[LLVMdev] sample-code for alias-analysis

[Nick Lewycky]

RAAD B wrote:
> 
> Hi,
> 
> i need a sample-code, for which the llvm alias-analysis finds a 
> *must-aliases*.
> I have tried codes like followings. In all cases, i see just 
> *may-aliases* when i use "opt -aa-eval -print-all-alias-modref-info foo.bc":

Your examples are all .c files. What did your .ll look like? Here's the 
problem:

$ llvm-g++ -O0 ex1.c -S -o - -emit-llvm
define void @_Z3foov() nounwind {
entry:
         %r = alloca i32*                ; <i32**> [#uses=1]
         %i = alloca i32         ; <i32*> [#uses=2]
         %"alloca point" = bitcast i32 0 to i32          ; <i32> [#uses=0]
         store i32 2, i32* %i, align 4
         store i32* %i, i32** %r, align 4
         br label %return

return:         ; preds = %entry
         ret void
}

Obviously %r and %i don't alias: they're two separate heap allocations. 
What you really want to do is run through some optimizations like 
mem2reg and friends. But on that same example, it'll produce this:

$ llvm-g++ -O2 ex1.c -S -o - -emit-llvm
define void @_Z3foov() nounwind readnone {
entry:
         ret void
}

The must-alias result is really hard to produce from a .c file because 
it inherently means that one pointer is *exactly* equal to the other in 
the sense that you could replace one with the other. One of them would 
wind up being deleted.

You can cook up a .ll example easily enough, or I suppose you could 
fiddle with .c examples piped through select optimizations to produce an 
example that works for you.

Nick

> Regards
> Raad
> 
> 
> 1 ==========================================
> void foo() {
>     int i = 2;
>     int& r = i;
> }
> 
> 2 ===========================================
> void foo(){
>     int gi;
>     int *gip1 = &gi;
>     int *gip2 = &gi;
> }
> 
> 3 ==========================================
> void foo (double * fa){
>     int fi, fj;
>     fi = 0;
>     fj = 2;
>     if (! fa)
>         fj = fj + 4;
>     fa[fi] = fa[fi + fj];
> }
> 
> 
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