llvm dev - Feb 2020 - Differentiate array access at IR level

Hi LLVM community,

I am trying to differentiate access to different array elements, for example:

for (int i = 1; i < 10; i++) {

    a[i] = a[i] + 10;

     b[i] = a[i - 1]  * 2;

}

If it is possible to tell it loads/stores 3 different array elements: a[i], b[i]
and a[i - 1] at IR level?


Thanks for your time in advance!


Best,
Michael
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It sounds like alias analysis can help:
https://llvm.org/docs/AliasAnalysis.html

On Sun, Feb 16, 2020 at 10:03 PM Michael Meng via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi LLVM community,
>
> I am trying to differentiate access to different array elements, for
> example:
>
> for (int i = 1; i < 10; i++) {
>
>     a[i] = a[i] + 10;
>
>      b[i] = a[i - 1]  * 2;
>
> }
>
> If it is possible to tell it loads/stores 3 different array elements:
> a[i], b[i] and a[i - 1] at IR level?
>
>
> Thanks for your time in advance!
>
>
> Best,
> Michael
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Note that a[i] are a[i-1] are accessing the same memory addresses,
just not in the same loop iteration. You may want to look into
DependenceInfo.

Michael

Am Mo., 17. Feb. 2020 um 00:04 Uhr schrieb Michael Meng via llvm-dev
<llvm-dev at lists.llvm.org>:
>
> Hi LLVM community,
>
> I am trying to differentiate access to different array elements, for
example:
>
> for (int i = 1; i < 10; i++) {
>
>     a[i] = a[i] + 10;
>
>      b[i] = a[i - 1]  * 2;
>
> }
>
> If it is possible to tell it loads/stores 3 different array elements: a[i],
b[i] and a[i - 1] at IR level?
>
>
> Thanks for your time in advance!
>
>
> Best,
> Michael
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Hi Michael,

Thanks for your reply!

Since I want to collect the counts of unique array access to array elements in
one iteration, so I consider a[i] and a[i - 1] as two accesses to different
array elements. Would it be possible to get the notion of "i - 1" at
IR level.

The IR looks like:

%11 = load float*, float** %a.addr, align 8, !dbg !954
%12 = load i32, i32* %i, align 4, !dbg !955
%sub = sub nsw i32 %12, 1, !dbg !956
%idxprom7 = sext i32 %sub to i64, !dbg !954
%arrayidx8 = getelementptr inbounds float, float* %11, i64 %idxprom7, !dbg !954

Working the the source level might be easier, but I wonder if it is possible to
know idxprom7 -> i - 1 with IR


Thanks again!
Michael
________________________________
From: Michael Kruse <llvmdev at meinersbur.de>
Sent: Tuesday, February 18, 2020 9:26 AM
To: Michael Meng <overrainbow2013 at hotmail.com>
Cc: via llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Differentiate array access at IR level

Note that a[i] are a[i-1] are accessing the same memory addresses,
just not in the same loop iteration. You may want to look into
DependenceInfo.

Michael

Am Mo., 17. Feb. 2020 um 00:04 Uhr schrieb Michael Meng via llvm-dev
<llvm-dev at lists.llvm.org>:
>
> Hi LLVM community,
>
> I am trying to differentiate access to different array elements, for
example:
>
> for (int i = 1; i < 10; i++) {
>
>     a[i] = a[i] + 10;
>
>      b[i] = a[i - 1]  * 2;
>
> }
>
> If it is possible to tell it loads/stores 3 different array elements: a[i],
b[i] and a[i - 1] at IR level?
>
>
> Thanks for your time in advance!
>
>
> Best,
> Michael
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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