[Bug target/110217] [avr] SREG: use BSET and BCLR instead of load/modify/write

["gjl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110217

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement

--- Comment #1 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
The only case where this might make sense is for bit 7 (the I-flag), however
the established coding style is to use cli() and sei() from AVR-LibC, cf.
documentation of #include <avr/interrupt.h>:

https://www.nongnu.org/avr-libc/user-manual/group__avr__interrupts.html

For more sophitsticated use cases there is even ATOMIC_BLOCK and friends
provided by #include <util/atomic.h>, cf:

https://www.nongnu.org/avr-libc/user-manual/group__util__atomic.html

This has the additional benefit of being more readable than bit manipulations.  

Apart from that, the proposed patch won't work for indirect addressing, or when
the compiler is turning direct addresses to indirect addresses (using CSE etc,
common subexpression elimination and similar strategies).

Also the patch relies on insn combine which only runs when optimization is on,
thus any application which relies on that optimization will glitch at -O0.

So I am inclined to "won't fix" this PR.

Maybe you just missed avr/interrupt.h and / or util/atomic.h ?

If you must not use AVR-LibC for some reason, then the next best approach is to
use __builtin_avr_sei(), cf. AVR built-in functions:

https://gcc.gnu.org/onlinedocs/gcc-13.1.0/gcc/AVR-Built-in-Functions.html

Or implement it as static inline function that does __asm volatile ("sei" :::
"memory") if you are not allowed to use built-ins.