[Bug tree-optimization/79460] gcc fails to optimise out a trivial additive loop for seemingly arbitrary numbers of iterations

[amker at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79460

amker at gcc dot gnu.org changed:

           What    |Removed                     |Added
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                 CC|                            |amker at gcc dot gnu.org

--- Comment #2 from amker at gcc dot gnu.org ---
(In reply to Raphael C from comment #1)
> After some experimentation (also carried out by Hagen von Eitzen), it seems
> that any limit of at least 72 which is also a multiple of 4 causes the same
> optimisation problem. That is the loop is *not* optimised out in these
> cases.  
> 
> Perhaps this is an example of one optimisation (SIMD vectorisation)
> conflicting with another?

IMHO, this is "scev propagation" on floating point values if ffast-math? is
enabled.  We shouldn't rely on vectorizer to propagate final value, it should
be done somewhere before vectorizer.  Thanks.