[LLVMdev] A question about induction variables

[Vladimir Prus]

Chris Lattner wrote:

> > int main()
> > {
> >     int r(0);
> >     for (int i = 0; i < 100; ++i)
> >         r += i;
> >         ;
> >     return r;
> > }
>
> When I compiled it, I got the following LLVM code:

The code I get is somewhat different:

int %main() {
entry:
	%tmp.1.i = load bool* %Initialized.0__		; <bool> [#uses=1]
	br bool %tmp.1.i, label %no_exit, label %endif.0.i

endif.0.i:		; preds = %entry
	store bool true, bool* %Initialized.0__
	br label %no_exit

no_exit:		; preds = %entry, %endif.0.i, %no_exit
	%i.0.0 = phi int [ %inc, %no_exit ], [ 0, %endif.0.i ], [ 0, %entry ]		; 
<int> [#uses=2]
	%r.0.0 = phi int [ %tmp.4, %no_exit ], [ 0, %endif.0.i ], [ 0, %entry ]		; 
<int> [#uses=1]
	%tmp.4 = add int %r.0.0, %i.0.0		; <int> [#uses=2]
	%inc = add int %i.0.0, 1		; <int> [#uses=2]
	%tmp.1 = setgt int %inc, 99		; <bool> [#uses=1]
	br bool %tmp.1, label %loopexit, label %no_exit

loopexit:		; preds = %no_exit
	ret int %tmp.4
}

but still there's a loop.

> > I compile it and then run:
> >    analyze -indvars x.bc
> > which prints:
> >    Printing analysis 'Induction Variable Analysis' for function 'main':
>
> When I did this, I got the following:
>
> $ analyze -indvars t.ll
> Printing analysis 'Induction Variable Analysis' for function 'main':
> Canonical Induction Variable: int %i.0.0:
>         %i.0.0 = phi int [ 0, %entry ], [ %inc, %no_exit ]              ;
> <int> [#uses=2] Start = int 0  Step = int 1
>
> ... so it seems to work.  Are you sure that your x.bc file contains the
> correct LLVM code?  

Yes, I've just tried again, with the same results.

> Hrm, I'm really not sure.  It works for me :)

Well, I guess I'll just try the CVS version... the version built from the 
trunk has the same behaviour.

I'm using the prebuilt frontend which is part of 1.2 release, though. Maybe 
this can explain the difference in output.

- Volodya



>
> -Chris