Skip to main content

C++: Counting Function Calls

How many function calls are involved in executing this piece of C++ (from a QT project):
/**
* Given a QString, safely escape it properly for sh. For example, given
* $`"\a\" return \$\`\"\a\\".
*/
QString
ConfIO::writeString(const QString s)
{
QString ret;

for (int i = 0; i < s.length(); i++)
{
QChar c = s[i];

if (c == '$' || c == '`' || c == '"' || c == '\\')
ret += '\\';
ret += c;
}

return ret;
}
If you don't count any function calls made by .length(), etc., I've counted
21 so far!

Comments

Will Moffat said…
Hmm, my C++ is really rusty.
* 2 calls to instantiate ret and c
* 3 operator= or +=
* 4 operator==
* 1 operator[]
So I only get 10, what am I missing?
Doug Napoleone said…
I count even less with a proper compile. Compilers these days are very good at inlining. Though QT is not known for using the best compiler options, nor does it do profile guided optimizations.

In short, don't bother trying to count the function calls you think you see. Count the ones which are actually there with proper profiling systems.

Unfortunately g++ makes this harder than it should as it inserts the _penter and _pexit calls even for inlined functions. This means you are best off using the intel profiler tools on linux. On windows you can have fun building your own profiler (not an easy task, but you can come up with something very powerful which is what we did at work). Work better than the intel tools IMHO.

We have custom string and array classes/templates, but the compiler turns every function into an inlined block for release builds (including the constructor on windows, something g++ does not seem to do, but icl (intels compiler) does. Enabling SSE2 instructions also helps out quite a bit (means the emitted ASM will not run on chips w/o SSE2).

As a result our compiled int8 array (same as char array in the end) can do array/vector math performing 16 operations at a time, as that is the SSE2 ASM which is generated, inlined, w/o writing any specialized code to do such (which we had at one point). The compiler is now smart enough to do it for us just by looking at the 'for(int i=0; i<o.numElem(); i++)...' code and most of the time do it better. This frees up the developers to work on the real hard problems.

Granted it took some time to develop our tools and tests so that we could properly determine when and where the compiler was either helping or hurting us, and understanding the differences between the compilers we are using.
Having a proper test framework with a proper profiling/timing system is crucial to any project; no matter the language.
jjinux said…
Will, I wrote that years ago, and I can't remember now ;)

Doug, great comment! It seems every year I learn even more about how pathetically little I know about C++.
Unknown said…
With no inlining this is what I see:

2x Explicit Constructors for ret and c
1x for length
1x for s[i]
6x Implicit Constructors (s[i], '$', ...)
3x operator=, operator+= (usually QChar c = ... gets turned into a constructor call, saving a call)
4x operator==

Its fun playing similar games with the number of code paths due to short circuit evaluation and exceptions.

Popular posts from this blog

Drawing Sierpinski's Triangle in Minecraft Using Python

In his keynote at PyCon, Eben Upton, the Executive Director of the Rasberry Pi Foundation, mentioned that not only has Minecraft been ported to the Rasberry Pi, but you can even control it with Python. Since four of my kids are avid Minecraft fans, I figured this might be a good time to teach them to program using Python. So I started yesterday with the goal of programming something cool for Minecraft and then showing it off at the San Francisco Python Meetup in the evening.

The first problem that I faced was that I didn't have a Rasberry Pi. You can't hack Minecraft by just installing the Minecraft client. Speaking of which, I didn't have the Minecraft client installed either ;) My kids always play it on their Nexus 7s. I found an open source Minecraft server called Bukkit that "provides the means to extend the popular Minecraft multiplayer server." Then I found a plugin called RaspberryJuice that implements a subset of the Minecraft Pi modding API for Bukkit s…

Apple: iPad and Emacs

Someone asked my boss's buddy Art Medlar if he was going to buy an iPad. He said, "I figure as soon as it runs Emacs, that will be the sign to buy." I think he was just trying to be funny, but his statement is actually fairly profound.

It's well known that submitting iPhone and iPad applications for sale on Apple's store is a huge pain--even if they're free and open source. Apple is acting as a gatekeeper for what is and isn't allowed on your device. I heard that Apple would never allow a scripting language to be installed on your iPad because it would allow end users to run code that they hadn't verified. (I don't have a reference for this, but if you do, please post it below.) Emacs is mostly written in Emacs Lisp. Per Apple's policy, I don't think it'll ever be possible to run Emacs on the iPad.

Emacs was written by Richard Stallman, and it practically defines the Free Software movement (in a manner of speaking at least). Stal…

ERNOS: Erlang Networked Operating System

I've been reading Dreaming in Code lately, and I really like it. If you're not a dreamer, you may safely skip the rest of this post ;)

In Chapter 10, "Engineers and Artists", Alan Kay, John Backus, and Jaron Lanier really got me thinking. I've also been thinking a lot about Minix 3, Erlang, and the original Lisp machine. The ideas are beginning to synthesize into something cohesive--more than just the sum of their parts.

Now, I'm sure that many of these ideas have already been envisioned within Tunes.org, LLVM, Microsoft's Singularity project, or in some other place that I haven't managed to discover or fully read, but I'm going to blog them anyway.

Rather than wax philosophical, let me just dump out some ideas:Start with Minix 3. It's a new microkernel, and it's meant for real use, unlike the original Minix. "This new OS is extremely small, with the part that runs in kernel mode under 4000 lines of executable code." I bet it&…