arm64: Investigate using adds and changing the subsequent compare suffix to match expected dotnet behaviour

[jonathandavies-arm]

If we have the following instructions

add w0, w0, w1
cmp w0, #0
cset w0, gt

we would like to use the adds instruction so that the flags are set during the adds and we don't need a separate cmp instruction.

The issue is that without changing the cset suffix we will get an incorrect results if the add results in an overflow. We would need to check that the result is positive and not zero.

See these 2 comments:
#121380 (comment)
#121380 (comment)

This should also be done for subs and negs.

[tannergooding]

The set of conditions are:

mnemonic	meaning	condition flags
eq	equal	Z == 1
ne	not equal	Z == 0
cs or hs	carry set	C == 1
cc or lo	carry clear	C == 0
mi	minus, negative	N == 1
pl	plus, positive or zero	N == 0
vs	overflow	V == 1
vc	no overflow	V == 0
hi	unsigned higher	(C == 1) && (Z == 0)
ls	unsigned lower or same	!((C == 1) && (Z == 0))
ge	signed greater than or equal	N == V
lt	signed less than	N != V
gt	signed greater than	(Z == 0) && (N == V)
le	signed less than or equal	!((Z == 0) && (N == V))

So while (a + b) > 0 won't be possible since it needs (N == 0) && (Z == 0). It is possible for some other cases like (a + b) >= 0, (a + b) > -1, (a + b) < 0, and (a + b) <= -1. Others may also exist.

-- For additional reference:

• N - set if the result is negative
• Z - set if the result is zero
• C - set if the result overflows for unsigned inputs
• V - set if the result overflows for signed inputs

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