RE: CRC32C Parallel Computation Optimization on ARM

On Date: Wed, 22 Nov 2023 15:06:18PM -0600, Nathan Bossart wrote:

>> On Date: Fri, 10 Nov 2023 10:36:08AM -0600, Nathan Bossart wrote:
>>>+__attribute__((target("+crc+crypto")))
>>>
>>>I'm not sure we can assume that all compilers will understand this, and I'm
>>>not sure we need it.
>>
>> CFLAGS_CRC is "-march=armv8-a+crc". Generally, if -march is supported,
>> __attribute__ is also supported.

>IMHO we should stick with CFLAGS_CRC for now. If we want to switch to
>using __attribute__((target("..."))), I think we should do so in a separate
>patch. We are cautious about checking the availability of an attribute
>before using it (see c.h), and IIUC we'd need to verify that this works for
>all supported compilers that can target ARM before removing CFLAGS_CRC
>here.

I agree.

>> In addition, I am not sure about the source file pg_crc32c_armv8.c, if
>> CFLAGS_CRC and CFLAGS_CRYPTO are needed at the same time, how should it
>> be expressed in the makefile?
>
>pg_crc32c_armv8.o: CFLAGS += ${CFLAGS_CRC} ${CFLAGS_CRYPTO}

It does not work correctly. CFLAGS ='-march=armv8-a+crc, -march=armv8-a+crypto', what actually works is '-march=armv8-a+crypto'.

We set a new variable CLAGS_CRC_CRYPTO,In configure.ac,

If test x"$CFLAGS_CRC" != x"" || test x"CFLAGS_CRYPTO" != x""; then
CLAGS_CRC_CRYPTO = '-march=armv8-a+crc+crypto'
fi

then in makefile,
pg_crc32c_armv8.o: CFLAGS +=${ CLAGS_CRC_CRYPTO }

And same thing in meson.build. In src/port/meson.build,

replace_funcs_pos = [
# arm / aarch64
['pg_crc32c_armv8', 'USE_ARMV8_CRC32C', 'crc_crypto'],
['pg_crc32c_armv8', 'USE_ARMV8_CRC32C_WITH_RUNTIME_CHECK', 'crc_crypto'],
['pg_crc32c_armv8_choose', 'USE_ARMV8_CRC32C_WITH_RUNTIME_CHECK', 'crc_crypto'],
['pg_crc32c_sb8', 'USE_ARMV8_CRC32C_WITH_RUNTIME_CHECK'],
]
'pg_crc32c_armv8' also needs 'crc_crypto' when 'USE_ARMV8_CRC32C'.

Looking forward to your feedback, thanks!

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