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ambika

2010-Feb-14 16:39 UTC

[LLVMdev] A very basic doubt about LLVM Alias Analysis

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.
 
Duncan Sands wrote:> Hi ambika,
>
>> main() {
>>     int i,*k;
>>     k = &i;
>
>> then why dont i get {k -> i} ie k is aliased to i in any of the 
>> analysis.
>
> only pointers can alias each other, and i is not a pointer.
>
> Ciao,
>
> Duncan.

Duncan Sands

2010-Feb-14 17:34 UTC

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[LLVMdev] A very basic doubt about LLVM Alias Analysis

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.

Török Edwin

2010-Feb-14 17:36 UTC

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[LLVMdev] A very basic doubt about LLVM Alias Analysis

On 02/14/2010 06:39 PM, ambika wrote:> 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;
> }
>   
Your example is too simple, i,j,k don't exist after running mem2reg.
And well, without running mem2reg i and j will be stack variables, so
you need to look for the value that is loaded from them.
That is the real pointer:
  AliasSet[0x0x1631400,3] may alias, Mod/Ref   Pointers: (i32* %k, 4),
(i32* %tmp, 4), (i32* %tmp2, 4)

Here %tmp is i (  %tmp = load i32** %i ), %tmp2 is j ( %tmp2 = load
i32** %j).

If you make it a bit more complicated:
int main(int argc, char **argv)
{
        int *i=0,*j=0,k;
        if (argc > 1) {
          i=&k;
        } else {
          j=&k;
        }
        k=4;
        printf("%d,%d,%d,%p,%p",*i,*j,k,i,j);
        return 0;
}

Then you get this:
  AliasSet[0x0xb8e6a0,3] may alias, Mod/Ref   Pointers: (i32* %k, 4),
(i32* %i.0, 4), (i32* %j.0, 4)

ambika

2010-Feb-14 17:55 UTC

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[LLVMdev] A very basic doubt about LLVM Alias Analysis

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.

ambika

2010-Feb-15 09:33 UTC

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[LLVMdev] A very basic doubt about LLVM Alias Analysis

I tried the program u mentioned but still did not get the result you are 
getting. I think there must be some mistake in compiling.
The commands which I am using are :

To create bc file:

llvm-gcc -emit-llvm -c -o s.bc s.c -O1

and to get alias result :

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


Török Edwin wrote:> On 02/14/2010 06:39 PM, ambika wrote:
>   
>> 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;
>> }
>>   
>>     
>
> Your example is too simple, i,j,k don't exist after running mem2reg.
> And well, without running mem2reg i and j will be stack variables, so
> you need to look for the value that is loaded from them.
> That is the real pointer:
>   AliasSet[0x0x1631400,3] may alias, Mod/Ref   Pointers: (i32* %k, 4),
> (i32* %tmp, 4), (i32* %tmp2, 4)
>
> Here %tmp is i (  %tmp = load i32** %i ), %tmp2 is j ( %tmp2 = load
> i32** %j).
>
> If you make it a bit more complicated:
> int main(int argc, char **argv)
> {
>         int *i=0,*j=0,k;
>         if (argc > 1) {
>           i=&k;
>         } else {
>           j=&k;
>         }
>         k=4;
>         printf("%d,%d,%d,%p,%p",*i,*j,k,i,j);
>         return 0;
> }
>
> Then you get this:
>   AliasSet[0x0xb8e6a0,3] may alias, Mod/Ref   Pointers: (i32* %k, 4),
> (i32* %i.0, 4), (i32* %j.0, 4)
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

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