commit ff9ea922480d2abb960294eb1dde674d79577f8b
author Mark Rutland <mark.rutland@arm.com> Fri Dec 10 15:14:07 2021 +0000
committer SX <neoroms69@gmail.com> Sat Oct 12 03:56:53 2024 +0530
tree fbf9a7bc360244bded9a5edc9e4971c453e6e2bd
parent 492a908677a66a99a5e159f2dbc8fb0bde12dbf3

arm64: atomics lse: define SUBs in terms of ADDs

The FEAT_LSE atomic instructions include atomic ADD instructions
(`stadd*` and `ldadd*`), but do not include atomic SUB instructions, so
we must build all of the SUB operations using the ADD instructions. We
open-code these today, with each SUB op implemented as a copy of the
corresponding ADD op with a leading `neg` instruction in the inline
assembly to negate the `i` argument.

As the compiler has no visibility of the `neg`, this leads to less than
optimal code generation when generating `i` into a register. For
example, __les_atomic_fetch_sub(1, v) can be compiled to:

	mov     w1, #0x1
	neg     w1, w1
	ldaddal w1, w1, [x2]

This patch improves this by replacing the `neg` with negation in C
before the inline assembly block, e.g.

	i = -i;

This allows the compiler to generate `i` into a register more optimally,
e.g.

	mov     w1, #0xffffffff
	ldaddal w1, w1, [x2]

With this change the assembly for each SUB op is identical to the
corresponding ADD op (including barriers and clobbers), so I've removed
the inline assembly and rewritten each SUB op in terms of the
corresponding ADD op, e.g.

| static inline void __lse_atomic_sub(int i, atomic_t *v)
| {
| 	__lse_atomic_add(-i, v);
| }

For clarity I've moved the definition of each SUB op immediately after
the corresponding ADD op, and used a single macro to create the RETURN
forms of both ops.

This is intended as an optimization and cleanup.
There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20211210151410.2782645-3-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>