[llvm-dev] LLVM Alias Analysis problem

[Doerfert, Johannes Rudolf via llvm-dev]

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.

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;
> }
> 

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-- 

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov
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