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.
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.
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.
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.
>
> 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"
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.
