[PATCH v2 2/3] Add predict_doloop_p target hook

[Jeff Law]

On 5/13/19 9:09 PM, linkw@linux.ibm.com wrote:
> From: Kewen Lin <linkw@linux.ibm.com>
> 
> Previous version link for background:
> https://gcc.gnu.org/ml/gcc-patches/2019-04/msg00912.html
> 
> This hook is to predict whether one loop in gimple will
> be transformed to low-overhead loop later in RTL, and
> designed to be called in ivopts.
> 
> "Since the low-overhead loop optimize transformation is
> based on RTL, some of those checks are hard to be imitated
> on gimple, so it's not possible to predict the current
> loop will be transformed exactly in middle-end.  But if we
> can have most loop predicted precisely, it would be helpful.
> It highly depends on target hook fine tuning. It's
> acceptable to have some loops which can be transformed to
> low-overhead loop but we don't catch.  But we should try
> our best to avoid to predict some loop as low-overhead loop
> but which isn't."
> 
> Bootstrapped and regression testing passed on powerpc64le.
> 
> Is it ok for trunk?
> 
> gcc/ChangeLog
> 
> 2019-05-13  Kewen Lin  <linkw@gcc.gnu.org>
> 
> 	PR middle-end/80791
> 	* target.def (predict_doloop_p): New hook.
> 	* targhooks.h (default_predict_doloop_p): New declaration.
> 	* targhooks.c (default_predict_doloop_p): New function.
> 	* doc/tm.texi.in (TARGET_PREDICT_DOLOOP_P): New hook.
> 	* doc/tm.texi: Regenerate.
> 	* config/rs6000/rs6000.c (invalid_insn_for_doloop_p): New function.
> 	(costly_iter_for_doloop_p): Likewise.
> 	(rs6000_predict_doloop_p): Likewise.
> 	(TARGET_PREDICT_DOLOOP_P): New macro.
Trying to guess what the target is going to do, then changing the
structure of the loops in gimple based on that guess -- is that really a
good idea.  It's fairly counter to many of the design goals around gimple.

jeff