053f58bab3
The FEAT_LSE atomic instructions include LD* instructions which return
the original value of a memory location can be used to directly
implement FETCH opertations. Each RETURN op is implemented as a copy of
the corresponding FETCH op with a trailing instruction to generate the
new value of the memory location. We only directly implement
*_fetch_add*(), for which we have a trailing `add` instruction.
As the compiler has no visibility of the `add`, this leads to less than
optimal code generation when consuming the result.
For example, the compiler cannot constant-fold the addition into later
operations, and currently GCC 11.1.0 will compile:
return __lse_atomic_sub_return(1, v) == 0;
As:
mov w1, #0xffffffff
ldaddal w1, w2, [x0]
add w1, w1, w2
cmp w1, #0x0
cset w0, eq // eq = none
ret
This patch improves this by replacing the `add` with C addition after
the inline assembly block, e.g.
ret += i;
This allows the compiler to manipulate `i`. This permits the compiler to
merge the `add` and `cmp` for the above, e.g.
mov w1, #0xffffffff
ldaddal w1, w1, [x0]
cmp w1, #0x1
cset w0, eq // eq = none
ret
With this change the assembly for each RETURN op is identical to the
corresponding FETCH op (including barriers and clobbers) so I've removed
the inline assembly and rewritten each RETURN op in terms of the
corresponding FETCH op, e.g.
| static inline void __lse_atomic_add_return(int i, atomic_t *v)
| {
| return __lse_atomic_fetch_add(i, v) + i
| }
The new construction does not adversely affect the common case, and
before and after this patch GCC 11.1.0 can compile:
__lse_atomic_add_return(i, v)
As:
ldaddal w0, w2, [x1]
add w0, w0, w2
... while having the freedom to do better elsewhere.
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/20211210151410.2782645-6-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
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.