On 2024-10-04 09:52, David Marchand wrote:
On Fri, Sep 20, 2024 at 12:36 PM Mattias Rönnblom
<mattias.ronnb...@ericsson.com> wrote:
Provide build option to have functions in <rte_memcpy.h> delegate to
the standard compiler/libc memcpy(), instead of using the various
custom DPDK, handcrafted, per-architecture rte_memcpy()
implementations.
A new meson build option 'use_cc_memcpy' is added. By default, the
traditional, custom DPDK rte_memcpy() implementation is used.
The performance benefits of the custom DPDK rte_memcpy()
implementations have been diminishing with every compiler release, and
with current toolchains the use of a custom memcpy() implementation
may even be a liability.
An additional benefit of this change is that compilers and static
analysis tools have an easier time detecting incorrect usage of
rte_memcpy() (e.g., buffer overruns, or overlapping source and
destination buffers).
Signed-off-by: Mattias Rönnblom <mattias.ronnb...@ericsson.com>
Acked-by: Morten Brørup <m...@smartsharesystems.com>
I like this patch and the direction we are taking: stop reinvent
memcpy and rely on compiler to optimize it.
I have some comments on the implementation.
- When I splitted headers in the early days of dpdk, the intention
with arch-specific headers in EAL was to have them include the generic
one, in all cases.
It seems that, over time, x86 rte_memcpy.h (at least) deviated from
this and stopped including generic/rte_memcpy.h...
So in this current patch, I expect every arch specific headers first
include generic/rte_memcpy.h, regardless of any arch-specific define
coming from the configuration.
An additional note on this, ARM32 and ARM64 have their own
implementation in rte_memcpy_32.h resp. rte_memcpy_64.h, and I would
check RTE_USE_CC_MEMCPY in each of them rather than in the top as
ARM32 and ARM64 are like two different arches.
- Now, looking at what was available for arches so far in DPDK:
* ARM was relying by default on compiler implementation, with specific
implementations for ARM32 and ARM64 available (see for more details
below) => possible values (default first) RTE_USE_CC_MEMCPY = true /
false
* loongarch was relying on compiler implementation, with no specific
implementations, => RTE_USE_CC_MEMCPY = true
* ppc was relying on arch specific implementation, => RTE_USE_CC_MEMCPY = false
* risc was relying on compiler implementation, with no specific
implementations, => RTE_USE_CC_MEMCPY = true
* x86 was relying on arch specific implementation, => RTE_USE_CC_MEMCPY = false
We can't get a unified default value for a meson option and keep
compat for all arches (except maybe introduce a "auto" value).
What if you just renamed RTE_USE_CC_MEMCPY to
RTE_ALWAYS_USE_CC_MEMCPY
RTE_FORCE_CC_MEMCPY
Then the naming would better match a scenario where cc memcpy may be the
only option.
Plus, disabling RTE_USE_CC_MEMCPY on loongarch and risc makes no
sense, as there was never a specific implementation.
My suggestion is to drop the meson option and instead just set
RTE_USE_CC_MEMCPY in config/$arch/meson.build.
Testers / interested users may edit config/$arch/meson.build on their own.
- Additionnally, ARM people have introduced arch-specific
implementation config options for memcpy in ARM32 resp. ARM64:
RTE_ARCH_ARM_NEON_MEMCPY resp. RTE_ARCH_ARM64_MEMCPY.
RTE_USE_CC_MEMCPY can replace those two options (we may keep some
compat in case someone relied on those defines for arm).
That removes the need for a RTE_CC_MEMCPY define.
More comments below:
[snip]
diff --git a/doc/guides/rel_notes/release_24_11.rst
b/doc/guides/rel_notes/release_24_11.rst
index 0ff70d9057..8be000294d 100644
--- a/doc/guides/rel_notes/release_24_11.rst
+++ b/doc/guides/rel_notes/release_24_11.rst
@@ -55,6 +55,26 @@ New Features
Also, make sure to start the actual text at the margin.
=======================================================
+* **Compiler memcpy replaces custom DPDK implementation.**
+
+ The memory copy functions of ``<rte_memcpy.h>`` now optionally
+ delegates to the standard memcpy() function, implemented by the
+ compiler and the C runtime (e.g., libc).
+
+ In this release of DPDK, the handcrafted, per-architecture memory
+ copy implementations are still the default. Compiler memcpy is
+ enabled by setting the new ``use_cc_memcpy`` build option to true.
+
+ The performance benefits of the custom DPDK rte_memcpy()
+ implementations have been diminishing with every new compiler
+ release, and with current toolchains the use of a custom memcpy()
+ implementation may even result in worse performance than the
+ standard memcpy().
+
+ An additional benefit of using compiler memcpy is that compilers and
+ static analysis tools have an easier time detecting incorrect usage
+ of rte_memcpy() (e.g., buffer overruns, or overlapping source and
+ destination buffers).
As explained in the RN comments, an entry should use the form:
* **Add a title in the past tense with a full stop.**
Add a short 1-2 sentence description in the past tense.
The description should be enough to allow someone scanning
the release notes to understand the new feature.
It seems this note is a copy/paste of the commit log, please adjust
the title and make the description shorter.
Removed Items
-------------
[snip]
diff --git a/lib/eal/include/generic/rte_memcpy.h
b/lib/eal/include/generic/rte_memcpy.h
index e7f0f8eaa9..cfb0175bd2 100644
--- a/lib/eal/include/generic/rte_memcpy.h
+++ b/lib/eal/include/generic/rte_memcpy.h
@@ -5,12 +5,19 @@
#ifndef _RTE_MEMCPY_H_
#define _RTE_MEMCPY_H_
+#ifdef __cplusplus
+extern "C" {
+#endif
+
/**
* @file
*
* Functions for vectorised implementation of memcpy().
*/
+#include <stdint.h>
+#include <string.h>
I don't think those includes should go in a extern "C" { block.
+
/**
* Copy 16 bytes from one location to another using optimised
* instructions. The locations should not overlap.
@@ -35,8 +42,6 @@ rte_mov16(uint8_t *dst, const uint8_t *src);
static inline void
rte_mov32(uint8_t *dst, const uint8_t *src);
-#ifdef __DOXYGEN__
-
This strange check was added as not all architectures provide
rte_mov48 (/me slaps Adrien and Thomas).
I think the CI reported no issue because of a problem in the next
patch where all that is tested is RTE_USE_CC_MEMCPY = true
combination.
Still, the overall goal of this work is to drop the whole rte_memcpy
thing in the future, so I think we can live with this #ifdef
__DOXYGEN__ non sense hiding the absence of rte_mov48 in x86...
/**
* Copy 48 bytes from one location to another using optimised
* instructions. The locations should not overlap.
@@ -49,8 +54,6 @@ rte_mov32(uint8_t *dst, const uint8_t *src);
static inline void
rte_mov48(uint8_t *dst, const uint8_t *src);
-#endif /* __DOXYGEN__ */
-
/**
* Copy 64 bytes from one location to another using optimised
* instructions. The locations should not overlap.
@@ -87,8 +90,6 @@ rte_mov128(uint8_t *dst, const uint8_t *src);
static inline void
rte_mov256(uint8_t *dst, const uint8_t *src);
-#ifdef __DOXYGEN__
-
/**
* Copy bytes from one location to another. The locations must not overlap.
*
@@ -111,6 +112,52 @@ rte_mov256(uint8_t *dst, const uint8_t *src);
static void *
rte_memcpy(void *dst, const void *src, size_t n);
-#endif /* __DOXYGEN__ */
Removing this DOXYGEN here should be ok.
CI will tell us.
diff --git a/lib/eal/x86/include/meson.build b/lib/eal/x86/include/meson.build
index 52d2f8e969..09c2fe2485 100644
--- a/lib/eal/x86/include/meson.build
+++ b/lib/eal/x86/include/meson.build
@@ -16,6 +16,7 @@ arch_headers = files(
'rte_spinlock.h',
'rte_vect.h',
)
+
Unrelated change.
arch_indirect_headers = files(
'rte_atomic_32.h',
'rte_atomic_64.h',
