[Bug tree-optimization/84512] [8 Regression] Missed optimization: should be precalculated in compile-time

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84512

Richard Biener  changed:

   What|Removed |Added

 Status|ASSIGNED|RESOLVED
 Resolution|--- |FIXED

--- Comment #3 from Richard Biener  ---
Fixed.

[Bug tree-optimization/84512] [8 Regression] Missed optimization: should be precalculated in compile-time

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84512

--- Comment #2 from Richard Biener  ---
Author: rguenth
Date: Tue Feb 27 15:25:33 2018
New Revision: 258036

URL: https://gcc.gnu.org/viewcvs?rev=258036=gcc=rev
Log:
2018-02-27  Richard Biener  

PR tree-optimization/84512
* tree-vect-loop.c (vect_compute_single_scalar_iteration_cost):
Do not use the estimate returned from record_stmt_cost for
the scalar iteration cost but sum properly using add_stmt_cost.

* gcc.dg/tree-ssa/pr84512.c: New testcase.

Added:
trunk/gcc/testsuite/gcc.dg/tree-ssa/pr84512.c
Modified:
trunk/gcc/ChangeLog
trunk/gcc/testsuite/ChangeLog
trunk/gcc/tree-vect-loop.c

[Bug tree-optimization/84512] [8 Regression] Missed optimization: should be precalculated in compile-time

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84512

Richard Biener  changed:

   What|Removed |Added

 Status|UNCONFIRMED |ASSIGNED
Version|tree-ssa|8.0
   Keywords||missed-optimization
   Last reconfirmed||2018-02-27
 Blocks||53947
   Assignee|unassigned at gcc dot gnu.org  |rguenth at gcc dot 
gnu.org
 Ever confirmed|0   |1
Summary|Missed optimization: should |[8 Regression] Missed
   |be precalculated in |optimization: should be
   |compile-time|precalculated in
   ||compile-time
   Target Milestone|--- |8.0

--- Comment #1 from Richard Biener  ---
Confirmed.  This is another case where we vectorize one loop but not the other
and DOM doesn't handle removing vector loads against scalar stores.  Later
store-merging aggregates the stores but nothig performs CSE after it.

The vectorizer decides that vectorizing the reduction is profitable while
vectorizing the init is not:

t.c:4:3: note: Cost model analysis:
  Vector inside of loop cost: 68
  Vector prologue cost: 8
  Vector epilogue cost: 128
  Scalar iteration cost: 16
  Scalar outside cost: 0
  Vector outside cost: 136
  prologue iterations: 0
  epilogue iterations: 2
t.c:4:3: note: cost model: the vector iteration cost = 68 divided by the scalar
iteration cost = 16 is greater or equal to the vectorization factor = 4.
t.c:4:3: note: not vectorized: vectorization not profitable.

With -fno-vect-cost-model we vectorize both loops and optimize the function
like clang does.

The issue with the cost model here is that for the scalar iteration cost
we end up using builtin_vectorization_cost () while for the vector cost
we use add_stmt_cost.  Only the latter makes a difference between the
different kind of operations.

I have a patch.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations