[Rd] Matrix issues when building R with znver3 architecture under GCC 11

[Kieran Short]

Hello,

I'm new to this list, and have subscribed particularly because I've come
across an issue with building R from source with an AMD-based Zen
architecture under GCC11. Please don't attack me for my linux operating
system choice, but it is Ubuntu 20.04 with Linux Kernel 5.10.102.1 -
microsoft-standard-WSL2. I've built GCC11 using GCC8 (the standard GCC
under Ubuntu20.04 WSL release), under Windows11 with wslg. WSL2/g runs as a
hypervisor with ports to all system resources including display, GPU (cuda,
etc).

The reason why I am posting this email is that I am trying to compile R
using the AMD Zen3 platform architecture rather than x86/64, because it has
processor-specific optimizations that improve performance over the standard
x86/64 in benchmarks. The Zen3 architecture optimizations are not available
in earlier versions of GCC (actually, they have possibly been backported to
GCC10 now). Since Ubuntu 20.04 doesn't have GCC11, I compiled the GCC11
compiler using the native GCC8.

The GCC11 I have built can build R 4.1.3 with a standard x86-64
architecture and pass all tests with "make check-all".
I configured that with:
> ~/R/R-4.1.3/configure CC=gcc-11.2 CXX=g++-11.2 FC=gfortran-11.2
CXXFLAGS="-O3 -march=x86-64" CFLAGS="-O3 -march=x86-64" FFLAGS="-O3
-march=x86-64" --enable-memory-profiling --enable-R-shlib
and built with
> make -j 32 -O
> make check-all
## PASS.

So I can build R in my environment with GCC11.
In configure, I am using references to "gcc-11.2" "gfortran-11.2" and
"g++-11.2" because I compiled GCC11 compilers with these suffixes.

Now, I'm using a 32 thread (16 core) AMD Zen3 CPU (a 5950x), and want to
use it to its full potential. Zen3 optimizations are available as a
-march=znver3 option n GCC11. The znver3 optimizations improve performance
in Phoronix Test Suite benchmarks (I'm not aware of anyone that has
compiled R with them). See:
https://www.phoronix.com/scan.php?page=article&item=amd-5950x-gcc11

However, the R 4.1.3 build (made with "make -j 32 -O"), configured with
-march=znver3, produces an R that fails "make check-all".

> ~/R/R-4.1.3/configure CC=gcc-11.2 CXX=g++-11.2 FC=gfortran-11.2
CXXFLAGS="-O2 -march=znver3" CFLAGS="-O2 -march=znver3" FFLAGS="-O2
-march=znver3" --enable-memory-profiling --enable-R-shlib
or
> ~/R/R-4.1.3/configure CC=gcc-11.2 CXX=g++-11.2 FC=gfortran-11.2
CXXFLAGS="-O3 -march=znver3" CFLAGS="-O3 -march=znver3" FFLAGS="-O3
-march=znver3" --enable-memory-profiling --enable-R-shlib

The fail is always in the factorizing.R Matrix.R tests, and in particular,
there are a number of errors and a fatal error.
I have attached the output because I cannot really understand what is going
wrong. But results returned from matrix calculations are obviously odd with
-march=znver3 in GCC 11. There is another backwards-compatible architecture
option "znver2" and this has EXACTLY the same result.

While there are other warrnings and errors (many in assert.EQ() ), the
factorizing.R script continues. The fatal error (at line 2662 in the
attached factorizing.Rout.fail text file) is:

> ## problematic rank deficient rankMatrix() case -- only seen in large
cases ??
> Z. <- readRDS(system.file("external", "Z_NA_rnk.rds", package="Matrix"))
> tools::assertWarning(rnkZ. <- rankMatrix(Z., method = "qr")) # gave errors
Error in assertCondition(expr, classes, .exprString = d.expr) :
  Failed to get warning in evaluating rnkZ. <- rankMatrix(Z., method  ...
Calls: <Anonymous> -> assertCondition
Execution halted

Can anybody shed light on what might be going on here? 'make check-all'
passes all the other checks. It is just factorizing.R in Matrix that fails
(other matrix tests run ok).
Sorry this is a bit long-winded, but I thought details might be important.

best regards,
Kieran