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Shouldn't inline functions be faster?

16 posts in this topic

So there is an exercise that says I should create a program to measure which function is faster,a normal one,or a inline one.

 

Here's the code:

 


#include <iostream>
#include <string>
#include <assert.h>

#include <ctime>
using namespace std;

clock_t t;
inline void f1(){
	t=clock()-t;
	if(t!=0)
	cout << "f1 " << t << endl;
}

void f2(){
    t=clock()-t;
	if(t!=0)
	cout <<"f2 " << t << endl;
}
int main() {
	for(int i = 0;i<10000;i++){
		t=clock();
		f1();
	}
	for(int i = 0;i<10000;i++){
		t=clock();
		f2();
	}

} 

 

 

The problem is f1 appears 4 times and f2 appears 2 times.As you can see I set the functions so they'll show text only if the time difference is bigger than 0.

 

Why does the inline function get executed slower?! Shouldn't it be faster? 

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It should be faster, not by much but a little, if you call them lots of times you should see a slight increase in performance.  However the compiler is free to chose to inline or not to inline, whether you declare it as inline or not.  For example with optimisations on, your compiler may inline functions that you didn't declare inline.  And also choose to not inline if you declared as inline.

 

That said, your clock probably doesn't have the accuracy needed for this test.  Try using c++11 high_precision_clock instead ( <chrono> ).  Or if not using c++11, try win32 clock QUERY functions.

Edited by EddieV223
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That said, your clock probably doesn't have the accuracy needed for this test. Try using c++11 high_precision_clock instead ( ). Or if not using c++11, try win32 clock QUERY functions.

 

If you're using Visual Studio 12 then high_precision_clock won't work unless they've fixed it in one of the updates. It shipped with a placeholder version that used a low-resolution clock. If you're on Windows/Visual Studio 12, you should use QueryPerformanceCounter instead.

 

Somewhat related are the contents of this thread, in particular my last post and my post preceding it touch a little bit on why inlining and other micro-optimizations aren't so simple to reason about, and from there you can infer why its a good thing that the inline keyword is only a hint to the compiler, rather than a direct command.

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I did my own test a while back on inlining functions, compiled a similar test exe, and disassembled it to look at the actual output from the compiler - only to find that the code, for both functions, and for calling them, was identical.. eg: there was no discernable difference between using inline and not inline at the machine code level.. All of this I assume is the result of what everybody is saying about it here.

 

So, I just #define function-like macros to produce virtually the same result as inlining functions. At least, from what I've read, it's virtually the same as inlining functions. Either way, it works how I want it to at the machine level.

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If you are using Microsoft visual studio you can force use '__forceinline' keyword, meanwhile 'inline' just gives the compiler a "hint". Inlining a function removes the call/ret overhead generated by compiler, but creates a larger executable image. The only time i use inlining is small tight loops where the overhead would cost too many cpu cycles, otherwise i rely on the compiler to make the right decision.
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Function inlining can cost you performance because it increases the code size an thus the the strain on the I-cache.

However if used correctly it can significantly increase the performance for two reasons:

1. The compiler can optimize across the boundaries of the function. In your case you won't be seeing this, because there is virtually nothing outside the function except for the loop.

2. You don't need the the instructions for calling the function, passing parameters, creating a stack frame, etc. You only see a benefit from this, if this overhead is actually a large percentage of what the function does. This is typically the case for getters and setters which would boil down to about 1 instruction if not for that overhead. However in your case, you are doing a syscall in that function worth a couple of thousand instructions, so the overhead of not inlining is insignificant.

 

Bottom line, in the scenario you chose, inlining should not give you any visible performance advantage.

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You shouldn't treat inline as a performance directive.  Much like the old auto and register keywords, its initial use is pretty much depricated.  Rather think of it in terms of a linkage directive, like extern, or static.  The inline keyword allows you to define a function in the header, and it quite useful in that regard.  As far as performance concerns, modern compilers with global optimizations do not need a hint to know when or when not to considering inlining.

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If you are using Microsoft visual studio you can force use '__forceinline' keyword

Actually you can't. Even __forceinline is just a strong suggestion. From the MSDN documentation on __forceinline:

The compiler treats the inline expansion options and keywords as suggestions. There is no guarantee that functions will be inlined. You cannot force the compiler to inline a particular function, even with the __forceinline keyword.


It's a very strong suggestion. It overrides the compiler's own analysis and it will do it unless it's impossible to inline the function, for example if it is recursive or virtual.

Because it is such a strong suggestion __forceinline should be used with caution and after profiling. Unlike inline itself which means nothing but 'I put this function in a header'.

For this test you may very well need __forceinline and possibly __declspec(noinline) too. Check in the debugger that it's doing what you expect.

The method of timing looks unreliable. It's most likely random whether a tick occurs between those two points. Try timing the whole loop instead.
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Inspecting the compiled assembler code should be the first step of any performance test: if the code is identical there is nothing to measure.
With GCC, you can simply pass option -S to get assembler code.

You should also test completely separate programs, not one program with two functions: the compiler could do something different because there are two identical functions in the same compilation unit.
The test program can be written as 
INLINESPEC void testFunction(){
...
}

int main (){
...
testFunction()
...
}
 
You can define INLINESPEC as "inline", an empty string, or the fancy compiler-specific attributes through compiler commandline options to get each variant.
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To understand why inlining sometimes is faster and sometimes isn't you need to have a solid understanding of what a C++ compiler is doing under the hood and how your code is transformed into assembler. There can be quite a few surprises happening when you look at the assembler code of a particular function, as a single float divide can spawn SSE2 assembler if compiling with that option on, even in non vectorised code.

 

http://www.altdevblogaday.com/2013/01/05/cc-low-level-curriculum-part-10-user-defined-types/ the articles referenced in this link will give you a good explanation of what is going on under the hood if you want to know more.

 

Inspecting the compiled assembler code should be the first step of any performance test: if the code is identical there is nothing to measure.
With GCC, you can simply pass option -S to get assembler code.

You should also test completely separate programs, not one program with two functions: the compiler could do something different because there are two identical functions in the same compilation unit.
The test program can be written as 

INLINESPEC void testFunction(){
...
}

int main (){
...
testFunction()
...
}
 
You can define INLINESPEC as "inline", an empty string, or the fancy compiler-specific attributes through compiler commandline options to get each variant.

Actually it shouldn't be this works for this case but if you find that your application is running slow the first thing you should do is run a profiler and find where the hotspot is in your application. Looking at the generated assembler is a last resort as a C++ compiler for an out-of-order CPU is better at optimising this then you are, on inline CPU's hand optimised assembler can be faster. Nowadays optimisations are more about data locality then about instruction level otimisations, hitting a cache miss is more costly than having a slightly unoptimised instruction order, especially on out-of-order CPU's. The compiler is not the only place where optimisations to your code happen even during runtime a modern CPU is reordering the way your instuctions are issued to the ALU and you have no control over this.

 

http://channel9.msdn.com/Shows/Going+Deep/Cpp-and-Beyond-2012-Herb-Sutter-atomic-Weapons-1-of-2 even though this presentation is mostly about Parallel programming in the first bit he tells you what happens after your code is compiled and run on a modren CPU and wich transformations it can apply to that code.

Edited by NightCreature83
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The problem is f1 appears 4 times and f2 appears 2 times.As you can see I set the functions so they'll show text only if the time difference is bigger than 0.
 
Why does the inline function get executed slower?! Shouldn't it be faster? 

 

Disregarding all the issues others have mentioned about the way you measure time, doesn't that result say that the inlined f2 executes faster then f1?

The less times time difference is > 0, the faster the loop runs...

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The problem is f1 appears 4 times and f2 appears 2 times.As you can see I set the functions so they'll show text only if the time difference is bigger than 0.

Why does the inline function get executed slower?! Shouldn't it be faster?


Disregarding all the issues others have mentioned about the way you measure time, doesn't that result say that the inlined f2 executes faster then f1?
The less times time difference is > 0, the faster the loop runs...

That would be a good point if f2 were the inlined one.
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