USE MINGW-W64

1. Install mingw-w64 (Follow the "Sourceforge" link) and install it to the default Program Files based path.

2. Select the latest "version" (for GCC), change "architecture" from i686 (32-bit) to x86_64 (64-bit), and change "threads" to win32.

3. Add the bin folder to the beginning of your Git BASH (MINGW64) path:

    $ export PATH="c:/Program Files/mingw-w64/x86_64-8.1.0-win32-seh-rt_v6-rev0/mingw64/bin:$PATH"
   

   You'll, of course, need to adjust the above path to match the version of your of your particular install:

    $ export PATH="c:/Program Files/mingw-w64/<YOUR_ARCH_VERSION_REVISION_FOLDER>/mingw64/bin:$PATH"
   

Alternatively

You may be able to just install it directly into the same folder as Git: C:\Program Files\Git.

But I haven't tried this alternative approach and I wouldn't recommend it because it is likely to cause issues when updating and may confuse uninstallers, leaving remnants. (if the two installations don't interfere to begin with)

[ Edit: GNU Make is installed as mingw32-make.exe. You can make a copy of it, somewhere in your $PATH which you have permission to, and rename it to make. Git Bash seems to respect that it's executable, even without an extension, which is handy if you don't want it picked up outside of the MinGW environment (even though it may be in your PATH). ]

You can browse using gitweb at https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git

From there, goto tree views and navigate to the parts of the AVR backend:

• gcc/config/avr/: The GCC AVR backend
• libgcc/config/avr/: The AVR specific part of the libgcc runtime
• gcc/testsuite/gcc.target/avr/: AVR-specific parts of the test suite
• gcc/common/config/avr/: Part of the backend that's common to the compiler proper (cc1, cc1plus) and the compiler driver (avr-gcc, avr-g++).

Most convenient is to browse on file, so you would

$ git clone git://gcc.gnu.org/git/gcc.git SomeLocalDir

When you are interested in a specific branch or tag, like branches/releases/gcc-14 or tags/releases/gcc-14.1.0, you can navigate to that ref and browse from there.

What is the relation to LLVM?

I don't understand that question. LLVM is a different compiler (infrastructure) with it's own runtime. It has nothing to do with GCC (claims at least).

What do "they" use for regression testing architecture specific routines;

See Installing GCC: Testing → How to test GCC on a simulator → avr → AVRtest → README: Running the avr-gcc Testsuite using the AVRtest Simulator

is there something I could easily use myself?

Yes it's easy enough to compile AVRtest, for example. Notice that AVRtest is just an AVR core simulator; no peripherals are simulated. When you prefer avr-gdb, see SimulAVR and AVaRICE.

Since the rest of my system was presumably built with the version of GCC that arch ships (11, I believe),

Most of it. You probably have a few odd packages left that were built with older versions. You very likely have some that were built with Clang.

Won't everything no longer have the right libc?

libc is not part of the compiler. It comes from the Glibc project.

And aside from a few exceptions, C libraries retain compatibility for a long time, changing their 'soname' versions infrequently – Glibc has been at libc.so.6 since around 1997; binaries built then should still generally work with libc.so.6 from today.