Related
Consider the following code snippet
#include <vector>
#include <cstdlib>
void __attribute__ ((noinline)) calculate1(double& a, int x) { a += x; };
void __attribute__ ((noinline)) calculate2(double& a, int x) { a *= x; };
void wrapper1(double& a, int x) { calculate1(a, x); }
void wrapper2(double& a, int x) { calculate2(a, x); }
typedef void (*Func)(double&, int);
int main()
{
std::vector<std::pair<double, Func>> pairs = {
std::make_pair(0, (rand() % 2 ? &wrapper1 : &wrapper2)),
std::make_pair(0, (rand() % 2 ? &wrapper1 : &wrapper2)),
};
for (auto& [a, wrapper] : pairs)
(*wrapper)(a, 5);
return pairs[0].first + pairs[1].first;
}
With -O3 optimization the latest gcc and clang versions do not optimize the pointers to wrappers to pointers to underlying functions. See assembly here at line 22:
mov ebp, OFFSET FLAT:wrapper2(double&, int) # tmp118,
which results later in call + jmp, instead of just call had the compiler put a pointer to the calculate1 instead.
Note that I specifically asked for no-inlined calculate functions to illustrate; doing it without noinline results in another flavour of non-optimization where compiler will generate two identical functions to be called by pointer (so still won't optimize, just in a different fashion).
What am I missing here? Is there any way to guide the compiler short of manually plugging in the correct functions (without wrappers)?
Edit 1. Following suggestions in the comments, here is a disassembly with all functions declared static, with exactly the same result (call + jmp instead of call).
Edit 2. Much simpler example of the same pattern:
#include <vector>
#include <cstdlib>
typedef void (*Func)(double&, int);
static void __attribute__ ((noinline)) calculate(double& a, int x) { a += x; };
static void wrapper(double& a, int x) { calculate(a, x); }
int main() {
double a = 5.0;
Func f;
if (rand() % 2)
f = &wrapper; // f = &calculate;
else
f = &wrapper;
f(a, 0);
return 0;
}
gcc 8.2 successfully optimizes this code by throwing pointer to wrapper away and storing &calculate directly in its place (https://gcc.godbolt.org/z/nMIBeo). However changing the line as per comment (that is, performing part of the same optimization manually) breaks the magic and results in pointless jmp.
You seem to be suggesting that &calculate1 should be stored in the vector instead of &wrapper1. In general this is not possible: later code might try to compare the stored pointer against &calculate1 and that must compare false.
I further assume that your suggestion is that the compiler might try to do some static analysis and determine that the function pointers values in the vector are never compared for equality with other function pointers, and in fact that none of the other operations done on the vector elements would produce a change in observable behaviour; and therefore in this exact program it could store &calculate1 instead.
Usually the answer to "why does the compiler not perform some particular optimization" is that nobody has conceived of and implemented that idea. Another common reason is that the static analysis involved is, in the general case, quite difficult and might lead to a slowdown in compilation with no benefit in real programs where the analysis could not be guaranteed to succeed.
You are making a lot of assumptions here. Firstly, your syntax. The second is that compilers are perfect in the eye of the beholder and catch everything. The reality is that it is easy to find and hand optimize compiler output, it is not difficult to write small functions to trip up a compiler that you are well in tune with or write a decent size application and there will be places where you can hand tune. This is all known and expected. Then opinion comes in where on my machine my blah is faster than blah so it should have made these instructions instead.
gcc is not a great compiler for performance, on some targets it has been getting worse for a number of major revs. It is pretty good at what it does, better than pretty good, it deals with a number of pre processors/languages has a common middle and a number of backends. Some backends get better optimization applied front to back others are just hanging on for the ride.There were a number of other compilers that could produce code that could easily outperform gcc.
These were mostly pay-for compilers. More than an individual would pay out of pocket: used car prices, sometimes recurring annually.
There are things that gcc can optimize that are simply amazing and times that it totally goes in the wrong direction. Same goes for clang, often they do similar jobs with similar output, sometimes do some impressive things sometimes just go off into the weeds. I now find it more fun to manipulate the optimizer to make it do good or bad things rater than worry about why didn't it do what I "think" it should have done on a particular occasion. If I need that code faster I take the compiled output and hand fix it and use it as an assembly function.
You get what you pay for with gcc, if you were to look deep in its bowels you will find it is barely held together with duct tape and bailing wire (llvm is catching up). But for a free tool it does a simply amazing job, it is so widely used that you can get free support just about anywhere. We are sadly well into a time where folks think that because gcc interprets the language in a certain way that is how the language is defined and sadly that is not remotely true. But so many folks don't try other compilers to find out what "implementation defined" really means.
Last and most important, it's open source, if you want to "fix" an optimization then just do it. Keep that fix for yourself, post it, or try to push it upstream.
Chandler Carruth introduced two functions in his CppCon2015 talk that can be used to do some fine-grained inhibition of the optimizer. They are useful to write micro-benchmarks that the optimizer won't simply nuke into meaninglessness.
void clobber() {
asm volatile("" : : : "memory");
}
void escape(void* p) {
asm volatile("" : : "g"(p) : "memory");
}
These use inline assembly statements to change the assumptions of the optimizer.
The assembly statement in clobber states that the assembly code in it can read and write anywhere in memory. The actual assembly code is empty, but the optimizer won't look into it because it's asm volatile. It believes it when we tell it the code might read and write everywhere in memory. This effectively prevents the optimizer from reordering or discarding memory writes prior to the call to clobber, and forces memory reads after the call to clobber†.
The one in escape, additionally makes the pointer p visible to the assembly block. Again, because the optimizer won't look into the actual inline assembly code that code can be empty, and the optimizer will still assume that the block uses the address pointed by the pointer p. This effectively forces whatever p points to be in memory and not not in a register, because the assembly block might perform a read from that address.
(This is important because the clobber function won't force reads nor writes for anything that the compilers decides to put in a register, since the assembly statement in clobber doesn't state that anything in particular must be visible to the assembly.)
All of this happens without any additional code being generated directly by these "barriers". They are purely compile-time artifacts.
These use language extensions supported in GCC and in Clang, though. Is there a way to have similar behaviour when using MSVC?
† To understand why the optimizer has to think this way, imagine if the assembly block were a loop adding 1 to every byte in memory.
Given your approximation of escape(), you should also be fine with the following approximation of clobber() (note that this is a draft idea, deferring some of the solution to the implementation of the function nextLocationToClobber()):
// always returns false, but in an undeducible way
bool isClobberingEnabled();
// The challenge is to implement this function in a way,
// that will make even the smartest optimizer believe that
// it can deliver a valid pointer pointing anywhere in the heap,
// stack or the static memory.
volatile char* nextLocationToClobber();
const bool clobberingIsEnabled = isClobberingEnabled();
volatile char* clobberingPtr;
inline void clobber() {
if ( clobberingIsEnabled ) {
// This will never be executed, but the compiler
// cannot know about it.
clobberingPtr = nextLocationToClobber();
*clobberingPtr = *clobberingPtr;
}
}
UPDATE
Question: How would you ensure that isClobberingEnabled returns false "in an undeducible way"? Certainly it would be trivial to place the definition in another translation unit, but the minute you enable LTCG, that strategy is defeated. What did you have in mind?
Answer: We can take advantage of a hard-to-prove property from the number theory, for example, Fermat's Last Theorem:
bool undeducible_false() {
// It took mathematicians more than 3 centuries to prove Fermat's
// last theorem in its most general form. Hardly that knowledge
// has been put into compilers (or the compiler will try hard
// enough to check all one million possible combinations below).
// Caveat: avoid integer overflow (Fermat's theorem
// doesn't hold for modulo arithmetic)
std::uint32_t a = std::clock() % 100 + 1;
std::uint32_t b = std::rand() % 100 + 1;
std::uint32_t c = reinterpret_cast<std::uintptr_t>(&a) % 100 + 1;
return a*a*a + b*b*b == c*c*c;
}
I have used the following in place of escape.
#ifdef _MSC_VER
#pragma optimize("", off)
template <typename T>
inline void escape(T* p) {
*reinterpret_cast<char volatile*>(p) =
*reinterpret_cast<char const volatile*>(p); // thanks, #milleniumbug
}
#pragma optimize("", on)
#endif
It's not perfect but it's close enough, I think.
Sadly, I don't have a way to emulate clobber.
I'm doing a bit of hands on research surrounding the speed benefits of making a function inline. I don't have the book with me, but one text I was reading, was suggesting a fairly large overhead cost to making function calls; and when ever executable size is either negligible, or can be spared, a function should be declared inline, for speed.
I've written the following code to test this theory, and from what I can tell, there is no speed benifit from declaring a function as inline. Both functions, when called 4294967295 times, on my computer, execute in 196 seconds.
My question is, what would be your thoughts as to why this is happening? Is it modern compiler optimization? Would it be the lack of large calculations taking place in the function?
Any insight on the matter would be appreciated. Thanks in advance friends.
#include < iostream >
#include < time.h >
// RESEARCH Jared Thomson 2010
////////////////////////////////////////////////////////////////////////////////
// Two functions that preform an identacle arbitrary floating point calculation
// one function is inline, the other is not.
double test(double a, double b, double c);
double inlineTest(double a, double b, double c);
double test(double a, double b, double c){
a = (3.1415 / 1.2345) / 4 + 5;
b = 9.999 / a + (a * a);
c = a *=b;
return c;
}
inline
double inlineTest(double a, double b, double c){
a = (3.1415 / 1.2345) / 4 + 5;
b = 9.999 / a + (a * a);
c = a *=b;
return c;
}
// ENTRY POINT Jared Thomson 2010
////////////////////////////////////////////////////////////////////////////////
int main(){
const unsigned int maxUINT = -1;
clock_t start = clock();
//============================ NON-INLINE TEST ===============================//
for(unsigned int i = 0; i < maxUINT; ++i)
test(1.1,2.2,3.3);
clock_t end = clock();
std::cout << maxUINT << " calls to non inline function took "
<< (end - start)/CLOCKS_PER_SEC << " seconds.\n";
start = clock();
//============================ INLINE TEST ===================================//
for(unsigned int i = 0; i < maxUINT; ++i)
test(1.1,2.2,3.3);
end = clock();
std::cout << maxUINT << " calls to inline function took "
<< (end - start)/CLOCKS_PER_SEC << " seconds.\n";
getchar(); // Wait for input.
return 0;
} // Main.
Assembly Output
PasteBin
The inline keyword is basically useless. It is a suggestion only. The compiler is free to ignore it and refuse to inline such a function, and it is also free to inline a function declared without the inline keyword.
If you are really interested in doing a test of function call overhead, you should check the resultant assembly to ensure that the function really was (or wasn't) inlined. I'm not intimately familiar with VC++, but it may have a compiler-specific method of forcing or prohibiting the inlining of a function (however the standard C++ inline keyword will not be it).
So I suppose the answer to the larger context of your investigation is: don't worry about explicit inlining. Modern compilers know when to inline and when not to, and will generally make better decisions about it than even very experienced programmers. That's why the inline keyword is often entirely ignored. You should not worry about explicitly forcing or prohibiting inlining of a function unless you have a very specific need to do so (as a result of profiling your program's execution and finding that a bottleneck could be solved by forcing an inline that the compiler has for some reason not done).
Re: the assembly:
; 30 : const unsigned int maxUINT = -1;
; 31 : clock_t start = clock();
mov esi, DWORD PTR __imp__clock
push edi
call esi
mov edi, eax
; 32 :
; 33 : //============================ NON-INLINE TEST ===============================//
; 34 : for(unsigned int i = 0; i < maxUINT; ++i)
; 35 : blank(1.1,2.2,3.3);
; 36 :
; 37 : clock_t end = clock();
call esi
This assembly is:
Reading the clock
Storing the clock value
Reading the clock again
Note what's missing: calling your function a whole bunch of times
The compiler has noticed that you don't do anything with the result of the function and that the function has no side-effects, so it is not being called at all.
You can likely get it to call the function anyway by compiling with optimizations off (in debug mode).
Both the functions could be inlined. The definition of the non-inline function is in the same compilation unit as the usage point, so the compiler is within its rights to inline it even without you asking.
Post the assembly and we can confirm it for you.
EDIT: the MSVC compiler pragma for banning inlining is:
#pragma auto_inline(off)
void myFunction() {
// ...
}
#pragma auto_inline(on)
Two things could be happening:
The compiler may either be inlining both or neither functions. Check your compiler documentation for how to control that.
Your function may be complex enough that the overhead of doing the function call isn't big enough to make a big difference in the tests.
Inlining is great for very small functions but it's not always better. Code bloat can prevent the CPU from caching code.
In general inline getter/setter functions and other one liners. Then during performance tuning you can try inlining functions if you think you'll get a boost.
Your code as posted contains a couple oddities.
1) The math and output of your test functions are completely independent of the function parameters. If the compiler is smart enough to detect that those functions always return the same value, that might give it incentive to optimize them out entirely inline or not.
2) Your main function is calling test for both the inline and non-inline tests. If this is the actual code that you ran, then that would have a rather large role to play in why you saw the same results.
As others have suggested, you would do well to examine the actual assembly code generated by the compiler to determine that you're actually testing what you intended to.
Um, shouldn't
//============================ INLINE TEST ===================================//
for(unsigned int i = 0; i < maxUINT; ++i)
test(1.1,2.2,3.3);
be
//============================ INLINE TEST ===================================//
for(unsigned int i = 0; i < maxUINT; ++i)
inlineTest(1.1,2.2,3.3);
?
But if that was just a typo, would recommend that look at a dissassembler or reflector to see if the code is actually inline or still stack-ed.
If this test took 196 seconds for each loop, then you must not have turned optimizations on; with optimizations off, generally compilers don't inline anything.
With optimization on, however, the compiler is free to notice that your test function can be completely evaluated at compile time, and crush it down to "return [constant]" -- at which point, it may well decide to inline both functions since they're so trivial, and then notice that the loops are pointless since the function value is not used, and squash that out too! This is basically what I got when I tried it.
So either way, you're not testing what you thought you tested.
Function call overhead ain't what it used to be, compared to the overhead of blowing out the level-1 instruction cache, which is what aggressive inlining does to you. You can easily find reports online of gcc's -Os option (optimize for size) being a better default choice for large projects than -O2, and the big reason for that is that -O2 inlines a lot more aggressively. I would expect it is much the same with MSVC.
The only way I know of to guarantee a function is inline is to #define it
For example:
#define RADTODEG(x) ((x) * 57.29578)
That said, the only time I would bother with such a function would be in an embedded system. On a desktop/server the performance difference is negligible.
Run it in a debugger and have a look at the generated code to see if your function is always or never inlined. I think it's always a good idea to have a look at the assembler code when you want more knowledge about the optimization the compiler does.
Apologies for a small flame ...
Compilers think in assembly language. You should too. Whatever else you do, just step through the code at the assembler level. Then you'll know exactly what the compiler did.
Don't think of performance in absolute terms like "fast" or "slow". It's all relative, percentage-wise. The way software is made fast is by removing, in successive steps, things that take too large a percent of the time.
Here's the flame: If a compiler can do a pretty good job of inlining functions that clearly need it, and if it can do a really good job of managing registers, I think that's just what it should do. If it can do a reasonable job of unrolling loops that clearly could use it, I can live with that. If it's knocking itself out trying to outsmart me by removing function calls that I clearly wrote and intended to be called, or scrambling my code sanctimoniously trying to save a JMP when that JMP occupies 0.000001% of running time (the way Fortran does), I get annoyed, frankly.
There seems to be a notion in the compiler world that there's no such thing as an unhelpful optimization. No matter how smart the compiler is, real optimization is the programmer's job, and nobody else's.
I'm thinking of using pure/const functions more heavily in my C++ code. (pure/const attribute in GCC)
However, I am curious how strict I should be about it and what could possibly break.
The most obvious case are debug outputs (in whatever form, could be on cout, in some file or in some custom debug class). I probably will have a lot of functions, which don't have any side effects despite this sort of debug output. No matter if the debug output is made or not, this will absolutely have no effect on the rest of my application.
Or another case I'm thinking of is the use of some SmartPointer class which may do some extra stuff in global memory when being in debug mode. If I use such an object in a pure/const function, it does have some slight side effects (in the sense that some memory probably will be different) which should not have any real side effects though (in the sense that the behaviour is in any way different).
Similar also for mutexes and other stuff. I can think of many complex cases where it has some side effects (in the sense of that some memory will be different, maybe even some threads are created, some filesystem manipulation is made, etc) but has no computational difference (all those side effects could very well be left out and I would even prefer that).
So, to summarize, I want to mark functions as pure/const which are not pure/const in a strict sense. An easy example:
int foo(int) __attribute__((const));
int bar(int x) {
int sum = 0;
for(int i = 0; i < 100; ++i)
sum += foo(x);
return sum;
}
int foo_callcounter = 0;
int main() {
cout << "bar 42 = " << bar(42) << endl;
cout << "foo callcounter = " << foo_callcounter << endl;
}
int foo(int x) {
cout << "DEBUG: foo(" << x << ")" << endl;
foo_callcounter++;
return x; // or whatever
}
Note that the function foo is not const in a strict sense. Though, it doesn't matter what foo_callcounter is in the end. It also doesn't matter if the debug statement is not made (in case the function is not called).
I would expect the output:
DEBUG: foo(42)
bar 42 = 4200
foo callcounter = 1
And without optimisation:
DEBUG: foo(42) (100 times)
bar 42 = 4200
foo callcounter = 100
Both cases are totally fine because what only matters for my usecase is the return value of bar(42).
How does it work out in practice? If I mark such functions as pure/const, could it break anything (considering that the code is all correct)?
Note that I know that some compilers might not support this attribute at all. (BTW., I am collecting them here.) I also know how to make use of thes attributes in a way that the code stays portable (via #defines). Also, all compilers which are interesting to me support it in some way; so I don't care about if my code runs slower with compilers which do not.
I also know that the optimised code probably will look different depending on the compiler and even the compiler version.
Very relevant is also this LWN article "Implications of pure and constant functions", especially the "Cheats" chapter. (Thanks ArtemGr for the hint.)
I'm thinking of using pure/const functions more heavily in my C++ code.
That’s a slippery slope. These attributes are non-standard and their benefit is restricted mostly to micro-optimizations.
That’s not a good trade-off. Write clean code instead, don’t apply such micro-optimizations unless you’ve profiled carefully and there’s no way around it. Or not at all.
Notice that in principle these attributes are quite nice because they state implied assumptions of the functions explicitly for both the compiler and the programmer. That’s good. However, there are other methods of making similar assumptions explicit (including documentation). But since these attributes are non-standard, they have no place in normal code. They should be restricted to very judicious use in performance-critical libraries where the author tries to emit best code for every compiler. That is, the writer is aware of the fact that only GCC can use these attributes, and has made different choices for other compilers.
You could definitely break the portability of your code. And why would you want to implement your own smart pointer - learning experience apart? Aren't there enough of them available for you in (near) standard libraries?
I would expect the output:
I would expect the input:
int bar(int x) {
return foo(x) * 100;
}
Your code actually looks strange for me. As a maintainer I would think that either foo actually has side effects or more likely rewrite it immediately to the above function.
How does it work out in practice? If I mark such functions as pure/const, could it break anything (considering that the code is all correct)?
If the code is all correct then no. But the chances that your code is correct are small. If your code is incorrect then this feature can mask out bugs:
int foo(int x) {
globalmutex.lock();
// complicated calculation code
return -1;
// more complicated calculation
globalmutex.unlock();
return x;
}
Now given the bar from above:
int main() {
cout << bar(-1);
}
This terminates with __attribute__((const)) but deadlocks otherwise.
It also highly depends on the implementation. For example:
void f() {
for(;;)
{
globalmutex.unlock();
cout << foo(42) << '\n';
globalmutex.lock();
}
}
Where the compiler should move the call foo(42)? Is it allowed to optimize this code? Not in general! So unless the loop is really trivial you have no benefits of your feature. But if your loop is trivial you can easily optimize it yourself.
EDIT: as Albert requested a less obvious situation, here it comes:
F
or example if you implement operator << for an ostream, you use the ostream::sentry which locks the stream buffer. Suppose you call pure/const f after you released or before you locked it. Someone uses this operator cout << YourType() and f also uses cout << "debug info". According to you the compiler is free to put the invocation of f into the critical section. Deadlock occurs.
I would examine the generated asm to see what difference they make. (My guess would be that switching from C++ streams to something else would yield more of a real benefit, see: http://typethinker.blogspot.com/2010/05/are-c-iostreams-really-slow.html )
I think nobody knows this (with the exception of gcc programmers), simply because you rely on undefined and undocumented behaviour, which can change from version to version. But how about something like this:
#ifdef NDEBUG \
#define safe_pure __attribute__((pure)) \
#else \
#define safe_pure \
#endif
I know it's not exactly what you want, but now you can use the pure attribute without breaking the rules.
If you do want to know the answer, you may ask in the gcc forums (mailing list, whatever), they should be able to give you the exact answer.
Meaning of the code: When NDEBUG (symbol used in assert macros) is defined, we don't debug, have no side effects, can use pure attribute. When it is defined, we have side effects, so it won't use pure attribute.
This question already has answers here:
Closed 13 years ago.
Possible Duplicate:
Can a recursive function be inline?
What are the trade offs of making recursive functions inline.
Recursive functions that can be optimised by tail-end recursion can certainly be inlined. If the last thing a function does is call itself, then it can be converted into a plain loop.
Arbitrary recursive functions can't be inlined for the same reason a snake can't swallow its own tail.
[Edit: just noticed that although your title says "be inlined", your actual question says "making functions inline". The two effectively have nothing to do with one another, they just have confusingly similar names. In modern compilers, the primary effect of inline is the thing that originally in C99 was (I think) just a necessary detail to make inline work at all: to permit multiple definitions of a symbol with external linkage. That's because modern compilers don't pay a whole lot of attention to the programmer's opinion of whether a function should be inlined. They do pay some, though, so the confusion of concepts persists. I've answered the question in the title, which is the decision the compiler makes, not the question in the body, which is the decision the programmer makes.]
Inlining is not necessarily an all-or-nothing deal. One strategy which compilers use to decide whether to inline, is to keep inlining function calls until the resulting code is "too big". "Big" is defined by some hopefully sensible heuristic.
So consider the following recursive function (which deliberately is not simply tail-recursive):
int triangle(int n) {
if (n == 1) return 1;
return n + triangle(n-1);
}
If it's called like this:
int t100() {
return triangle(100);
}
Then there's no particular reason in principle that the usual rules that the compiler uses for inlining shouldn't result in this:
int t100() {
// inline call to triangle(100)
int result;
if (100 == 1) { result = 1; } else {
// inline call to triangle(99)
int t99;
if (100 - 1 == 1) { t99 = 1; } else {
// inline call to triangle(98)
int t98;
if (100 - 1 - 1 == 1) { t98 = 1; } else {
// oops, "too big", no more inlining
t98 = triangle(100 - 1 - 1 - 1) + 98;
}
t99 = t98 + 99;
}
result = t99 + 100;
}
return result;
}
Obviously the optimiser will have a field day with that, so it's much "smaller" than it looks:
int t100() {
return triangle(97) + 297;
}
The code in triangle itself could be "unrolled" a few steps by a few levels of inlining, in exactly the same way, except that it doesn't have the benefits of constants:
int triangle(int n) {
if (n == 1) return 1;
if (n == 2) return 3;
if (n == 3) return 6;
return triangle(n-3) + 3*n - 3;
}
I doubt whether compilers actually do this, though, I don't think I've ever noticed it [Edit: MSVC does if you tell it to, thanks peterchen].
There's an obvious potential benefit in saving call overhead, but as against that people don't really expect recursive functions to get inlined, and there's no particular guarantee that the usual inlining heuristics will perform well with recursive functions (where there are two different places, the call site and the recursive call, that might be inlined, with different benefits in each case). Furthermore, it's difficult at compile time to estimate how deep the recursion will go, and the inline heuristics might like to take account of the call depth to make decisions. So it may be that the compiler just doesn't bother.
Functional language compilers are typically a lot more aggressive dealing with recursion than C or C++ compilers. The relevant trade-off there is that so many functions written in functional languages are recursive, that performance might be hopeless if the compiler couldn't optimise tail-recursion. So Lisp programmers typically rely on good optimisation of recursive functions, whereas C and C++ programmers typically don't.
If your compiler does not support it, you can try manually inlining instead...
int factorial(int n) {
int result = 1;
if (n-- == 0) {
return result;
} else {
result *= 1;
if (n-- == 0) {
return result;
} else {
result *= 2;
if (n-- == 0) {
return result;
} else {
result *= 3;
if (n-- == 0) {
return result;
} else {
result *= 4;
if (n-- == 0) {
return result;
} else {
// ...
}
}
}
}
}
}
See the problem yet?
Tail recursion (a special case of recursion) it's possible to be inlined by smart compilers.
Now, hold on. A tail-recursive function could be unrolled and inlined pretty easily. Apparently there are compilers that do this, but I am not aware of specifics.
Of course. Any function can be inlined if it makes sense to do it:
int f(int i)
{
if (i <= 0) return 1;
else return i * f(i - 1);
}
int main()
{
return f(10);
}
pseudo assembly (f is inlined in main):
main:
mov r0, #10 ; Pass 10 to f
f:
cmp r0, #0 ; arg <= 0? ...
bge 1l
mov r0, #1 ; ... is so, return 1
ret
1:
mov r0, -(sp) ; if not, save arg.
dec r0 ; pass arg - 1 to f
call f ; just because it's inlined doesn't mean I can't call it.
mul r0, (sp)+ ; compute the result
ret ; done.
;-)
When you call an ordinary function when you change command sequential execution order and jump(call or jmp) into some address where the function resides. Inlining mean that you place in all occurences of this function the commands of this function, so you don't have a one place where you could jump, also other types of optimisations can be used, like elemination of pushing/popping function parameters.
When you know, that the recursive chain will in normal cases be not so long, you could do inlining upto a predefined level (I don't know, if any existing compiler is intelligent enough for this today).
Inlining a recursive function is much like unrolling a loop. You will end up with much duplicate code -- but in some cases it could be worthwhile:
The number of recursive calls (the length of the chain) is normally short (in cases it gets longer than predefined, just do normal recursion)
The overhead for the functions calls is relatively big compared to the logic -- so do some "unrolling" for example five instances and end up doing a recursive call again -- this would lead to saving 80% of the call overhead.
Off course the tail-recursive special-case -- but this was mentioned by others.
Of course can be declared inline. The inline keyword is just a hint to the compiler. In many case the compiler just ignore it and depending on the compiler this could be one of this situatios.
Some compilers cna turn tail recursion into plain loops, and thus inline them normally.
Non-tail recursion could be inlined up to a given depth, usually decided by the compiler.
I've never encountered a practical application for that, as the cost of call isn't high enough anymore to offset the increase in code size.
[edit] (to clarify that: even though I like to toy with these things, and often check what code my compiler generates for "funny stuff" just out of curiosity, I haven't encountered a use case where any such unrolling helped significantly. This doesn't mean they don't exist or couldn't be constructed.
The only place where it would help is precalculating low iterations during compile time. However, in my experience this immensely increases compile times for often negligible runtime performance benefits.
Note that Visual Studio 2008 (and earlier) gives you quite some control over this:
#pragma inline_recursion(on)
#pragma inline_depth(N)
__forceinline
Be careful with the latter, it can easily overload the compiler :)
Inline means that on each place a call to a function marked as inline gets done, the compiler places a copy of the said function code there. This avoids function calling mechanisms, and it's usual argument stack pushing-poping, saving time in gazillion-calls-per-second situations. You see the consequences to static variables and stuff like that? all gone...
So, if you had an inlined recursive call, either your compiler is super smart and figures whether the number of copies is deterministic, of it will say "Cannot make it inline", because it wouldn't know when to stop.