Re: [PATCH] Add loongarch native checksum implementation.

On Thu, Jun 8, 2023 at 12:24 PM YANG Xudong <yangxudong@ymatrix.cn> wrote:
>
> This patch tries to add loongarch native crc32 check with crcc.*
> instructions to postgresql.
>
> The patch is tested on my Loongson 3A5000 machine with Loong Arch Linux
> and GCC 13.1.0 / clang 16.0.0 with
>
> - default ./configure
> - default meson setup

I took a quick look at this, and it seems mostly in line with other
architectures we support for CRC. I have a couple questions and comments:

configure.ac:

+AC_SUBST(CFLAGS_CRC)

This seems to be an unnecessary copy-paste. I think we only need one, after
all checks have run.

meson.build

+  if cc.links(prog, name: '__builtin_loongarch_crcc_w_b_w,
__builtin_loongarch_crcc_w_h_w, __builtin_loongarch_crcc_w_w_w, and
__builtin_loongarch_crcc_w_d_w without -march=loongarch64',
+      args: test_c_args)
+    # Use LoongArch CRC instruction unconditionally
+    cdata.set('USE_LOONGARCH_CRC32C', 1)
+    have_optimized_crc = true
+  elif cc.links(prog, name: '__builtin_loongarch_crcc_w_b_w,
__builtin_loongarch_crcc_w_h_w, __builtin_loongarch_crcc_w_w_w, and
__builtin_loongarch_crcc_w_d_w with -march=loongarch64',
+      args: test_c_args + ['-march=loongarch64'])
+    # Use LoongArch CRC instruction unconditionally

For x86 and Arm, if it fails to link without an -march flag, we allow for a
runtime check. The flags "-march=armv8-a+crc" and "-msse4.2" are for
instructions not found on all platforms. The patch also checks both ways,
and each one results in "Use LoongArch CRC instruction unconditionally".
The -march flag here is general, not specific. In other words, if this only
runs inside "+elif host_cpu == 'loongarch64'", why do we need both with
-march and without?

Also, I don't have a Loongarch machine for testing. Could you show that the
instructions are found in the binary, maybe using objdump and grep? Or a
performance test?

In the future, you may also consider running the buildfarm client on a
machine dedicated for testing. That will give us quick feedback if some
future new code doesn't work on this platform. More information here:

https://wiki.postgresql.org/wiki/PostgreSQL_Buildfarm_Howto

--
John Naylor
EDB: http://www.enterprisedb.com