[llvm-dev] Can't get an induction variable by getInductionVariable API

[Philip Reames via llvm-dev]

If this particular API is too restrictive for your use case, you can 
also consider the following two approaches:

 1. Walk the phi nodes in the header, and ask SCEV for the node
    representing each.  For any which is an AddRec within the current
    loop, you've found an induction variable.
 2. If you're using IVs to analyze loop behavior, consider using the
    exit count APIs instead.  They handle all the complexities of multi
    exit loops which can be tedious to get right by hand.

Philip

p.s. The point Florian made about rotated loops is also spot on.

On 10/23/2020 1:23 AM, ZIN MOON via llvm-dev wrote:
> Dear All
>
> I have a question about the getInductionVariable API.
> I was trying to get an induction variable (indvar) in the code below 
> using the getInductionVariable API (LoopInfo class).
>
> *C file*
> void test(int val) {
>   int sum = 0, i = 0;
>   for (i = 0; i < val; i++) {
>       sum += i;
>   }
> }
>
> *IR (with mem2reg pass)*
> ; Function Attrs: nounwind
> define dso_local void @test(i32 %0) #0 {
>   br label %2
> 2:                                                ; preds = %6, %1
>   %.01 = phi i32 [ 0, %1 ], [ %5, %6 ]
>   %.0 = phi i32 [ 0, %1 ], [ %7, %6 ]
>   %3 = icmp slt i32 %.0, %0
>   br i1 %3, label %4, label %8 <- *Why don't check this BranchInst ?*
> 4:                                                ; preds = %2
>   %5 = add nsw i32 %.01, %.0
>   br label %6
> 6:                                                ; preds = %4
>   %7 = add nsw i32 %.0, 1
> *br label %2  <- checked it whether isCondition by getLatchCmpInst 
> API(called by getInductionVariable API)*
> 8:                                                ; preds = %2
>   ret void
> }
>
> Then I got null (no indvar) because BranchInst's number of operands 
> was less than 2.(red block).
>
> A CmpInst(compare with induction variable) is contained in the header 
> of the loop.
> But the getInductionVariable API checks only in a latch block(contains 
> a branch back to the header of the loop)
> is there a missing case that contains CmpInst in the header?
>
> And if I use the getCanonicalInductionVariable API, I can get the 
> variable 'i'(%. 01) as an indvar.
>
> I checked the code implementing getInductionVariable and 
> getCanonicalInductionVariable.
> It seems that indvar and canonical indvar differ only in the starting 
> number (0 or none).
> Why not use the same way to detect the indvar between 
> getInductionVariable and getCanonicalInductionVariable.
>
> BR
> ZIN
>
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