[LLVMdev] Re: __main() function and AliasSet

[Nai XIA]

Oh, I appologize that I should not have asked about __main() ---- it appears
in FAQ.
But the question remains that why call to __main() can alias stack location?
I think the memory location pointed by data_X pointers are not visible to
__main().
In comparison, calls to printf() do not have similar effect.




On 5/14/06, Nai Xia <nelson.xia at gmail.com> wrote:
>
> 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
>
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