[Bug middle-end/116065] [13/14/15 Regression] Function attribute optimize() might make ISA target attribute broken

[haochen.jiang at intel dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116065

--- Comment #4 from Haochen Jiang <haochen.jiang at intel dot com> ---
It seems to me gcc-13-3950-g071e428c24e is the guilty commit.

071e428c24ee8c1ed062597a093708bba29509c9 is the first bad commit
commit 071e428c24ee8c1ed062597a093708bba29509c9
Author: Hongyu Wang <hongyu.wang@intel.com>
Date:   Thu Sep 8 16:52:02 2022 +0800

    Enable small loop unrolling for O2

    Modern processors has multiple way instruction decoders
    For x86, icelake/zen3 has 5 uops, so for small loop with <= 4
    instructions (usually has 3 uops with a cmp/jmp pair that can be
    macro-fused), the decoder would have 2 uops bubble for each iteration
    and the pipeline could not be fully utilized.

    Therefore, this patch enables loop unrolling for small size loop at O2
    to fullfill the decoder as much as possible. It turns on rtl loop
    unrolling when targetm.loop_unroll_adjust exists and O2 plus speed only.
    In x86 backend the default behavior is to unroll small loops with less
    than 4 insns by 1 time.

    This improves 548.exchange2 by 9% on icelake and 7.4% on zen3 with
    0.9% codesize increment. For other benchmarks the variants are minor
    and overall codesize increased by 0.2%.

    The kernel image size increased by 0.06%, and no impact on eembc.

    gcc/ChangeLog:

            * common/config/i386/i386-common.cc (ix86_optimization_table):
            Enable small loop unroll at O2 by default.
            * config/i386/i386.cc (ix86_loop_unroll_adjust): Adjust unroll
            factor if -munroll-only-small-loops enabled and -funroll-loops/
            -funroll-all-loops are disabled.
            * config/i386/i386.h (struct processor_costs): Add 2 field
            small_unroll_ninsns and small_unroll_factor.
            * config/i386/i386.opt: Add -munroll-only-small-loops.
            *
doc/gcc/gcc-command-options/machine-dependent-options/x86-options.rst:
            Document -munroll-only-small-loops.
            * doc/gcc/gcc-command-options/option-summary.rst: Likewise.
            * loop-init.cc (pass_rtl_unroll_loops::gate): Enable rtl
            loop unrolling for -O2-speed and above if target hook
            loop_unroll_adjust exists.
            (pass_rtl_unroll_loops::execute): Set UAP_UNROLL flag
            when target hook loop_unroll_adjust exists.
            * config/i386/x86-tune-costs.h: Update all processor costs
            with small_unroll_ninsns = 4 and small_unroll_factor = 2.

    gcc/testsuite/ChangeLog:

            * gcc.dg/guality/loop-1.c: Add additional option
            -mno-unroll-only-small-loops.
            * gcc.target/i386/pr86270.c: Add -mno-unroll-only-small-loops.
            * gcc.target/i386/pr93002.c: Likewise.