[llvm-dev] SimplifyCFG, llvm.loop and latch blocks

[Jeroen Dobbelaere via llvm-dev]

Hi Hiroshi,

thank you for the reply.

I am aware of the general rule that Metadata can be dropped.
I am wondering if that should still hold for (at least) some of the loop annotations.

In this particular case, there is also a 'real bug' where loop metadata indicating that a loop
that is 'progressing but should not be unrolled' is, after some transformations, suddenly attached to
a different, infinite loop (aka not progressing).

Greetings,

Jeroen Dobbelaere

> From: Hiroshi Yamauchi
>
> I think that metadata / annotation preservation isn't perfect and is on a best-effort basis in general,
> unfortunately, as the IR spec may say that >it's okay to drop them. But contributions would be welcome to improve it.
>
>> On Wed, Apr 7, 2021 at 10:17 AM Jeroen Dobbelaere via llvm-dev <mailto:llvm-dev at lists.llvm.org> wrote:
>> Hi all,
>>
>> https://llvm.org/docs/LangRef.html#llvm-loop  states that a llvm.loop is put on the 'branch instruction of the loop's latch block'.
>> 
>> With 'SimplifyCFG', I have come in a situation where a !llvm.loop is associated to
>> a basicblock that is found to be a latch block for two different loops.
>> 
>> The original annotation of one of those loops disappeared in the process.
>> (See  https://www.godbolt.org/z/5r1T5e9fs and look for the disappearing !llvm.loop !10  in BB 'do.cond')
>> 
>> Then later on,  'Canonicalize natural loops' splits the latch block up, so that it now is only
>> the latch block for a single loop, but the original annotation is moved to the wrong loop.
>> 
>> Questions:
>> - The branch in 'do.cond' is simplified, but I have the impression that at the same time, the loop annotation is omitted.
>>   This sounds like a bug ?
>> - Is it a correct assumption that we should not merge blocks if both have a branch instruction with a !llvm.loop annotation set ?