[U-Boot] [PATCH 2/3] string: Provide a slimmed-down memset()

Heiko Stuebner
2017-03-28T12:34:46Z

Am Montag, 27. März 2017, 23:16:45 CEST schrieb Alexander Graf:
>
> On 27/03/2017 17:17, Heiko Stuebner wrote:
> > Am Montag, 27. März 2017, 09:14:47 CEST schrieb Alexander Graf:
> >>
> >> On 27/03/2017 01:38, Simon Glass wrote:
> >>> Most of the time the optimised memset() is what we want. For extreme
> >>> situations such as TPL it may be too large. For example on the 'rock'
> >>> board, using a simple loop saves a useful 48 bytes. With gcc 4.9 and
> >>> the rodata bug, this patch is enough to reduce the TPL image below the
> >>> limit.
> >>>
> >>> Signed-off-by: Simon Glass <sjg@chromium.org>
> >>> ---
> >>>
> >>> lib/Kconfig | 9 +++++++++
> >>> lib/string.c | 6 ++++--
> >>> 2 files changed, 13 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/lib/Kconfig b/lib/Kconfig
> >>> index 65c01573e1..5bf512d8c0 100644
> >>> --- a/lib/Kconfig
> >>> +++ b/lib/Kconfig
> >>> @@ -52,6 +52,15 @@ config LIB_RAND
> >>> help
> >>> This library provides pseudo-random number generator functions.
> >>>
> >>> +config FAST_MEMSET
> >>> + bool "Use an optimised memset()"
> >>> + default y
> >>> + help
> >>> + The faster memset() is the arch-specific one (if available) enabled
> >>> + by CONFIG_USE_ARCH_MEMSET. If that is not enabled, we can still get
> >>> + better performance by write a word at a time. Disable this option
> >>> + to reduce code size slightly at the cost of some speed.
> >>
> >> The comment sounds slightly confused - it took me a few times of reading
> >> it until I grasped what it was trying to tell me :).
> >>
> >>> +
> >>> source lib/dhry/Kconfig
> >>>
> >>> source lib/rsa/Kconfig
> >>> diff --git a/lib/string.c b/lib/string.c
> >>> index 67d5f6a421..159493ed17 100644
> >>> --- a/lib/string.c
> >>> +++ b/lib/string.c
> >>> @@ -437,8 +437,10 @@ char *strswab(const char *s)
> >>> void * memset(void * s,int c,size_t count)
> >>> {
> >>> unsigned long *sl = (unsigned long *) s;
> >>> - unsigned long cl = 0;
> >>> char *s8;
> >>> +
> >>> +#ifdef CONFIG_FAST_MEMSET
> >>> + unsigned long cl = 0;
> >>> int i;
> >>>
> >>> /* do it one word at a time (32 bits or 64 bits) while possible */
> >>> @@ -452,7 +454,7 @@ void * memset(void * s,int c,size_t count)
> >>> count -= sizeof(*sl);
> >>> }
> >>> }
> >>> - /* fill 8 bits at a time */
> >>> +#endif /* fill 8 bits at a time */
> >>
> >> So while this is all neat, a few ideas:
> >>
> >> 1) Would having memset in a header improve things even more? After all,
> >> each external function call clobbers registers that you need to
> >> save/restore...
> >
> > I'd guess it really depends on the size constraints. The regular
> > libgeneric memset compiles on my rk3188 tpl to a total of
> > 64bytes on both gcc-4.9 and gcc-6.3 while Simon's fast-memset
> > comes down to 14bytes on my rk3188.
> >
> > On the rk3188 the only memset user is board_init_f, so here memset
> > is called only once without needing to save registers and I'd guess if an
> > implementation really is that size-constrained to worry about 50bytes
> > this one caller will probably always be the only one?
>
> I'm not sure I follow. If you put it into a header, the compiler has a
> better chance of evicting untaken code paths and optimize register usage
> over object linked variants (unless you use GOLD). I was mostly
> wondering whether that would already give you the savings without
> introducing a complicated #ifdef that is going to bitrot over time :).

On rk3188-tpl that small non-fast memset gets compiled to (bfd linker):
100809aa <board_init_f_init_reserve>:
100809aa: b510 push {r4, lr}
100809ac: 22c0 movs r2, #192 ; 0xc0
100809ae: 2100 movs r1, #0
100809b0: 4604 mov r4, r0
100809b2: f000 f804 bl 100809be <memset>
100809b6: 34c0 adds r4, #192 ; 0xc0
100809b8: f8c9 4090 str.w r4, [r9, #144] ; 0x90
100809bc: bd10 pop {r4, pc}

100809be <memset>:
100809be: 4402 add r2, r0
100809c0: 4603 mov r3, r0
100809c2: 4293 cmp r3, r2
100809c4: d100 bne.n 100809c8 <memset+0xa>
100809c6: 4770 bx lr
100809c8: f803 1b01 strb.w r1, [r3], #1
100809cc: e7f9 b.n 100809c2 <memset+0x4>

not saving any outside registers, as it's used only once at all and what
I was trying to say was that in cases where we worry about having the
tiniest memset possible, I guess that will most likely stay the only call.

But I may have been dug into the rk3188 tpl-specifics to long, to see
other possible cases right now :-) .

> I'm just slightly worried about the massive number of preprocessor
> excludes that happen in U-Boot in general. It seems like something
> that's really hard to ever have full testing coverage on.

That's essentially what I was worried about as well, seeing that memset
can be provided by different sources it seems.
There is the libgeneric memset we're having here and also the arch-
specific memset (way faster but also again way bigger) and without using
either, one could also provide some completely separate implementation
at the moment.

So having one version in a header would probably also incur some sort of
ifdef voodoo?

> >> 2) How much would GOLD save you? Have you tried? U-Boot is small enough
> >> of a code base that global optimizations should be able to give
> >> significant size savings.
> >
> > I think the issue that this is trying to solve is to allow more
> > toolchains to be used and thus make rebuilds on changes work on a lot
> > of boards at the same time with random toolchains.
> >
> > gcc-6.3 already produces way smaller results (well within the size
> > constraints the rk3188 has) than for example the gcc-4.9 used by
> > buildman as baseline toolchain.
>
> Ah, I see. So 4.9 does not have -lto? There's a good chance my gut
> feeling that GOLD actually saves anything is wrong - I don't know. Has
> anyone done the numbers? Then we would have something to actually base
> gut feeling on.

It looks like the u-boot Makefile makes explicitly sure to use GNU ld.
So I didn't try to dig deeper into this :-) .

> Size is always a serious constraint in U-Boot, especially in SPL
> environments. If we can include one more tool in our portfolio to
> optimize size across the board, I'm all for it. This patch just feels
> slightly short-term - but I'm definitely not nack'ing it :).

Heiko