[Bug tree-optimization/103195] [12 Regression] tfft2 text grows by 70% with -Ofast since r12-5113-gd70ef65692fced7a

["jamborm at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103195

Martin Jambor <jamborm at gcc dot gnu.org> changed:

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

--- Comment #7 from Martin Jambor <jamborm at gcc dot gnu.org> ---
(In reply to hubicka from comment #6)
> Yep, unit growths do not apply for very small units.  ipa-cp heuristics
> still IMO needs work and be based on relative speedups rather then
> absolute for the cutoffs.

I'll be happy to discuss how to change IPA-CP cost/benefit
estimations (ideally in person next to a whiteboard) but I am somewhat
wary of making the heuristics even more similar to the inliner as then
all clones will be inlined.

Moreover, the problem here is that the constant leads to a loop with
known number of iterations, even if it is executed with expected
relative frequency of just 0.47, and the extra points it gets for this
help it get over the limit.  (Would you suggest to scale the bonus by
the nonspec time somehow?)

The bonus for known loop iteration count is arguably too large.  In
current master, exchange2_r would be fine with less than half, even in
the PGO case.  We could also scale the bonus not linearly by estimated
frequency of the loop but by... I don't know... its square :-)

Looking at the code I also discovered that I (probably) forgot to
delete the old addition of a constant bonus and so we scale by
1+frequency rather than the frequency.

The simplest way of dealing with this issue would be to have a cut-off
frequency for the bonus, like the following which declares all loops
that are executed with estimated frequency less than 1/2 as
uninteresting:

diff --git a/gcc/ipa-cp.cc b/gcc/ipa-cp.cc
index 453e9c93cc3..c9f419c5b34 100644
--- a/gcc/ipa-cp.cc
+++ b/gcc/ipa-cp.cc
@@ -3257,15 +3257,14 @@ hint_time_bonus (cgraph_node *node, const
ipa_call_estimates &estimates)
 { 
   int result = 0;
   ipa_hints hints = estimates.hints;
-  if (hints & (INLINE_HINT_loop_iterations | INLINE_HINT_loop_stride))
-    result += opt_for_fn (node->decl, param_ipa_cp_loop_hint_bonus);
-  
   sreal bonus_for_one = opt_for_fn (node->decl, param_ipa_cp_loop_hint_bonus);

-  if (hints & INLINE_HINT_loop_iterations)
+  if (hints & INLINE_HINT_loop_iterations
+      && estimates.loops_with_known_iterations >= sreal (1, -1))
     result += (estimates.loops_with_known_iterations * bonus_for_one).to_int
();

-  if (hints & INLINE_HINT_loop_stride)
+  if (hints & INLINE_HINT_loop_stride
+      && estimates.loops_with_known_strides >= sreal (1, -1))
     result += (estimates.loops_with_known_strides * bonus_for_one).to_int ();

   return result;