Hi Doug,
On Tue, Mar 23, 2021 at 10:10 PM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
> On Sun, Mar 21, 2021 at 8:02 PM Laurent Pinchart
> <laurent.pinchart+renesas@xxxxxxxxxxxxxxxx> wrote:
> >
> > The valid rates are stored in an array of 8 booleans. Replace it with a
> > bitmask to save space.
>
> I'm curious: do you have evidence that this does anything useful? I
> guess you're expecting it to save .text space, right? Stack usage and
> execution time differences should be irrelevant--it's not in a
> critical section and the difference should be tiny anyway. As far as
> .text segment goes, it's not obvious to me that the compiler will use
> fewer instructions to manipulate bits compared to booleans.
>
> Doing a super simple "ls -ah" on vmlinux (unstripped):
>
> Before: 224820232 bytes
> After: 224820376 bytes
>
> ...so your change made it _bigger_. OK, so running "strip
> --strip-debug" on those:
>
> Before: 26599464 bytes
> After: 26599464 bytes
I've been surprised by the counter-intuitive impact of similar changes
before, too. The result may also differ a lot between arm32 or arm64.
> ...so exactly the same. I tried finding some evidence using "readelf -ah":
>
> Before:
> [ 2] .text PROGBITS ffffffc010010000 00020000
> 0000000000b03508 0000000000000000 WAX 0 0 65536
> [ 3] .rodata PROGBITS ffffffc010b20000 00b30000
> 00000000002e84b3 0000000000000000 WAMS 0 0 4096
>
> After:
> [ 2] .text PROGBITS ffffffc010010000 00020000
> 0000000000b03508 0000000000000000 WAX 0 0 65536
> [ 3] .rodata PROGBITS ffffffc010b20000 00b30000
> 00000000002e84b3 0000000000000000 WAMS 0 0 4096
>
> Maybe you have some evidence showing an improvement? Ah, OK. I
> disassembled ti_sn_bridge_enable() and your patch saves 12 bytes, but
> I guess maybe alignment washes it out in reality...
Yes, arm64 is bad w.r.t. this.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
