[LLVMdev] Question about IndVarSimplify

[Eli Friedman]

On Sun, Jun 19, 2011 at 7:21 AM, Sorin Baltateanu
<baltateanu.sorin at gmail.com> wrote:
> Hi guys,
>
> I am trying to use the indvars pass, but i don't see any changes in the IL
> representation.
>
> For this simple function
>
> void init_array(int k)
> {
>     int A[20];
>     for (int i=2; i<10; i++)
>         A[i] = 0;
> }
>
> i run clang test-simple.c -S -emit-llvm -o test.il and i obtain the
> following il representation
>
> define void @init_array(i32 %k) nounwind {
> entry:
>   %k.addr = alloca i32, align 4
>   %A = alloca [20 x i32], align 4
>   %i = alloca i32, align 4
>   store i32 %k, i32* %k.addr, align 4
>   store i32 2, i32* %i, align 4
>   br label %for.cond
>
> for.cond:                                         ; preds = %for.inc, %entry
>   %tmp = load i32* %i, align 4
>   %cmp = icmp slt i32 %tmp, 10
>   br i1 %cmp, label %for.body, label %for.end
>
> for.body:                                         ; preds = %for.cond
>   %tmp1 = load i32* %i, align 4
>   %arrayidx = getelementptr inbounds [20 x i32]* %A, i32 0, i32 %tmp1
>   store i32 0, i32* %arrayidx, align 4
>   br label %for.inc
>
> for.inc:                                          ; preds = %for.body
>   %tmp2 = load i32* %i, align 4
>   %inc = add nsw i32 %tmp2, 1
>   store i32 %inc, i32* %i, align 4
>   br label %for.cond
>
> for.end:                                          ; preds = %for.cond
>   ret void
> }
>
> Running opt -loop-simplify < test.il -S gives me the same output, which is
> what i would expect.
> The problem is that running opt -indvars < test.il -S gives, again, the same
> output, but i would expect that i starts from 0 and is, eventualy,  defined
> using a PHYNode.
>
> What it's wrong with what i'm doing?

You aren't running a normal pass pipeline; in particular, not running
mem2reg/scalarrepl first prevents a lot of optimizations from working
as expected.

-Eli