[llvm-dev] LLVM Alias Analysis problem (original) (raw)

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 testclean.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 testclean.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 testclean.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 = ''  >> target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"  >> target triple = "x8664-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 -------------- next part -------------- A non-text attachment was scrubbed... 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