[Bug string/30994] New: REP MOVSB performance suffers from page aliasing on Zen 4

["bmerry at sarao dot ac.za" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30994

            Bug ID: 30994
           Summary: REP MOVSB performance suffers from page aliasing on
                    Zen 4
           Product: glibc
           Version: 2.38
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: string
          Assignee: unassigned at sourceware dot org
          Reporter: bmerry at sarao dot ac.za
  Target Milestone: ---

When (dst-src)&0xFFF is small (but non-zero), the REP MOVSB path in memcpy
performs extremely poorly (as much as 25x slower than the alternative path).
I'm observing this on Zen 4 (Epyc 9374F). I'm running Ubuntu 22.04 with a glibc
hand-built from glibc-2.38.9000-185-g2aa0974d25.

To reproduce:
1. Download the microbench at
https://github.com/ska-sa/katgpucbf/blob/6176ed2e1f5eccf7f2acc97e4779141ac794cc01/scratch/memcpy_loop.cpp
2. Compile it with the adjacent Makefile (tl;dr: g++ -std=c++17 -O3 -pthread -o
memcpy_loop memcpy_loop.cpp)
3. Run ./memcpy_loop -t mmap -f memcpy -b 8192 -p 100000 -D 1 -r 5
4. Run GLIBC_TUNABLES=glibc.cpu.x86_rep_movsb_threshold=10000 ./memcpy_loop -t
mmap -f memcpy -b 8192 -p 100000 -D 1 -r 5

Step 3 reports a rate of 4.2 GB/s, while step 4 (which disables the rep_movsb
path) reports a rate of 111 GB/s. The test uses 8192-byte memory copies, where
the source is page-aligned and the destination starts 1 byte into a page.

I'll also attach the bench-memcpy-large.out, which shows similar results.

I've previously filed this as an Ubuntu bug
(https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/2030515) but it doesn't
seem to have received much attention.

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