[AVR] Simplify SPWRITE on devices with a small stack

[benshi001]

As shown in #147210, it is unnecessary to disable/restore interrupt on devices with a small stack.

[benshi001]

There are similar problems in SPREAD.

[tomtor]

SPREAD (which has no cli) is OK as it is because we need a 16bit SP to store (and calculate offsets) in the 16bit Y (FP) register on ALL devices. We could replace the read of SPH by an immediate load of 0 for NOSPH devices, but that just complicates the code, because reading SPH on those devices just returns 0. According to the attiny13 and attiny25 manuals reading those non implemented registers is fine.

So I suggest just skipping the cli in SPWRITE for NOSPH devices as only change (and adapt the test) to close this issue.

@benshi001 If you agree I will write a PR.

[tomtor]

Hmmm: avr-gcc -S -Os loop.c -mmcu=attiny25

        in r28,__SP_L__
        clr r29
        subi r28,lo8(-(-20))
        out __SP_L__,r28

So gcc DOES the clr instead of the read SPH. I looked at llvm sources, and this will require a lot of additional code (using subi). I do not think that is sound from a software engineering point of view. subi and sbiw are both 2 bytes, so we do not gain a reduction in code size.

Clang with my proposed change of just SPWRITE:

  e6:   cd b7           in      r28, 0x3d       ; 61
  e8:   de b7           in      r29, 0x3e       ; 62
  ea:   64 97           sbiw    r28, 0x14       ; 20
  ec:   cd bf           out     0x3d, r28       ; 61

[benshi001]

This bug is not urgent (and also new target features). I must pay my limited attention to decide #145291 ASAP, since that affects so much, including -mrelax and-lto .