llvm dev - May 2009 - [LLVMdev] difference between alias set tracker and alias analysis evaluator

Hi all,

I have a problem with alias aliasing results returned by the alias set 
tracker and the alias analysis evaluator.

so i call opt tool to run my module with the following options 
"-anders-aa -aa-eval -print-alias-sets -print-all-alias-modref-info"
i use 2 classes to print alias analysis, the AliasSetTracket and 
AliasAnalysisEvaluator.
but they give different results.

here is a sample program:
int main(){
    int a[5];
    struct {
            char    id[10];
            int     line;
    } sym;
    sym.line =a[1] + 10;
    a[3] = 3;
    sym.id[5] = a[3];
    return 0;
}

and here is the ll file
define i32 @main() nounwind {
entry:
    %retval = alloca i32        ; <i32*> [#uses=2]
    %sym = alloca %struct..0anon        ; <%struct..0anon*> [#uses=2]
    %a = alloca [5 x i32]        ; <[5 x i32]*> [#uses=3]
    %0 = alloca i32        ; <i32*> [#uses=2]
    %"alloca point" = bitcast i32 0 to i32        ; <i32>
[#uses=0]
    %1 = getelementptr [5 x i32]* %a, i32 0, i64 1        ; <i32*>
[#uses=1]
    %2 = load i32* %1, align 4        ; <i32> [#uses=1]
    %3 = add i32 %2, 10        ; <i32> [#uses=1]
    %4 = getelementptr %struct..0anon* %sym, i32 0, i32 1        ; 
<i32*> [#uses=1]
    store i32 %3, i32* %4, align 4
    %5 = getelementptr [5 x i32]* %a, i32 0, i64 3        ; <i32*>
[#uses=1]
    store i32 3, i32* %5, align 4
    %6 = getelementptr [5 x i32]* %a, i32 0, i64 3        ; <i32*>
[#uses=1]
    %7 = load i32* %6, align 4        ; <i32> [#uses=1]
    %8 = trunc i32 %7 to i8        ; <i8> [#uses=1]
    %9 = getelementptr %struct..0anon* %sym, i32 0, i32 0        ; <[10 
x i8]*> [#uses=1]
    %10 = getelementptr [10 x i8]* %9, i32 0, i64 5        ; <i8*>
[#uses=1]
    store i8 %8, i8* %10, align 1
    store i32 0, i32* %0, align 4
    %11 = load i32* %0, align 4        ; <i32> [#uses=1]
    store i32 %11, i32* %retval, align 4
    br label %return

return:        ; preds = %entry
    %retval1 = load i32* %retval        ; <i32> [#uses=1]
    ret i32 %retval1
}

the AliasSetTracker gives the following result:
Alias Set Tracker: 6 alias sets for 7 pointer values.
  AliasSet[0x2ce5d30,1] must alias, Ref       Pointers: (i32* %1, 4)
  AliasSet[0x2ce5ab0,1] must alias, Mod       Pointers: (i32* %4, 4)
  AliasSet[0x2ce59c0,2] must alias, Mod/Ref   Pointers: (i32* %5, 4), 
(i32* %6, 4)
  AliasSet[0x2ce5910,1] must alias, Mod       Pointers: (i8* %10, 1)
  AliasSet[0x2ce5820,1] must alias, Mod/Ref   Pointers: (i32* %0, 4)
  AliasSet[0x2ce4f00,1] must alias, Mod/Ref   Pointers: (i32* %retval, 4)

the alias analysis evaluator gives the following results (i removed all 
the NoAlias results)
  MayAlias:    [5 x i32]* %a, i32* %1
  MayAlias:    %struct..0anon* %sym, i32* %4
  MayAlias:    [5 x i32]* %a, i32* %5
  MayAlias:    [5 x i32]* %a, i32* %6
  MustAlias:    i32* %5, i32* %6
  MustAlias:    %struct..0anon* %sym, [10 x i8]* %9
  MayAlias:    %struct..0anon* %sym, i8* %10
  MayAlias:    [10 x i8]* %9, i8* %10

so my questions finally are:
- why is there is a difference between the 2 results,
- how can i get the alias set tracker to give me all the aliasing 
results returned by alias analysis evaluator


thank you

On May 29, 2009, at 3:35 AM, Amr Yehia wrote:
>
> so my questions finally are:
> - why is there is a difference between the 2 results,
> - how can i get the alias set tracker to give me all the aliasing
> results returned by alias analysis evaluator
>
This is an inherent limitation of aliassettracker: it unions the  
results of alias queries into "sets".  Because of this, transitive  
results cannot be distinguished.  For example, if "A mayalias B", and
"B mayalias C" alias set tracker will tell you that "a and c are
in
the same set" even if AliasAnalysis knows they don't alias.  You can  
use AliasAnalysis directly or use MemoryDependenceAnalysis for more  
precise results.

-Chris