I configured Emacs 27.2 RC2 on macOS 11.2 ARM64 (arm-apple-darwin20.3.0) 
using "./configure --with-gnutls=ifavailable" (plain "./configure" 
failed, which was disconcerting). When I ran "make" the compiler complained:

editfns.c:2063:18: warning: unsequenced modification and access to 
'sa_avail' [-Wunsequenced]

This correctly diagnoses undefined behavior in that C function. I didn't 
investigate what happens on that platform is as it's easier to fix the 
problem on all platforms. I propose installing the attached 
obviously-safe patch into the emacs-27 branch.

There were several deprecation warnings when compiling but I assume we 
don't care about them in this older branch.

"make check" reported four unexpected failures. Perhaps a macOS expert 
could be persuaded to look into them. A tarball of the failed logs is 
attached.