[llvm-dev] Check if an SCC is a Loop

[Stefanos Baziotis via llvm-dev]

Hi everyone,

Recently we needed to know whether an SCC is a loop in a patch:
https://reviews.llvm.org/D74691#1878465

First of all, do you think this is a useful addition to the SCC interface?

If yes, yhe solution that we ended up doing is:
- Get LoopInfo for the function.
- Given an SCC, take one random block of it. Then, call `getLoopFor()` with
that block.
If we don't get a loop back, then this SCC is definitely not a loop.
- Otherwise, we compare the size of the SCC with that of the loop and there
are 3 cases:
  - They're equal: this SCC is a loop.
  - The SCC is smaller: This SCC is not a loop because `getLoopFor()` gives
the innermost loop. So, if this is smaller, it is an SCC inside a loop.
  - The SCC is bigger: In that case, we can't decide since let's say the
SCC has blocks A, B, C. And the loop has blocks A, B. But blocks A, B, C
might also make a loop. However, since `getLoopFor()` gives us the
innermost loop, it will give us A, B. So, in that case, use
`getParentLoop()` repeatedly and repeat the procedure for the parent loops.

Do you think there's a better way ? I was thinking that maybe there was a
more straightforward way by trying to verify whether the criteria for a
loop are satisfied (i.e. there's a block that dominates all the other
blocks).

Best,
Stefanos Baziotis
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