Re: [PATCH] trivial: the memset operation on a automatic array variable should be optimized out by data initialization
- From: Oleg Verych <olecom@xxxxxxxxxxxxxx>
- Date: Mon, 25 Jun 2007 00:25:18 +0200
On Sun, Jun 24, 2007 at 08:58:10PM +0800, rae l wrote:
On 6/23/07, Oleg Verych <olecom@xxxxxxxxxxxxxx> wrote:
Why not just show actual objdump output on code (maybe with differenthere is the objdump output of the two object files:
oxygen atoms used in gcc), rather than *talking* about optimization and
standards, hm?
As you could see, the older one used 0x38 bytes stack space while the
new one used 0x28 bytes,
and the object code is two bytes less,
Actually more: $((0x3d9 - 0x3ce))
I think all these benefits are the gcc's __builtin_memset optimization
than the explicit call to memset.
... or from complex memset() implementation (some chips even didn't do
`rep' fast enough somehow). Maybe code like below will be acceptable for
both optimizers and maintainers?
|-*-
unsigned long max_zone_pfns[MAX_NR_ZONES] = {
[ZONE_DMA] = MAX_DMA_PFN,
[ZONE_DMA32] = MAX_DMA32_PFN,
[ZONE_NORMAL] = end_pfn,
[ZONE_MOVABLE] = 0UL
};
|-*-
$ objdump -d /tmp/init.orig.o|grep -A23 -nw '<paging_init>'[]
547- 3d9: c3 retq[]
545- 3ce: c3 retq[]
with '{}' initializer, gcc will fill its memory with zeros.
I bet, that will be a key for success. And if you are interested in such
optimizations, why not to grep whole source tree for this kind of
things? I'm not sure one function in arch/x86_64 is only such
``unoptimized''.
And after doing that maybe you will see, that "{}" initializer can be
applied not only to integer values (you did init with of *long int*,
with *int*, btw), but to structs and others.
to other potential points to be optimized, I only see this trivial as
the first point, I wonder how people gives comments on this; and if
this optimization can be tested correctly, this can be done as an
optimization example and I'll try others.
Yes, comments and discussion is most important thing. But with such
propositions you will be better in the kernel-janitors list.
Thank you, I know it and I've already subscribed the linux kernel
Ahh, one more thing about _optimizing_ your time, i.e. not wasting one.
Add to CC list people, who already did reply on you patch. Otherwise
you are showing your disrespect for them and hiding from further
discussion.
mailing list(linux-kernel@xxxxxxxxxxxxxxx) so that I won't miss any
further discussion about it.
OK, but news<=>e-mail service, like Gmane is much nicer.
What about that?
I think you do not, but Linux development not have an automatic system
for patch tracking, so you are on your own with your text editor and
e-mail client on this. Please take care for your time.
Do you mean something such as git by "an automatic system"?
That was a side note.
____
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