On Tue, 11 Mar 2014 04:28:25 -0000, Chris Angelico <ros...@gmail.com>
wrote:
On Tue, Mar 11, 2014 at 2:38 PM, Ian Kelly <ian.g.ke...@gmail.com> wrote:
On Mon, Mar 10, 2014 at 7:45 PM, Chris Angelico <ros...@gmail.com>
wrote:
No no,
I could make this so much better by using the 80x86 "REP MOVSW"
command (or commands, depending on your point of view). That would be
so much better than all those separate operations the silly compiler
was doing! Roughly an hour of fiddling later, making sure it all still
worked correctly, I discover that... hmm, it's not actually any
faster.
Better to have tried and failed though than to have simply accepted
what the compiler was doing with no verification at all.
Maybe. But I've learned now that one guy who used to do assembly
language programming on an 8086 is unlikely to discover something that
the many authors of a C compiler haven't noticed. Yes, it's possible
there'll be something specific to my code, like if I'm doing a
strcpy-like operation that isn't *actually* strcpy (the function will
be optimized heavily, but a C-level loop might not be recognized), but
it's more likely the compiler knows better than I do.
That might be true for x86, but it isn't true for ARM for example.
Apparently it's algorithmically hard to generate ARM code that makes
efficient use of conditional instructions, and I can almost always write
tighter, faster code than the compiler generates in consequence. It's not
always worth the extra coding time (longer now that I'm not in practise),
but that's a separate matter.
--
Rhodri James *-* Wildebeest Herder to the Masses
--
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