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ef53245060
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>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.