[Bug middle-end/33699] [9/10/11/12 regression] missing optimization on const addr area store

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33699

Michael Meissner <meissner at gcc dot gnu.org> changed:

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

--- Comment #32 from Michael Meissner <meissner at gcc dot gnu.org> ---
I looked at adding the following powerpc patch that was proposed in March,
2021:
https://gcc.gnu.org/pipermail/gcc-patches/2021-March/566744.html

There are two parts to the patch, that are sort of unrelated.

The first part is to add minimum and maximum section anchor offset values and
use -fsection anchors.  I ran a spec 2017 benchmark on a pre-production power10
system, comparing my normal run times to run times with -fsection-anchors and
setting the minimum/maximum section anchor offsets.

Two benchmarks improved and two benchmarks regressed:

    xalancbmk_r: 1.75% regression
    cactuBSSN_r: 4.24% improvement
    blender_r: 1.92% regression
    roms_r: 1.05% improvement

I then built spec 2017 with just the part of setting const_anchor, but not the
section anchor minimum/maximum offsets.  Eight benchmarks did not build due to
assertion failures in cse.c:

    gcc_r
    exchange2_r
    cactuBSSN_r
    wrf_r
    blender_r
    cam4_r
    fotonik3d_r
    roms_r

If I specify the section anchor minimum/maximum offsets, add -fsection-anchors,
and set the const_anchor, all 23 INT+FP benchmarks build, but WRF_R does not
run correctly.  So without more debugging, I don't recommend setting
const_anchor.  It is probably useful to set the minimum/maximum section anchor
offsets in case people use -fsection-anchors.

As an aside, if we wanted to accept using constant addresses in the PowerPC, we
would need to recognize a constant address as being legitimate.  This may be
useful in some embedded environments where you have devices at certain memory
locations.  But somebody would need to add the support.