[llvm-dev] LLVM Alias Analysis problem
Артём Вопилов via llvm-dev
llvm-dev at lists.llvm.org
Mon Dec 17 13:13:18 PST 2018
Dear LLVM Developers,
Thank you for your last messages!
Now I attached to this email code of the original program, _full_ LLVM-IR and clean LLVM-IR.
I executed "opt -analyze -aa-eval -print-all-alias-modref-info" and "opt -analyze -aa-eval -print-alias-sets" on both generated LLVM-IR.
So, I also attached the results of running this commands.
I would like to ask you again the same question.
My goal is to find sets of pointers, that point to the same memory locations.
Am I right, that with Alias Analysis I cannot accomplish my aim? Is it possible to achieve it using LLVM?
Thank you.
I am looking forward to hearing from you!
Respectfully yours,
Artem Vopilov
14.12.2018, 21:04, "Doerfert, Johannes Rudolf" <jdoerfert at anl.gov>:
> On 12/14, Daniel Sanders wrote:
>> > On 14 Dec 2018, at 16:58, Doerfert, Johannes Rudolf via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> >
>> > Hi Artem,
>> >
>> > 1) Please do not CC all the llvm lists you can find. llvm-dev is the one
>> > for questions like this one, llvm-admin, mailman, bugs-admin, ... are
>> > not.
>> >
>> > 2) Please attach the _full_ LLVM-IR you used, the ".txt" file you
>> > attached is not complete and therefore not helpful. We can all
>> > open/read ".ll" files and so should you. As long as you pass the -S
>> > flag to opt (and clang in combination with -emit-llvm) you get human
>> > readable (=plain text) LLVM-IR as outpu.
>>
>> According to the function group comments, the missing line seems to be:
>> attributes #0 = { nounwind uwtable }
>>
>> @Артём: Hi,
>> One other thing llvm-dev will need to know is the version of LLVM you're using. It looks like it isn't a recent version since load instructions changed a while back. For example:
>> %0 = load i32* %a.addr, align 4
>> from your .txt is now:
>> %0 = load i32, i32* %a.addr, align 4
>> in recent LLVM-IR.
>
> Good catch!
>
> The full IR (at least for custom builds) will also include the llvm and
> clang version, another thing that is missing from the ".txt" ;)
>
>> > 3) You should probably run something like:
>> > clang -emit-llvm -S -O3 -mllvm -disable-llvm-optzns test.c -o test.ll
>> > to get your initial IR. Then you probably want to continue with
>> > opt -S -mem2reg -instcombine -simplifycfg test.ll -o test_clean.ll
>> > to clean up the IR a bit (especially to remove the stack locations
>> > introduced for variables and transform them into "proper SSA values".)
>> > Finally you run your analyzes as you did before
>> > opt -analyze -aa-eval -print-all-alias-modref-info test_clean.ll
>> > to get your results. If you need help understand the results, please
>> > let us know and provide enough information for us to comprehend your
>> > problem. You should also take a look at the test.ll and test_clean.ll
>> > and make sure you fully understand what the code representation
>> > means. As I noted in the other thread, your example is to simple,
>> > after the stack locations are promoted to registers, there is no
>> > "ptra/ptrb" that could alias anymore.
>> >
>> > I hope this helps.
>> >
>> > Cheers,
>> > Johannes
>> >
>> >
>> >
>> > On 12/14, Артём Вопилов via llvm-dev wrote:
>> >> Dear LLVM developers,
>> >>
>> >> My name is Artem Vopilov, I am a student at TU Darmstadt. I am writing to you again to ask about Alias Analysis.
>> >>
>> >> Now I attached IR code and C code of program I analyze with Alias Analysis.
>> >>
>> >> Running commands "opt -analyze -aa-eval -print-all-alias-modref-info" and for printing sets of alias "opt -analyze -aa-eval -print-alias-sets" gives me the results, that in main function variables %a, %0, %2, %4 alias.
>> >>
>> >> However I expected from Alias Analysis to indicate that pointers which point to same memory location alias, namely, %ptra and %ptrb.
>> >> I learnt from your last messages, that "A pointer is just a variable containing memory address, so pointer itself will never alias with other pointers, but the ‘pointee’ will alias with other memory addresses."
>> >>
>> >> My goal is to find sets of pointers, that point to the same memory locations.
>> >> Am I right, that with Alias Analysis I cannot accomplish my aim? Is it possible to achieve it using LLVM?
>> >> Thank you.
>> >>
>> >> I am looking forward to hearing from you!
>> >>
>> >> Respectfully yours,
>> >> Artem Vopilov
>> >
>> >> ; ModuleID = '<stdin>'
>> >> target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
>> >> target triple = "x86_64-unknown-linux-gnu"
>> >>
>> >> ; Function Attrs: nounwind uwtable
>> >> define i32 @main() #0 {
>> >> entry:
>> >> %retval = alloca i32, align 4
>> >> %a = alloca i32, align 4
>> >> %b = alloca i32, align 4
>> >> %ptra = alloca i32*, align 8
>> >> %ptrb = alloca i32*, align 8
>> >> %temp = alloca i32, align 4
>> >> %sum = alloca i32, align 4
>> >> store i32 0, i32* %retval
>> >> store i32 9, i32* %a, align 4
>> >> store i32 7, i32* %b, align 4
>> >> store i32* %a, i32** %ptra, align 8
>> >> store i32* %a, i32** %ptrb, align 8
>> >> %0 = load i32** %ptrb, align 8
>> >> %1 = load i32* %0, align 4
>> >> store i32 %1, i32* %b, align 4
>> >> %2 = load i32** %ptrb, align 8
>> >> store i32 4, i32* %2, align 4
>> >> %3 = load i32* %a, align 4
>> >> %4 = load i32** %ptrb, align 8
>> >> %5 = load i32* %4, align 4
>> >> %call = call i32 @addNumbers(i32 %3, i32 %5)
>> >> store i32 %call, i32* %sum, align 4
>> >> %6 = load i32* %sum, align 4
>> >> ret i32 %6
>> >> }
>> >>
>> >> ; Function Attrs: nounwind uwtable
>> >> define i32 @addNumbers(i32 %a, i32 %b) #0 {
>> >> entry:
>> >> %a.addr = alloca i32, align 4
>> >> %b.addr = alloca i32, align 4
>> >> %result = alloca i32, align 4
>> >> %notResult = alloca i32*, align 8
>> >> store i32 %a, i32* %a.addr, align 4
>> >> store i32 %b, i32* %b.addr, align 4
>> >> store i32* %b.addr, i32** %notResult, align 8
>> >> %0 = load i32* %a.addr, align 4
>> >> %1 = load i32** %notResult, align 8
>> >> %2 = load i32* %1, align 4
>> >> %add = add nsw i32 %0, %2
>> >> store i32 %add, i32* %result, align 4
>> >> %3 = load i32* %result, align 4
>> >> ret i32 %3
>> >> }
>> >
>> >> #include <stdio.h>
>> >>
>> >>
>> >> int addNumbers(int a, int b);
>> >>
>> >>
>> >> int main()
>> >> {
>> >> int a = 9, b = 7;
>> >> int *ptra, *ptrb;
>> >> int temp;
>> >> int sum;
>> >>
>> >> ptra = &a;
>> >> ptrb = ptra;
>> >> b = *ptrb;
>> >>
>> >> *ptrb = 4;
>> >>
>> >> sum = addNumbers(a, *ptrb);
>> >>
>> >> return sum;
>> >> }
>> >>
>> >>
>> >> int addNumbers(int a,int b)
>> >> {
>> >> int result;
>> >> int *notResult;
>> >>
>> >> notResult = &b;
>> >>
>> >> result = a + *notResult;
>> >>
>> >> return result;
>> >> }
>> >>
>> >
>> >> _______________________________________________
>> >> LLVM Developers mailing list
>> >> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>> >
>> >
>> > --
>> >
>> > Johannes Doerfert
>> > Researcher
>> >
>> > Argonne National Laboratory
>> > Lemont, IL 60439, USA
>> >
>> > jdoerfert at anl.gov <mailto:jdoerfert at anl.gov>
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>
> --
>
> Johannes Doerfert
> Researcher
>
> Argonne National Laboratory
> Lemont, IL 60439, USA
>
> jdoerfert at anl.gov
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