Re: always use runtime checks for CRC-32C instructions

From: Nathan Bossart <nathandbossart(at)gmail(dot)com>
To: Jeff Davis <pgsql(at)j-davis(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org, Xiang(dot)Gao(at)arm(dot)com
Subject: Re: always use runtime checks for CRC-32C instructions
Date: 2023-10-31 03:36:01
Message-ID: 20231031033601.GA68409@nathanxps13
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On Mon, Oct 30, 2023 at 01:48:29PM -0700, Jeff Davis wrote:
> I assume you are concerned about the call going through a function
> pointer? If so, is it possible that setting a flag and then branching
> would be better?
> Also, if it's a concern, should we also consider making an inlineable
> version of pg_comp_crc32c_sse42()?

I tested pg_waldump -z with 50M 65-byte records for the following
implementations on an ARM system:

* slicing-by-8 : ~3.08s
* proposed patches applied (runtime check) : ~2.44s
* only CRC intrinsics implementation compiled : ~2.42s
* forced inlining : ~2.38s

Avoiding the runtime check produced a 0.8% improvement, and forced inlining
produced another 1.7% improvement. In comparison, even the runtime check
implementation produced a 20.8% improvement over the slicing-by-8 one.

Nathan Bossart
Amazon Web Services:

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