[PATCH] D86844: [LoopDeletion] Allows deletion of possibly infinite side-effect free loops

[Dávid Bolvanský via Phabricator via cfe-commits]

xbolva00 added a comment.

In D86844#2484639 <https://reviews.llvm.org/D86844#2484639>, @atmnpatel wrote:

> In D86844#2484568 <https://reviews.llvm.org/D86844#2484568>, @fhahn wrote:
>
>> In D86844#2481922 <https://reviews.llvm.org/D86844#2481922>, @xbolva00 wrote:
>>
>>>   int a, b;
>>>   
>>>   int f(void) {
>>>       while (1) {
>>>           if (a != b) return 1;
>>>       }
>>>       return 0;
>>>   }
>>>   
>>>   int g(int a, int b) {
>>>       while (1) {
>>>           if (a != b) return 1;
>>>       }
>>>       return 0;
>>>   }
>>>
>>> LLVM does not catch these cases; gcc does.
>>>
>>> https://godbolt.org/z/jW7son
>>
>> Looks like `must progress` does not get added? If it gets added to the IR the loops get removed: https://godbolt.org/z/77v17P
>
> I might be misunderstanding the standard here but since 1 is a non-zero constant expression, it can't be assumed to terminate by the implementation right? The relevant section from C11 at least is "An iteration statement whose controlling expression is not a constant expression that performs [explanation of what it deems as progress] may be assumed by the implementation to terminate" (C11 6.8.5 p6). I think these cases fall outside of the scope of this particular change ...

For C yes, but are there such rules for C++? GCC in c++ mode does not check for non-zero constant expr and happily performs this optimization.


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