[PATCH] Remove empty loop with assumed finiteness (PR tree-optimization/89713)

[Jeff Law]

On 5/16/19 10:17 PM, Feng Xue OS wrote:
> This patch is meant to give user a way to optimize away those empty loops which are impossible to be recognized by compiler, such as C++ STL container-based loop,
> 
>     void f (std::map<int, int> &m)
> Â    {
> Â  Â      for (auto it = m.begin (); it != m.end (); ++it);
> Â    }
>  
> An option "-ffinite-loop" is added to tell compiler about finiteness of loops so that compiler can apply the optimization.
> 
> Feng
> 
> diff --git a/gcc/ChangeLog b/gcc/ChangeLog
> index d8bed3a..c55f2e6 100644
> --- a/gcc/ChangeLog
> +++ b/gcc/ChangeLog
> @@ -1,3 +1,18 @@
> +2019-05-16  Feng Xue  <fxue@os.amperecomputing.com>
> +
> +        PR tree-optimization/89713
> +        * doc/invoke.texi (-ffinite-loop): Document new option.
> +        * common.opt (-ffinite-loop): New option.
> +        * passes.def: Replace pass_cd_dce with pass_cd_dce2.
> +        * tree-pass.h (pass_cd_dce2): New declaration.
> +        * tree-ssa-dce.c (loop_has_true_exits): New function.
> +        (find_obviously_necessary_stmts): Add aggressive_loop_removal
> +        parameter.
> +        (perform_tree_ssa_dce, tree_ssa_cd_dce): Likewise.
> +        (class pass_cd_dce): Add new member aggressive_loop_removal.
> +        (pass_cd_dce::pass_cd_dce: Add alr parameter.
> +        (make_pass_cd_dce2): New function.
I'm not a big fan of this patch.  I'd rather be looking at either
improving our analysis or adding attributes to the loops to help the
analysis bits prove termination.

Jeff