[Bug tree-optimization/98598] Missed opportunity to optimize dependent loads in loops

[jiangning.liu at amperecomputing dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98598

--- Comment #5 from Jiangning Liu <jiangning.liu at amperecomputing dot com> ---
> It has to be done with care of course, cost modeling is difficult
> (we need to have a good estimate of n and m or need to version
> the whole nest).  That said, usually we attempt the reverse transform.

Before tuning the cost model good enough, we may implement this optimization by
adding a new optimization command line option. This won't hurt gcc, right?

> 
> My personal opinion is that hinting the user to possibly refactor
> his code (guided by profiling to be not too noisy) is much
> prefered to the idea that the compiler can ever apply such transform
> to the loops where it matters and not to the loops where it is
> harmful.

Sometimes, it is not always easy for the user to modify the code, and even the
user may be lazy and reluctant to change the code. This kind of Memory
Gathering Optimization can make end-user's life easier.