[llvm-dev] About LoopDeletion and infinite loops ... again! (RFC?)

Marcello Maggioni via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 29 19:17:09 PDT 2017


I read a bunch of discussions about the matter on this very mailing-list that are relatively recent or relatively old and I couldn’t find much agreement on the matter, so … here again :D

LoopDeletion and infinite loops …

Currently LoopDeletion bails if non-detectable trip count loops are encountered and that’s fine, there are languages where infinite loops without side effects cannot be removed.

If I read the C++ spec right though N4527 (sec 1.10)  point 27 says:

27  The implementation may assume that any thread will eventually do one of the following: 
27.1 — terminate,

27.2 — make a call to a library I/O function

(27.3)— read or modify a volatile object, or 

(27.4)— perform a synchronization operation or an atomic operation.

[Note: This is intended to allow compiler transformations such as removal of empty loops, even when 

termination cannot be proven. — end note ]

So, the spec seems to be explicitly calling out that removing infinite loops is ok unless they do any of the things mentioned there (at least that is my interpretation, is that correct?).

If that is the case we could be missing out for languages that have such a behavior (and in particular in C++).

I was wondering how it would be viewed the possibility of adding a flag to loop deletion that allows the removal of loops with loop counts that are not SCEVable.

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