[LLVMdev] How could I hide the visible string?

[Chris Lattner]

On Mon, 27 Sep 2004, Zhang Qiuyu wrote:

> Is there a way to modify the string such as char a or char b?  Could I
> use the way like "Replace an instruction with another Value" in Programm
> Manual? In fact, what I am interested in is string with visible
> expression, not all string, and I am trying to hide the orignal string
> by using simple way like XOR..

Yes, there is.  At the C level, what transformation do you want to do?
The LLVM code is a pretty straight-forward translation from the C code in
this case.

> Is there a way to reorder the basic blocks?

Yes, for an example pass that does this, take a look at
lib/Transforms/Scalar/BasicBlockPlacement.cpp

-Chris

> C Source code:
>
> #include <stdio.h>
> #include <stdlib.h>
>
> char a[20]="global string test\n";
>
> int main( int argc , char *argv[]){
>
>   char b[20]= "Internal string test";
>
>   printf("Global %s \n",a);
>   printf("Internal %s\n",b);
>
> }
>
>
> IR code :
>
> target endian = little
> target pointersize = 32
>  %struct..TorRec = type { int, void ()* }
>  %struct.TorRec = type { int, void ()* }
> %a = internal global [20 x sbyte] c"global string test\0A\00"  ; <[20 x sbyte]*> [#uses=1]
> %.ctor_1 = internal global [20 x sbyte] c"Internal string test"  ; <[20 x sbyte]*> [#uses=1]
> %.str_1 = internal constant [12 x sbyte] c"Global %s \0A\00"  ; <[12 x sbyte]*> [#uses=1]
> %.str_2 = internal constant [13 x sbyte] c"Internal %s\0A\00"  ; <[13 x sbyte]*> [#uses=1]
> %Initialized.0__ = internal global bool false  ; <bool*> [#uses=2]
>
> implementation   ; Functions:
>
> declare int %printf(sbyte*, ...)
>
> int %main(int %argc, sbyte** %argv) {
> entry:
>  %b = alloca [20 x sbyte]  ; <[20 x sbyte]*> [#uses=1]
>  %tmp.1.i = load bool* %Initialized.0__  ; <bool> [#uses=1]
>  br bool %tmp.1.i, label %__main.entry, label %endif.0.i
>
> endif.0.i:  ; preds = %entry
>  store bool true, bool* %Initialized.0__
>  br label %__main.entry
>
> __main.entry:  ; preds = %entry, %endif.0.i
>  %tmp.0 = getelementptr [20 x sbyte]* %b, uint 0, uint 0  ; <sbyte*> [#uses=2]
>  call void %llvm.memcpy( sbyte* %tmp.0, sbyte* getelementptr ([20 x sbyte]*  %.ctor_1, long 0, long 0), uint 20, uint 1 )
>  %tmp.3 = call int (sbyte*, ...)* %printf( sbyte* getelementptr ([12 x sbyte]*  %.str_1, long 0, long 0), sbyte* getelementptr ([20 x sbyte]*  %a, long 0, long 0) )  ; <int> [#uses=0]
>  %tmp.6 = call int (sbyte*, ...)* %printf( sbyte* getelementptr ([13 x sbyte]*  %.str_2, long 0, long 0), sbyte* %tmp.0 )  ; <int> [#uses=0]
>  ret int 0
> }
>
> declare void %llvm.memcpy(sbyte*, sbyte*, uint, uint)
>
>

-Chris

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