Hi!

Maxim Cournoyer <maxim.cournoyer@gmail.com> skribis:

> Our OpenBLAS package uses DYNAMIC_ARCH=1 to provide optimizations for
> all supported targets, at least of x86 and x86_64.  In theory that seems
> OK, but in practice the builds differ depending on the host CPU.

What follows is the log of an investigation that didn’t find the root
cause, but perhaps it’ll give us ideas…

Right now the build results of ci.guix and bordeaux.guix differ:

--8<---------------cut here---------------start------------->8---
$ guix describe
Generacio 202	Jan 30 2022 23:57:03	(nuna)
  guix 43dd34c
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 43dd34c7777a212c99a97da7a2c237158faa9a1b
ludo@ribbon ~/src/guix$ guix challenge openblas
/gnu/store/ras6dprsw3wm3swk23jjp8ww5dwxj333-openblas-0.3.18 contents differ:
  no local build for '/gnu/store/ras6dprsw3wm3swk23jjp8ww5dwxj333-openblas-0.3.18'
  https://ci.guix.gnu.org/nar/lzip/ras6dprsw3wm3swk23jjp8ww5dwxj333-openblas-0.3.18: 0m1jlc26yrwxn8gxwpj8452kw4g84ywclh0hnab93873ifz87s5c
  https://bordeaux.guix.gnu.org/nar/lzip/ras6dprsw3wm3swk23jjp8ww5dwxj333-openblas-0.3.18: 1d0m9v3kpsqzplpl1law2lfhm6rrbhkkqsvh19dlg9wx45vbbvjb
  differing file:
    /lib/libopenblasp-r0.3.18.so

1 store items were analyzed:
  - 0 (0.0%) were identical
  - 1 (100.0%) differed
  - 0 (0.0%) were inconclusive
--8<---------------cut here---------------end--------------->8---

To get an idea, I thought we could compare the two build logs:

  https://ci.guix.gnu.org/log/ras6dprsw3wm3swk23jjp8ww5dwxj333-openblas-0.3.18
  https://bordeaux.guix.gnu.org/build/3fab433c-e7d3-498d-86f8-4bcd5da9c4db

(Protip: I found the second one via
<http://data.guix.gnu.org/gnu/store/ras6dprsw3wm3swk23jjp8ww5dwxj333-openblas-0.3.18>.)

The “ar  -ru ../libopenblasp-r0.3.18.a …” are apparently the same in
both cases, which rules out the simple case of unsorted .o files.

The .so on ci.guix is slightly bigger:

--8<---------------cut here---------------start------------->8---
$ wget -qO - https://ci.guix.gnu.org/nar/lzip/ras6dprsw3wm3swk23jjp8ww5dwxj333-openblas-0.3.18| lzip -d | guix archive -x /tmp/o1
$ wget -qO - https://bordeaux.guix.gnu.org/nar/lzip/ras6dprsw3wm3swk23jjp8ww5dwxj333-openblas-0.3.18| lzip -d | guix archive -x /tmp/o2
$ ls -l /tmp/{o1,o2}/lib/libopenblasp-r0.3.18.so
-r-xr-xr-x 1 ludo users 40538768 Jan  1  1970 /tmp/o1/lib/libopenblasp-r0.3.18.so
-r-xr-xr-x 1 ludo users 40436368 Jan  1  1970 /tmp/o2/lib/libopenblasp-r0.3.18.so
--8<---------------cut here---------------end--------------->8---

Both have the same symbols though, and in the same order:

--8<---------------cut here---------------start------------->8---
$ diff -u <(objdump -T  /tmp/o1/lib/libopenblasp-r0.3.18.so |cut -c 60-  ) <(objdump -T /tmp/o2/lib/libopenblasp-r0.3.18.so |cut -c60- )
$ echo $?
0
--8<---------------cut here---------------end--------------->8---

… which suggests they include code optimized for the same
micro-architectures because symbols include the name of the
micro-architecture:

--8<---------------cut here---------------start------------->8---
$ objdump -T  /tmp/o1/lib/libopenblasp-r0.3.18.so |cut -c 60-|tail -10
  csymm3m_RU
  cgemv_c_BARCELONA
  csymv_U_HASWELL
  dtrmm_iltncopy_CORE2
  LAPACKE_dsytrs2
  openblas_num_threads_env
  csycon_rook_
  csytri_rook_


--8<---------------cut here---------------end--------------->8---

Some of the offsets differ though: