LLVM Alias Analysis (Load and store from same address is not showed up in same set)

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1

Hi
I have this simple c code for which I would like to use for alias analysis.

#include <stdio.h>

#include <stdlib.h>

static int (*fp) (void);

void ind_call (int (*compr)(void)){

fp = compr;

fp();

}

int hello(void){

return printf(“hello world\n”);

}

int main(){

ind_call(hello);

return 0;

}

So, I do the following:

bin/opt -basicaa -cfl-anders-aa -print-alias-sets bin/test/test.bc

I get the following result:

Alias sets for function ‘ind_call’:

Alias Set Tracker: 2 alias sets for 2 pointer values.

AliasSet[0x91a5820, 1] must alias, Mod/Ref Pointers: (i32 ()** %compr.addr, 8)

AliasSet[0x91a58c0, 2] may alias, Mod/Ref Pointers: (i32 ()** @fp, 8)

1 Unknown instructions: i32 %call

Alias sets for function ‘hello’:

Alias Set Tracker: 1 alias sets for 0 pointer values.

AliasSet[0x91a5910, 1] may alias, Mod/Ref

1 Unknown instructions: i32 %call

Alias sets for function ‘main’:

Alias Set Tracker: 2 alias sets for 1 pointer values.

AliasSet[0x91a5a50, 1] must alias, Mod Pointers: (i32* %retval, 4)

AliasSet[0x91a5aa0, 1] may alias, Mod/Ref

1 Unknown instructions: void

My question is from c code when we can notice that fp and compr target to same address (because of fp = compr), why they are not in the same alias set. As you can see, for function ind_call, the alias set that contain fp is along, while from my understanding there should be another alias compr.

For better understanding, I also include the LLVM IR here (generated by LLVM-7.0):

2

LLVM’s alias analysis is not very smart; it depends on other optimization passes to produce reasonable results. You should at least run mem2reg before trying to do any alias analysis queries. -Eli

3

Okay, I have tried the following:

bin/opt -mem2reg -basicaa -cfl-anders-aa -print-alias-sets bin/test/test.bc

But, the result is same as last time. Do you mean something like this to try? Can you suggest what else could be an important factor for this issue?

4

The functions in your IR are marked optnone because you compiled it at -O0, so mem2reg will refuse to transform them. If you need clang to generate unoptimized IR without the optnone flag, you can use “clang -O2 -mllvm -disable-llvm-optzns”. -Eli