Using basicaa alias analysis pass

Hi

I am completely new to LLVM, and I am trying to explore the alias analysis part of it. It seems to me that -basicaa is the most simple alias analysis pass in LLVM. So I would like to try and make it work (to see some alias analysis results of some sample bit code).

What I have done is that I

—make lib/Analysis/BasicAliasAnalysis.cpp into a .so file
—write a sample c program, hello.c, with the following code

#include<stdio.h>

int main()
{
int a;
int *p;
p = &a;
a = 10;
printf(“Hello World”);
return 0;
}

—compile the c program into bit code by doing
clang -c -emit-llvm hello.c -o hello.bc

—run the bit code through basicaa pass
opt -load lib/AliasAnalysis.so -basicaa < hello.bc > result.bc

—display the resulted bit code
llvm-dis result.bc -o -

However, the resulted bit code result.bc is exactly the same as the original bit code hello.bc. That is no alias analysis information is printed on the screen or recorded in the resulting result.bc file.

Could someone explain what I should do to get some results from basicaa alias analysis pass?

Thank you very much. Your help is much appreciated!

Sheng-Hsiu

Hi Sheng-Hsiu,

Hi

I am completely new to LLVM, and I am trying to explore the alias analysis
part of it. It seems to me that -basicaa is the most simple alias analysis
pass in LLVM. So I would like to try and make it work (to see some alias
analysis results of some sample bit code).

What I have done is that I

---make lib/Analysis/BasicAliasAnalysis.cpp into a .so file
---write a sample c program, hello.c, with the following code

#include<stdio.h>

int main()
{
    int a;
    int *p;
    p = &a;
    a = 10;
    printf("Hello World");
    return 0;
}

---compile the c program into bit code by doing
clang -c -emit-llvm hello.c -o hello.bc

---run the bit code through basicaa pass
opt -load lib/AliasAnalysis.so -basicaa < hello.bc > result.bc

---display the resulted bit code
llvm-dis result.bc -o -

However, the resulted bit code result.bc is exactly the same as the original
bit code hello.bc. That is no alias analysis information is printed on the
screen or recorded in the resulting result.bc file.

Could someone explain what I should do to get some results from basicaa
alias analysis pass?

Alias analyses (and for that matter, other kinds of analyses) don't do
any transformation on their own, but merely provide the analysis
result for other transformations to use. See the related
documentation page (http://llvm.org/docs/AliasAnalysis.html) for more
information.

There are several tools to help evaluate alias analysis results, (in
part) documented on that same page; a good starting point would be
-aa-eval, and the associated options (-print-no-aliases, etc.. See
lib/Analysis/AliasAnalysisEvaluator.cpp).

-Ahmed

Hi,

I think you're looking for the -print-alias-sets option in opt

http://llvm.org/docs/AliasAnalysis.html#the-print-alias-sets-pass

Also two other small remarks:

1)
The alias analysis is a pass provided by LLVM itself so you won't have to load it as a library

2)
Alias analysis only does analysis and this information gets used by other passes. Only running the alias analysis won't change the program. Opt has an option -disable-output, which disables the bitcode output (it will still print the debug output though).

That would make your opt command:
> opt -load lib/AliasAnalysis.so -basicaa < hello.bc > result.bc

More like this:
opt -basicaa -print-alias-sets -disable-output hello.bc

Cheers,
  Roel

Specifying -basicaa only tells opt to run the basic alias analysis pass.

However, since this is not a transformation pass, but, rather, an analysis that is meant to be used by transformations, you will not see any change in the bitcode.

If you want to see the results in a human-readable form, you can try something like:

opt -basicaa -aa-eval -print-all-alias-modref-info < hello.bc

Michael

Ahmed, Roel and Michael,

Thank you so much for clarifying this for me.

Now if I want to use the result of basicaa pass to do some transformation, I suppose I will need write a pass, correct? Is there a good document on this (specifically on writing a pass to act on alias analysis results) that I could refer to?

Thanks again!

Sheng-Hsiu

Hi Ahmed,

Yes, to do a new transformation using the alias analysis results you would need to write a pass.

I'd suggest you look at the "Writing an LLVM Pass" document [1] for information on how to get started with writing your pass and maybe have a look at the AliasAnalysisEvaluater pass [2] for an example on how you can extract information from the alias analysis.

Cheers,
  Roel

[1] http://llvm.org/docs/WritingAnLLVMPass.html
[2] http://llvm.org/docs/doxygen/html/AliasAnalysisEvaluator_8cpp_source.html