[LLVMdev] A very basic doubt about LLVM Alias Analysis

[ambika]

to compile it to bitcode I give the following command :

   llvm-gcc -emit-llvm -c -o s.bc s.c

and then I run different alias analysis passes like -anders-aa, -basicaa 
using following:

   opt -anders-aa -aa-eval -print-all-alias-modref-info  s.bc

 From this I get the following output:

Function: main: 8 pointers, 1 call sites
 NoAlias:    i32* %retval, i32** %j
 NoAlias:    i32* %retval, i32** %i
 NoAlias:    i32** %i, i32** %j
 NoAlias:    i32* %0, i32* %retval
 NoAlias:    i32* %0, i32** %j
 NoAlias:    i32* %0, i32** %i
 NoAlias:    i32* %2, i32* %retval
 NoAlias:    i32* %2, i32** %j
 NoAlias:    i32* %2, i32** %i
 NoAlias:    i32* %0, i32* %2
 NoAlias:    i32* %4, i32* %retval
 NoAlias:    i32* %4, i32** %j
 NoAlias:    i32* %4, i32** %i
 NoAlias:    i32* %0, i32* %4
 MayAlias:    i32* %2, i32* %4
 NoAlias:    i32* %k, i32* %retval
 NoAlias:    i32* %k, i32** %j
 NoAlias:    i32* %k, i32** %i
 NoAlias:    i32* %0, i32* %k
 MayAlias:    i32* %2, i32* %k
 MayAlias:    i32* %4, i32* %k
 NoAlias:    i32* %retval, i8* getelementptr inbounds ([9 x i8]* @.str, 
i32 0, i32 0)
 NoAlias:    i32** %j, i8* getelementptr inbounds ([9 x i8]* @.str, i32 
0, i32 0)
 NoAlias:    i32** %i, i8* getelementptr inbounds ([9 x i8]* @.str, i32 
0, i32 0)
 NoAlias:    i32* %0, i8* getelementptr inbounds ([9 x i8]* @.str, i32 
0, i32 0)
 NoAlias:    i32* %2, i8* getelementptr inbounds ([9 x i8]* @.str, i32 
0, i32 0)
 NoAlias:    i32* %4, i8* getelementptr inbounds ([9 x i8]* @.str, i32 
0, i32 0)
 NoAlias:    i32* %k, i8* getelementptr inbounds ([9 x i8]* @.str, i32 
0, i32 0)
 NoModRef:  Ptr: i32* %retval    <->  %6 = call i32 (i8*, ...)* 
@printf(i8* getelementptr inbounds ([9 x i8]* @.str, i32 0, i32 0), i32 
%5, i32 %3, i32 %1) nounwind ; <i32> [#uses=0]  NoModRef:  Ptr: i32** 
%j    <->  %6 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds 
([9 x i8]* @.str, i32 0, i32 0), i32 %5, i32 %3, i32 %1) nounwind ; 
<i32> [#uses=0]  NoModRef:  Ptr: i32** %i    <->  %6 = call i32 (i8*, 
...)* @printf(i8* getelementptr inbounds ([9 x i8]* @.str, i32 0, i32 
0), i32 %5, i32 %3, i32 %1) nounwind ; <i32> [#uses=0]  NoModRef:  Ptr: 
i32* %0    <->  %6 = call i32 (i8*, ...)* @printf(i8* getelementptr 
inbounds ([9 x i8]* @.str, i32 0, i32 0), i32 %5, i32 %3, i32 %1) 
nounwind ; <i32> [#uses=0]  NoModRef:  Ptr: i32* %2    <->  %6 = call 
i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([9 x i8]* @.str, i32 
0, i32 0), i32 %5, i32 %3, i32 %1) nounwind ; <i32> [#uses=0]  
NoModRef:  Ptr: i32* %4    <->  %6 = call i32 (i8*, ...)* @printf(i8* 
getelementptr inbounds ([9 x i8]* @.str, i32 0, i32 0), i32 %5, i32 %3, 
i32 %1) nounwind ; <i32> [#uses=0]  NoModRef:  Ptr: i32* %k    <->  %6 = 
call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([9 x i8]* 
@.str, i32 0, i32 0), i32 %5, i32 %3, i32 %1) nounwind ; <i32> 
[#uses=0]  NoModRef:  Ptr: i8* getelementptr inbounds ([9 x i8]* @.str, 
i32 0, i32 0)    <->  %6 = call i32 (i8*, ...)* @printf(i8* 
getelementptr inbounds ([9 x i8]* @.str, i32 0, i32 0), i32 %5, i32 %3, 
i32 %1) nounwind ; <i32> [#uses=0]===== Alias Analysis Evaluator Report 
=====
 28 Total Alias Queries Performed
 25 no alias responses (89.2%)
 3 may alias responses (10.7%)
 0 must alias responses (0.0%)
 Alias Analysis Evaluator Pointer Alias Summary: 89%/10%/0%
 8 Total ModRef Queries Performed
 8 no mod/ref responses (100.0%)
 0 mod responses (0.0%)
 0 ref responses (0.0%)
 0 mod & ref responses (0.0%)
 Alias Analysis Evaluator Mod/Ref Summary: 100%/0%/0%/0%



And here nowhere it shows even a may alias relation between i & j.
I am interpreting this by looking at No Alias/May Alias/Must Alias 
outputs shown infron of them. eg

 NoAlias:    i32** %i, i32** %j

I interpret it as no alias relation between i & j.



Duncan Sands wrote:
> Hi Ambika,
>
>> Oh m sorry for that mistake as I had points to in mind.
>> But still what about the following prog:
>>
>> int main()
>> {
>>        int *i,*j,k;
>>        i=&k;
>>        j=&k;
>>        k=4;
>>        printf("%d,%d,%d",*i,*j,k);
>>        return 0;
>> }
>>
>>
>> here too i dont get <i,j> alias each other.
>
> how are you compiling to bitcode, and how are you determining that alias
> analysis thinks they don't alias?
>
> Ciao,
>
> Duncan.