[LLVMdev] __main() function and AliasSet

[Nai Xia]

In a code segment of my pass plugin, I try to gather AliasSets for all StoreInst, LoadInst and CallInst instructions in a function. 
Some behaviors of the pass puzzled me. 
Below is the *.ll of the test program which I run the pass on, 
it was get with "llvm-gcc -Wl,--disable-opt" from a rather simple *.c program.

----------------------------------
; ModuleID = 'ptralias.bc'
target endian = little
target pointersize = 32
target triple = "i686-pc-linux-gnu"
deplibs = [ "c", "crtend" ]
%.str_1 = internal constant [25 x sbyte] c"ptra=0x ptrb=0x ptrc=0x\0A\00"		; <[25 x sbyte]*> [#uses=1]
%ptr = weak global void ()* null		; <void ()**> [#uses=0]

implementation   ; Functions:

declare int %printf(sbyte*, ...)

void %foo1() {
	ret void
}

void %foo2() {
	ret void
}

int %main(int %argc, sbyte** %argv) {
entry:
	%data_b = alloca int		; <int*> [#uses=2]
	%data_c = alloca int		; <int*> [#uses=1]
	%data_d = alloca int		; <int*> [#uses=3]
	%data_e = alloca int		; <int*> [#uses=2]
	%data_f = alloca int		; <int*> [#uses=2]
	call void %__main( )
	store int 2, int* %data_b
	store int 3, int* %data_c
	store int 4, int* %data_d
	store int 5, int* %data_e
	store int 6, int* %data_f
	switch int %argc, label %switchexit [
		 int 3, label %label.3
		 int 2, label %then.2
		 int 1, label %label.1
		 int 0, label %endif.2
	]

label.1:	; preds = %entry
	br label %switchexit

label.3:		; preds = %entry
	br label %then.2

switchexit:		; preds = %label.1, %entry
	%ptr_b.0 = phi int* [ %data_d, %label.1 ], [ null, %entry ]	; <int*>  [#uses=1]
	br label %endif.2

then.2:		; preds = %label.3, %entry
	%ptr_a.1.0 = phi int* [ %data_f, %label.3 ], [ %data_e, %entry ]		; <int*> [#uses=1]
	store int 0, int* %ptr_a.1.0
	br label %then.3

endif.2:		; preds = %switchexit, %entry
	%ptr_b.0.1 = phi int* [ %ptr_b.0, %switchexit ], [ %data_b, %entry ]		; <int*> [#uses=2]
	%tmp.12 = seteq int* %ptr_b.0.1, null		; <bool> [#uses=1]
	br bool %tmp.12, label %then.4, label %then.3

then.3:		; preds = %endif.2, %then.2
	%ptr_b.0.2 = phi int* [ %data_d, %then.2 ], [ %ptr_b.0.1, %endif.2 ]		; <int*> [#uses=1]
	store int 0, int* %ptr_b.0.2
	%tmp.1913 = call int (sbyte*, ...)* %printf( sbyte* getelementptr ([25 x sbyte]* %.str_1, int 0, int 0) )		; <int> [#uses=0]
	ret int 0

then.4:		; preds = %endif.2
	%tmp.19 = call int (sbyte*, ...)* %printf( sbyte* getelementptr ([25 x sbyte]* %.str_1, int 0, int 0) )		; <int> [#uses=0]
	ret int 0
}

void %__main() {
entry:
	ret void
}

----------------------------------
I think the right AliasSet information calculated for this program should be

  Information for alias set0:
      pointer0=data_b
      pointer1=data_d
      pointer2=ptr_b.0.2
  Information for alias set1:
      pointer0=data_c
  Information for alias set2:
  Information for alias set3:
      pointer0=data_e
      pointer1=data_f
      pointer2=ptr_a.1.0
  Information for alias set4:
  Information for alias set5:
    
,where the empty AliasSets I think should be "Forwarded".

However, the result of the pass was:

  Information for alias set0:
    pointer0=data_b
    pointer1=data_d
    pointer2=data_e
    pointer3=data_f
    pointer4=ptr_a.1.0
    pointer5=ptr_b.0.2
  Information for alias set1:
    pointer0=data_c
After I deleted "call void %__main( )" in %main(), the pass get the right answer.

So my question is:

1. What is the purpose for call to __main() ? __main() is just a empty function. My .c program only contains main().
2. My explanation for this behavior is that the CallSite of "call void %__main( )" alias some pointers in the program,
   however, __main() is obviously empty, why should the AliasAnalysis think that it may/must Mod/Ref any stack location of main()?

btw: the AliasAnalysis pass I used is -steens-aa and it also the same with -anders-aa.

-- 
Regards,
Nai