[Bug tree-optimization/94373] New: 548.exchange2_r run time is 7-12% worse than GCC 9 at -O2 and generic march/mtune

[jamborm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94373

            Bug ID: 94373
           Summary: 548.exchange2_r run time is 7-12% worse than GCC 9 at
                    -O2 and generic march/mtune
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jamborm at gcc dot gnu.org
                CC: rguenth at gcc dot gnu.org
            Blocks: 26163
  Target Milestone: ---
              Host: x86_64-linux
            Target: x86_64-linux

When compiled with just -O2, SPEC 2017 INTrate benchmark
548.exchange2_r runs slower than when compiled with GCC 9.2. It is:

-  8% slower on AMD Zen2-based server CPU (rev. 26b3e568a60)
- 12% slower on Intel Cascade Lake server CPU (rev. abe13e1847f)
-  7% slower on AMD Zen1-based server CPU (rev. 26b3e568a60)

During GCC 10 development cycle the benchmark was relatively noisy and
the run time was increasing in many small steps, but between October 7
and November 15 we were doing 3% better than GCC 9 (on Zen2).
Specifically the following commit brought about the improvement:

  commit 806bdf4e40d31cf55744c876eb9f17654de36b99
  Author: Richard Biener <rguenther@suse.de>
  Date:   Mon Oct 7 07:53:45 2019 +0000

    re PR tree-optimization/91975 (worse code for small array copy using
pointer arithmetic than array indexing)

    2019-10-07  Richard Biener  <rguenther@suse.de>

            PR tree-optimization/91975
            * tree-ssa-loop-ivcanon.c (constant_after_peeling): Consistently
            handle invariants.

    From-SVN: r276645

But it was undone by its revert:

  commit f0af4848ac40d2342743c9b16416310d61db85b5
  Author: Richard Biener <rguenther@suse.de>
  Date:   Fri Nov 15 09:09:16 2019 +0000

    re PR tree-optimization/92039 (Spurious -Warray-bounds warnings building
32-bit glibc)

    2019-11-15  Richard Biener  <rguenther@suse.de>

            PR tree-optimization/92039
            PR tree-optimization/91975
            * tree-ssa-loop-ivcanon.c (constant_after_peeling): Revert
            previous change, treat invariants consistently as non-constant.
            (tree_estimate_loop_size): Ternary ops with just the first op
            constant are not optimized away.

            * gcc.dg/tree-ssa/cunroll-2.c: Revert to state previous to
            unroller adjustment.
            * g++.dg/tree-ssa/ivopts-3.C: Likewise.

    From-SVN: r278281

On the Intel machine, reverting the revert fixes the regression too.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26163
[Bug 26163] [meta-bug] missed optimization in SPEC (2k17, 2k and 2k6 and 95)