Re: [PATCH 3/6] pack-bitmap.c: drop unnecessary 'inline's

From: Jeff King <peff@peff.net>
Date: Mar 21, 2023, 17:40 UTC
Message-ID: <20230321174033.GE3119834@coredump.intra.peff.net>
In-Reply-To: <2e3e3a7145a5851fcf5c485b38d14344c9b824d7.1679342296.git.me@ttaylorr.com>

On Mon, Mar 20, 2023 at 04:02:46PM -0400, Taylor Blau wrote:

> Both `read_be32()` and `read_u8()` are defined as inline dating back
> to b5007211b6 (pack-bitmap: do not use gcc packed attribute,
> 2014-11-27), though that commit does not hint at why the functions were
> defined with that attribute.

I think any non-header inline like this can implicitly be assumed to be "I thought it would make things faster". ;)

Show 22 quoted lines

> However (at least with GCC 12.2.0, at the time of writing), the
> resulting pack-bitmap.o contains the same instructions with or without
> the inline attribute applied to these functions:
> 
>     $ make O=3 pack-bitmap.o && mv pack-bitmap.o{,.before}
>     [ apply this patch ]
>     $ make O=3 pack-bitmap.o && mv pack-bitmap.o{,.after}
>     $ objdump -d pack-bitmap.o.before >before
>     $ objdump -d pack-bitmap.o.after >after
>     $ diff -u pack-bitmap.o.{before,after}
>     --- before	2023-03-15 18:54:17.021580095 -0400
>     +++ after	2023-03-15 18:54:21.853552218 -0400
>     @@ -1,5 +1,5 @@
> 
>     -pack-bitmap.o.before:     file format elf64-x86-64
>     +pack-bitmap.o.after:     file format elf64-x86-64
> 
>      Disassembly of section .text:
> 
> So defining these functions as inline is at best a noop, and at worst
> confuses the reader into thinking that there is some trickier reason
> that they are defined as inline when there isn't.

Nice digging. The "inline" is really just a hint to the compiler here, and obviously it does not need that hint. I do wonder if that is still true after you make them more complicated in a later patch in the series.

On the other hand, I doubt that these need to be very optimized at all. If there were a tight loop of single-byte reads, the function overhead might be noticeable. But generally we're reading only a few items from the beginning of each entry, and then reading the bulk of the data via ewah_read_mmap().

So I think the overall argument is "let the compiler decide what is good to inline and what is not".

-Peff