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Message-ID: <ZF/k0Xo76q60uk0M@moria.home.lan>
Date: Sat, 13 May 2023 15:28:17 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Eric Biggers <ebiggers@...nel.org>
Cc: Lorenzo Stoakes <lstoakes@...il.com>,
Christoph Hellwig <hch@...radead.org>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-bcachefs@...r.kernel.org,
Kent Overstreet <kent.overstreet@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Uladzislau Rezki <urezki@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec
On Fri, May 12, 2023 at 06:57:52PM -0700, Eric Biggers wrote:
> What I would suggest instead is preprocessing the list of 6 field lengths to
> create some information that can be used to extract all 6 fields branchlessly
> with no dependencies between different fields. (And you clearly *can* add a
> preprocessing step, as you already have one -- the dynamic code generator.)
>
> So, something like the following:
>
> const struct field_info *info = &format->fields[0];
>
> field0 = (in->u64s[info->word_idx] >> info->shift1) & info->mask;
> field0 |= in->u64s[info->word_idx - 1] >> info->shift2;
>
> ... but with the code for all 6 fields interleaved.
>
> On modern CPUs, I think that would be faster than your current C code.
>
> You could do better by creating variants that are specialized for specific
> common sets of parameters. During "preprocessing", you would select a variant
> and set an enum accordingly. During decoding, you would switch on that enum and
> call the appropriate variant. (This could also be done with a function pointer,
> of course, but indirect calls are slow these days...)
>
> For example, you mentioned that 8-byte packed keys is a common case. In that
> case there is only a single u64 to decode from, so you could create a function
> that just handles that case:
>
> field0 = (word >> info->shift) & info->mask;
>
> You could also create other variants, e.g.:
>
> - 16-byte packed keys (which you mentioned are common)
> - Some specific set of fields have zero width so don't need to be extracted
> (which it sounds like is common, or is it different fields each time?)
> - All fields having specific lengths (are there any particularly common cases?)
>
> Have you considered any of these ideas?
I like that idea.
Gonna hack some code... :)
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