Re: efficient function call

Victor Bazarov <v.bazarov@comcast.invalid>
Tue, 24 May 2011 15:07:34 -0400
On 5/24/2011 2:49 PM, hamze wrote:

On May 24, 4:52 pm, Victor Bazarov<v.baza...@comcast.invalid> wrote:

On 5/24/2011 8:52 AM, hamze wrote:

I had a small I/O job to do, with high frequency. it write 2 operand
and read 1.
previously I used macros to do this, but strange bugs specially in
optimization process appeared.
finally I decided to use functions instead of macros but function call
overhead is much much bigger than job's time itself.
what should I do to cape this problem? I mean an efficient function

Usually an inline function (a function declared 'inline' or a member of
the class defined in the class definition itself, which makes it
implicitly 'inline') can be optimized better by the compiler.

What makes you think the "function call overhead" is "much bigger"? Did
you measure it? How?

I have a timer ( counter ) on my system, I call it before and after
each function, different of value is elapsed clock.
when ever I use this timer for measurement, I turn optimization off to
exact measurement.

I.e. you're not measuring the same code that is going to be executed
(since you will compile that one with optimizations turned on). What's
the point?

I am working on an embedded system without any OS.

I am sure it's tough to develop on a system without any tools. If you
use your own timer for profiling, make sure the rest of the program is
as close as possible to the real thing, i.e. run it with your timer, but
fully optimized.

As soon as you try to measure the code's execution on the same level as
the code itself (down to, say, a function call), you will need to
actually change the code (by introducing your timer, or whatnot), and
you're going to affect the results. You can't get the exact (absolute)
timing, only the relative speed (some parts are slower than other).

You should work on optimizing only what makes the biggest difference,
and only one thing at a time. Divide a conquer. And only go as far as
you need to satisfy the overall speed of your program (which you should
check with your measurement mechanism disabled). IOW, make it as fast
as needed, but not faster. There is always the law of diminishing
returns that applies to such efforts.

with macro , as I measured my job is as little as 50 clock cycle, but
function call make it 3-4 times bigger.
thanks about inline, I will check it.

I do not respond to top-posted replies, please don't ask

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