[LLVMdev] Question about IndVarSimplify

[Nick Lewycky]

Sorin Baltateanu 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 <http://test.il> -S* gives me the
> same output, which is what i would expect.
> The problem is that running *opt -indvars < test.il <http://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?

Each optimization pass in LLVM does one job only. Indvars does induction 
variable canonicalization, not SSA construction, so it sees those loads 
and stores and assumes they can't be optimized because they already 
would have been by other optimizers. SSA construction is done by 
-mem2reg. You can run opt -mem2reg -indvars, or just opt -mem2reg then 
opt -indvars individually to see what effect each has.

Nick