just start learning how TBAA in clang works so I don't know why it
>> was implemented this way.
>>
>> On Fri, Dec 4, 2015 at 11:06 AM, Vaivaswatha Nagaraj via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi,
>>>
>>> I'm trying to optimize a simple C code and came across a situation where
>>> invariant code is not being moved out:
>>>
>>> On an -O3 compilation, I noticed that the "load" for the loop bounds
>>> (which remain invariant throughout) happens on each iteration of both the
>>> loops, even though it is not modified anywhere in the function "bigLoop".
>>> It seems that alias analysis is not able to say that the writes to one
>>> field in the structure does not impact the other field, leading to LICM
>>> being ineffective.
>>>
>>> Do any of the alias analyses currently have some kind of field
>>> sensitivity that can help in this case?
>>>
>>> ------------------------- test case ------------------------------------
>>> #include <stdlib.h>
>>> #include <stdio.h>
>>>
>>> #define SIZE 100
>>>
>>> struct AS {
>>>   int a[SIZE+4];
>>>   int size;
>>> } A;
>>>
>>> void bigLoop(void)
>>> {
>>>   unsigned i, j;
>>>
>>>   for (i = 0; i < A.size; i++) {
>>>     A.a[i+2] +=  A.a[i];
>>>   }
>>>   for (i = 0; i < A.size; i++) {
>>>     A.a[i+2] *=  A.a[i];
>>>   }
>>> }
>>>
>>> int main()
>>> {
>>>   A.size = random()%SIZE;
>>>   for (unsigned i = 0; i < A.size; i++) {
>>>     A.a[i] = random()%23;
>>>   }
>>>   bigLoop();
>>>   return 0;
>>> }
>>>
>>> Thanks,
>>>
>>>   - Vaivaswatha
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>
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