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". ;)
> 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
