Enabling -Winline on my project produces a whole lot of output which I don't really understand. Does anyone know how to use this output to figure out why my particular function wasn't inlined?
Well, according to my gcc man page...
The compiler uses a variety of
heuristics to determine whether or not
to inline a function. For example,
the compiler takes into account the
size of the function being inlined and
the amount of inlining that has
already been done in the current
function. Therefore, seemingly
insignificant changes in the source
program can cause the warnings
produced by -Winline to appear or
disappear.
I don't believe that you can force the compiler to inline your function; it's an implementation detail that could even change when the compiler is updated. Besides, as long as the compiler's choice causes your function to run faster, is there any particular reason that you care whether the function is actually inlined or not?
Of course, if you really want to inline your function for some reason, you could probably just use a macro to do so.
Related
How can I force the inlining of a function, but define it in a C++ file ?
This is a question that's been asked in the past, for example here: Moving inline methods from a header file to a .cpp files
The answers there, in short, go as follows: "inline used to mean [remove function call overhead at the expense of .text size], now it means [relax ODR], so don't use inline for anything that's not ODR related, the compiler knows better".
I'm aware of that, however in my somewhat exotic case, I don't care about performance.
I'm programming an embedded device and, should someone break through the other layers of security, I want to make it as obnoxious as possible to reverse engineer this part of the code, and one thing this implies is that I don't want function calls (that aren't called numerous times anyway) to expose the function boundaries, which are natural delimitations of pieces of code that achieve something on their own.
However, I would also like to keep my code orderly and not have code in my header files.
I see that I can use __attribute((force_inline)) to force inlining, but then I get warnings if those functions don't have an inline attribute too: warning: always_inline function might not be inlinable [-Wattributes]
Suppressing the attributes warning is an option, but I'd rather only take it once I'm sure there are no clean way to do this.
Hence the question: how can I have a forcibly inlined function whose declaration is in a header, but definition is in a source file, without suppressing all attributes warnings ? Is that impossible ?
Inlining can only be asked. Sometimes a bit forcefully. But you can never guarantee that the function WILL be inlined finally - because reasons, sometimes quite obscure ones.
Here what's MSVC documentation says (I've highlighted the important parts):
The compiler treats the inline expansion options and keywords as suggestions. There's no guarantee that functions will be inlined. You can't force the compiler to inline a particular function, even with the __forceinline keyword. When compiling with /clr, the compiler won't inline a function if there are security attributes applied to the function.
C++ standard says:
No matter how you designate a function as inline, it is a request that the compiler is allowed to ignore: the compiler might inline-expand some, all, or none of the places where you call a function designated as inline.
GCC documentation is a bit less crystal-clear about non-inlinable functions, but cases exists anyway.
The only "real" way to force inlining is quite ugly, since it rely on inlining it before compilation... Yeah, old-style preprocessor macros. The Evil Itself. Or by using a dirty hack with a #include replacing the function call (and inserting C++ code instead)... It may be a bit safer than a macro, regarding double evaluations, but other side-effects can be even worse since it must rely on "global" variables to work.
Does it worth the pain? Probably not. In particular for "obfuscation", because it won't be as "secure" as you think it will be. Yes, an explicit function call is easier to trace. But it won't change anything: reverse engineering don't rely on that to be done. In fact, obfuscation is near never a good (or even working...) solution. I used to think that... a long, very long time ago. I proved to myself that it was near useless. On my own "secured" code. Breaking the code took me much less time than it took me to "protect" it...
It is often said a function can be inlined only if it is known at the compile time how it will be called, or something to that effect. (pls correct/clarify if I am wrong)
So say if I have a function like this
void Calling()
{
if (m_InputState == State::Fast) //m_inputstate is a class member and is set by user
{
CallFastFunction();
}
else if (m_InputState == State::Slow)
{
CallSlowFunction();
}
}
as m_inputstate is set by end-user, can we say this variable is not known at the compile time hence calling() cant be inlined?
Inlining has nothing to do with the function input being known at compile time.
The compiler can (and almost certainly WILL) inline your Calling function. Depending on the availability of the source of CallFastFunction and CallSlowFunction, these may also be inlined.
If the compiler can determine the value of m_InputState, it will remove the if - but only if it's definitive that the value is one value.
For example, thing.m_InputState = State::Slow; thing.Calling(); will only compile in the "slow" call, without any conditonal code, where std::cin >> thing.m_InputState; thing.Calling(); will of course not.
If, through profile-based optimisation, the compiler knows how often each of the cases are chosen, it may also select which path ends up "next" in the order of the code, such that the most likely comes first (and it may also provide prefix or other indication to the processor that "you're likely going this way".
But inlining itself happens based on:
The code itself being available.
The size and number of calls to the function.
Arbitrary compiler decisions that we can't know (unless we're very familiar with the source-code of the compiler).
Modern compilers also support "link time optimisation", where the object files produced are only "half-done", such that the linker will actually produce the final code, and it can move code around and inline, just the same as the old-fashioned compiler, on the entire code that makes up the executable (all the code using link-time optimisation), allowing code that didn't have the same source file or not in a header file to still be inlined.
The fact that m_InputState is not known does not stop Calling() from being inlined.
Typically, if you want a function to be inlinable, it has to be declared in the same .cpp file that it is called in (no function prototypes). This depends a bit on the compiler though.
I have question when I compile an inline function in C++.
Can a recursive function work with inline. If yes then please describe how.
I am sure about loop can't work with it but I have read somewhere recursive would work, If we pass constant values.
My friend send me some inline recursive function as constant parameter and told me that would be work but that not work on my laptop, no error at compile time but at run time display nothing and I have to terminate it by force break.
inline f(int n) {
if(n<=1)
return 1;
else {
n=n*f(n-1);
return n;
}
}
how does this work?
I am using turbo 3.2
Also, if an inline function code is too large then, can the compiler change it automatically in normal function?
thanks
This particular function definitely can be inlined. That is because the compiler can figure out that this particular form of recursion (tail-recursion) can be trivially turned into a normal loop. And with a normal loop it has no problem inlining it at all.
Not only can the compiler inline it, it can even calculate the result for a compile-time constant without generating any code for the function.
With GCC 4.4
int fac = f(10);
produced this instruction:
movl $3628800, 4(%esp)
You can easily verify when checking assembly output, that the function is indeed inlined for input that is not known at compile-time.
I suppose your friend was trying to say that if given a constant, the compiler could calculate the result entirely at compile time and just inline the answer at the call site. c++0x actually has a mechanism for this called constexpr, but there are limits to how complex the code is allowed to be. But even with the current version of c++, it is possible. It depends entirely on the compiler.
This function may be a good candidate given that it clearly only references the parameter to calculate the result. Some compilers even have non-portable attributes to help the compiler decide this. For example, gcc has pure and const attributes (listed on that page I just linked) that inform the compiler that this code only operates on the parameters and has no side effects, making it more likely to be calculated at compile time.
Even without this, it will still compile! The reason why is that the compiler is allowed to not inline a function if it decides. Think of the inline keyword more of a suggestion than an instruction.
Assuming that the compiler doesn't calculate the whole thing at compile time, inlining is not completely possible without other optimizations applied (see EDIT below) since it must have an actual function to call. However, it may get partially inlined. In that case the compiler will inline the initial call, but also emit a regular version of the function which will get called during recursion.
As for your second question, yes, size is one of the factors that compilers use to decide if it is appropriate to inline something.
If running this code on your laptop takes a very long time, then it is possible that you just gave it very large values and it is simply taking a long time to calculate the answer... The code look ok, but keep in mind that values above 13! are going to overflow a 32-bit int. What value did you attempt to pass?
The only way to know what actually happens is to compile it an look at the assembly generated.
PS: you may want to look into a more modern compiler if you are concerned with optimizations. For windows there is MingW and free versions of Visual C++. For *NIX there is of course g++.
EDIT: There is also a thing called Tail Recursion Optimization which allows compilers to convert certain types of recursive algorithms to iterative, making them better candidates for inlining. (In addition to making them more stack space efficient).
Recursive function can be inlined to certain limited depth of recursion. Some compilers have an option that lets you to specify how deep you want to go when inlining recursive functions. Basically, the compiler "flattens" several nested levels of recursion. If the execution reaches the end of "flattened" code, the code calls itself in usual recursive fashion and so on. Of course, if the depth of recursion is a run-time value, the compiler has to check the corresponding condition every time before executing each original recursive step inside the "flattened" code. In other words, there's nothing too unusual about inlining a recursive function. It is like unrolling a loop. There's no requirement for the parameters to be constant.
What you mean by "I am sure about loop can't work" is not clear. It doesn't seem to make much sense. Functions with a loop can be easily inlined and there's nothing strange about it.
What are you trying to say about your example that "displays nothing" is not clear either. There is nothing in the code that would "display" anything. No wonder it "displays nothing". On top of that, you posted invalid code. C++ language does not allow function declarations without an explicit return type.
As for your last question, yes, the compiler is completely free to implement an inline function as "normal" function. It has nothing to do with function being "too large" though. It has everything to do with more-or-less complex heuristic criteria used by that specific compiler to make the decision about inlining a function. It can take the size into account. It can take other things into account.
You can inline recursive functions. The compiler normally unrolls them to a certain depth- in VS you can even have a pragma for this, and the compiler can also do partial inlining. It essentially converts it into loops. Also, as #Evan Teran said, the compiler is not forced to inline a function that you suggest at all. It might totally ignore you and that's perfectly valid.
The problem with the code is not in that inline function. The constantness or not of the argument is pretty irrelevant, I'm sure.
Also, seriously, get a new compiler. There's modern free compilers for whatever OS your laptop runs.
One thing to keep in mind - according to the standard, inline is a suggestion, not an absolute guarantee. In the case of a recursive function, the compiler would not always be able to compute the recursion limit - modern compilers are getting extremely smart, a previous response shows the compiler evaluating a constant inline and simply generating the result, but consider
bigint fac = factorialOf(userInput)
there's no way the compiler can figure that one out........
As a side note, most compilers tend to ignore inlines in debug builds unless specifically instructed not to do so - makes debugging easier
Tail recursions can be converted to loops as long as the compiler can satisfactorily rearrange the internal representation to get the recursion conditional test at the end. In this case it can do the code generation to re-express the recursive function as a simple loop
As far as issues like tail recursion rewrites, partial expansions of recursive functions, etc, these are usually controlled by the optimization switches - all modern compilers are capable of pretty signficant optimization, but sometimes things do go wrong.
Remember that the inline key word merely sends a request, not a command to the compiler. The compliler may ignore yhis request if the function definition is too long or too complicated and compile the function as normal function.
in some of the cases where inline functions may not work are
For functions returning values, if a loop, a switch or a goto exists.
For functions not returning values, if a return statement exists.
If function contains static variables.
If in line functions are recursive.
hence in C++ inline recursive functions may not work.
Inline functions are just a request to compilers that insert the complete body of the inline function in every place in the code where that function is used.
But how the compiler decides whether it should insert it or not? Which algorithm/mechanism it uses to decide?
Thanks,
Naveen
Some common aspects:
Compiler option (debug builds usually don't inline, and most compilers have options to override the inline declaration to try to inline all, or none)
suitable calling convention (e.g. varargs functions usually aren't inlined)
suitable for inlining: depends on size of the function, call frequency of the function, gains through inlining, and optimization settings (speed vs. code size). Often, tiny functions have the most benefits, but a huge function may be inlined if it is called just once
inline call depth and recursion settings
The 3rd is probably the core of your question, but that's really "compiler specific heuristics" - you need to check the compiler docs, but usually they won't give much guarantees. MSDN has some (limited) information for MSVC.
Beyond trivialities (e.g. simple getters and very primitive functions), inlining as such isn't very helpful anymore. The cost of the call instruction has gone down, and branch prediction has greatly improved.
The great opportunity for inlining is removing code paths that the compiler knows won't be taken - as an extreme example:
inline int Foo(bool refresh = false)
{
if (refresh)
{
// ...extensive code to update m_foo
}
return m_foo;
}
A good compiler would inline Foo(false), but not Foo(true).
With Link Time Code Generation, Foo could reside in a .cpp (without a inline declararion), and Foo(false) would still be inlined, so again inline has only marginal effects here.
To summarize: There are few scenarios where you should attempt to take manual control of inlining by placing (or omitting) inline statements.
The following is in the FAQ for the Sun Studio 11 compiler:
The compiler generates an inline function as an ordinary callable function (out of line) when any of the following is true:
You compile with +d.
You compile with -g.
The function's address is needed (as with a virtual function).
The function contains control structures the compiler can't generate inline.
The function is too complex.
According to the response to this post by 'clamage45' the "control structures that the compiler can't generate inline" are:
the function contains forbidden constructs, like loop, switch, or goto
Another list can be found here. As most other answers have specified the heuristics are going to be 100% compiler specific, from what I've read I think to ensure that a function is actually inlined you need to avoid:
local static variables
loop constructs
switch statements
try/catch
goto
recursion
and of course too complex (whatever that means)
All I know about inline functions (and a lot of other c++ stuff) is here.
Also, if you're focusing on the heuristics of each compiler to decide wether or not inlie a function, that's implementation dependant and you should look at each compiler's documentation. Keep in mind that the heuristic could also change depending on the level of optimitation.
I'm pretty sure most compilers decide based on the length of the function (when compiled) in bytes and how often it is used vs the optimization type (speed vs size).
I know only couple criteria:
If inline meets recursion - inline will be ignored.
switch/while/for in most cases cause compiler to ignore inline
It depends on the compiler. Here's (the first part of) what the GCC manual says:
-finline-limit=n
By default, GCC limits the size of functions that can be inlined.
This flag allows the control of this limit for functions that are
explicitly marked as inline (i.e., marked with the inline keyword
or defined within the class definition in c++). n is the size of
functions that can be inlined in number of pseudo instructions (not
counting parameter handling). The default value of n is 600.
Increasing this value can result in more inlined code at the cost
of compilation time and memory consumption. Decreasing usually
makes the compilation faster and less code will be inlined (which
presumably means slower programs). This option is particularly
useful for programs that use inlining heavily such as those based
on recursive templates with C++.
Inlining is actually controlled by a number of parameters, which
may be specified individually by using --param name=value. The
-finline-limit=n option sets some of these parameters as follows:
#item max-inline-insns-single
is set to I/2.
#item max-inline-insns-auto
is set to I/2.
#item min-inline-insns
is set to 130 or I/4, whichever is smaller.
#item max-inline-insns-rtl
is set to I.
See below for a documentation of the individual parameters
controlling inlining.
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way, it represents
a count of assembly instructions and as such its exact meaning
might change from one release to an another.
it inserts if you write "inline" to beginning of the function?
I have to check whether a function is being inlined by the compiler. Is there any way to do this without looking at assembly (which I don't read). I have no choice in figuring this out, so I would prefer if we could not discuss the wisdom of doing this. Thanks!
If you enable warnings C4714, C4710, and C4711, it should give you fairly detailed information about which functions are and aren't inlined.
Each call site may potentially be different.
The compiler may decide for certain parent methods it is worth inlining and for other parent methods that it is not worth inlining. Thus you can not actually determine the real answer without examing the assembley at each call site.
As a result any tools you use would potentially give you a misleading answer. If you use a tool that checks for the existance of symbol (it may be there because some call sites need it, but potentially it may be inlined at others). Conversely the lack of the symbol does not mean the method/function is not inlined it may be static (as in file static) and thus the compiler does not need to keep the symbol around (yet it was not inlined).
Using the /FAs compiler option to dump the asm with source code is the only way that I know of to be sure.
Note: if you want to force a function to be inline, just use __forceinline.
Generate a "MAP" file. This gives you the addresses of all non-inlined functions. If your function appears in this list, it's not inlined, otherwise it's either inlined or optimized out entirely (e.g. when it's not called at all).
If you really don't want to jump into assembly, declare the function as __forceinline, and if the executable gets larger, you know it wasn't being inlined.