llvm dev - May 2009 - [LLVMdev] Some questions on the output formats of AliasSetTracker

Dear Staff,
     Here are some questions on the output formats of AliasSetTracker.  
The code is as below:

int G1 = 9;
int G2 = 5;

int main() {
	int * XX;
	int * YY;

	XX = &G1;
	YY = XX;
	YY = &G2;
	XX = &G2;

}


The output for -anders-aa is:
Alias Set Tracker: 5 alias sets for 4 pointer values.
   AliasSet[0xea6fb0,0] may alias, Mod/Ref
     10 Call Sites: void ({ }*)* @llvm.dbg.func.start, void ({ }*, {  
}*)* @llvm.dbg.declare, void ({ }*, { }*)* @llvm.dbg.declare, void  
(i32, i32, { }*)* @llvm.dbg.stoppoint, void (i32, i32, { }*)*  
@llvm.dbg.stoppoint, void (i32, i32, { }*)* @llvm.dbg.stoppoint, void  
(i32, i32, { }*)* @llvm.dbg.stoppoint, void (i32, i32, { }*)*  
@llvm.dbg.stoppoint, void (i32, i32, { }*)* @llvm.dbg.stoppoint, void  
({ }*)* @llvm.dbg.region.end
   AliasSet[0xea7040,1] must alias, Mod/Ref   Pointers: (i32** %XX, 8)
   AliasSet[0xea6d40,1] must alias, Mod       Pointers: (i32** %YY, 8)
   AliasSet[0xea6ec0,1] must alias, Mod/Ref   Pointers: (i32* %0, 4)
   AliasSet[0xea67d0,1] must alias, Mod/Ref   Pointers: (i32* %retval, 4)



The output for -steens-aa in poolalloc module is:
Alias Set Tracker: 5 alias sets for 4 pointer values.
   AliasSet[0xeb0e20,0] may alias, Mod/Ref
     10 Call Sites: void ({ }*)* @llvm.dbg.func.start, void ({ }*, {  
}*)* @llvm.dbg.declare, void ({ }*, { }*)* @llvm.dbg.declare, void  
(i32, i32, { }*)* @llvm.dbg.stoppoint, void (i32, i32, { }*)*  
@llvm.dbg.stoppoint, void (i32, i32, { }*)* @llvm.dbg.stoppoint, void  
(i32, i32, { }*)* @llvm.dbg.stoppoint, void (i32, i32, { }*)*  
@llvm.dbg.stoppoint, void (i32, i32, { }*)* @llvm.dbg.stoppoint, void  
({ }*)* @llvm.dbg.region.end
   AliasSet[0xeb5a20,1] must alias, Mod/Ref   Pointers: (i32** %XX, 8)
   AliasSet[0xebb210,1] must alias, Mod       Pointers: (i32** %YY, 8)
   AliasSet[0xebabb0,1] must alias, Mod/Ref   Pointers: (i32* %0, 4)
   AliasSet[0xebac10,1] must alias, Mod/Ref   Pointers: (i32* %retval, 4)



However, I am a little confused with these output. In Both anders-aa  
and steens-aa, XX and YY should stay in the same AliasSet, and their  
alias relationship should be May Alias, right? Sorry I am just a  
freshman to the alias analysis area, please shed some light on the  
understanding of the output formats.

Best,
Heming

On Thu, May 7, 2009 at 5:07 PM,  <hc2428 at columbia.edu>
wrote:
> However, I am a little confused with these output. In Both anders-aa
> and steens-aa, XX and YY should stay in the same AliasSet, and their
> alias relationship should be May Alias, right?
%XX and %YY are the results of alloca instructions (i.e. equivalent to
the C expressions "&XX" and "&YY"), so they do not
in fact alias.

-Eli

Quoting Eli Friedman <eli.friedman at gmail.com>:

Dear Eli,
     Thanks very much for your reply. I have modified the XX and YY to  
be global variables, but the output of AliasSetTracker are still MUST  
alias:

Alias Set Tracker: 5 alias sets for 4 pointer values.
   AliasSet[0xea55d0,0] may alias, Mod/Ref
     8 Call Sites: void ({ }*)* @llvm.dbg.func.start, void (i32, i32,  
{ }*)* @llvm.dbg.stoppoint, void (i32, i32, { }*)*  
@llvm.dbg.stoppoint, void (i32, i32, { }*)* @llvm.dbg.stoppoint, void  
(i32, i32, { }*)* @llvm.dbg.stoppoint, void (i32, i32, { }*)*  
@llvm.dbg.stoppoint, void (i32, i32, { }*)* @llvm.dbg.stoppoint, void  
({ }*)* @llvm.dbg.region.end
   AliasSet[0xeb81e0,1] must alias, Mod/Ref   Pointers: (i32** @XX, 8)
   AliasSet[0xea5fc0,1] must alias, Mod       Pointers: (i32** @YY, 8)
   AliasSet[0xeab830,1] must alias, Mod/Ref   Pointers: (i32* %0, 4)
   AliasSet[0xea5e70,1] must alias, Mod/Ref   Pointers: (i32* %retval, 4)


I am sure that the XX and YY are global:
@XX = common global i32* null		; <i32**> [#uses=4]
@YY = common global i32* null		; <i32**> [#uses=3]

would you please tell me more details?

Thanks,
Heming

> On Thu, May 7, 2009 at 5:07 PM,  <hc2428 at columbia.edu> wrote:
>> However, I am a little confused with these output. In Both anders-aa
>> and steens-aa, XX and YY should stay in the same AliasSet, and their
>> alias relationship should be May Alias, right?
>
> %XX and %YY are the results of alloca instructions (i.e. equivalent to
> the C expressions "&XX" and "&YY"), so they do
not in fact alias.
>
> -Eli
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