[LLVMdev] A question about induction variables

[Chris Lattner]

On Wed, 31 Mar 2004, Vladimir Prus wrote:
> I've just downloaded the latest release of LLVM, and playing with the
> following simple example:
>
> 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:

int %main() {
entry:
	call void %__main( )
	br label %no_exit

no_exit:		; preds = %entry, %no_exit
	%i.0.0 = phi int [ 0, %entry ], [ %inc, %no_exit ]
	%r.0.0 = phi int [ 0, %entry ], [ %tmp.4, %no_exit ]
	%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
}

> 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?  This example certainly should work.

> and nothing else. It is a bit strange -- since 'i' is clearly an induction
> variable and the resulting loop in assembler should have some induction
> variable too. Does this mean that induction variable analysis is not fully
> implemented, or there's some error on my part?

The induction variable analysis in 1.2 is not the most powerful, but it
should certainly catch this case!

> If I don't use the 'r' variable, but just loop with empty body, the entire
> loop seems to be optimized out, but in this example the loop is still there
> in assembler.

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

-Chris

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