I always assumed that fortran passed entities "by reference" to a dummy argument. Then I got this answer (the actual argument of the answer was related, but not on this)
The standard never specifies that and,
indeed goes quite a lot out of its way
to avoid such specification. Although
yours is a common misconception, it
was not strictly accurate even in most
older compilers, particularly with
optimization turned on. A strict
pass-by-reference would kill many
common optimizations.
With recent standards,
pass-by-reference is all but
disallowed in some cases. The standard
doesn't use those words in its
normative text, but there are things
that would be impractical to implement
with pass-by-reference.
When you start getting into things
like pointers, the error of assuming
that everything is pass-by-reference
will start making itself more evident
than before. You'll have to drop that
misconception or many things wil
confuse you.
I think other people have answered the
rest of the post adequately. Some also
addressed the above point, but I
wanted to emphasize it.
See here for attribution.
According to this answer, then, there's nothing in the standard specifying how data are shipped from the caller to the callee. In practical terms, how this should be interpreted in terms of actually working with it (regardless of the practical effects resulting from how compilers implement the standard) in particular when it comes to intent() specification?
Edit: I'd like to clarify my question. What I am trying to understand is how the standard expects you to work when you are performing calls. Given that the actual compiler strategy used to pass entities is undefined by the standard, you cannot in principle (according to the standard) expect that passing an argument to a function will actually behave as a "pass-by-reference", with all its related side-effects, because this behavior is compiler and optimization dependent. I assume therefore the standard imposes you a programming style you have to follow to work regardless of the actual implementation strategy.
Yes, that's true. Fortran standard doesn't specify the exact evaluation strategy for different INTENT attributes. For example, for a dummy argument with INTENT(OUT) some Fortran compiler can use call-by-reference or call-by-copy-restore evaluation.
But I do not understand why do you really need to know which evaluation strategy is used? What is the reason? I think Fortran do it right. Humans concern is (high-level) argument intent, and computers concern is (low-level) evaluation strategy. "Render unto man the things which are man’s and unto the computer the things which are the computer’s." N. Wiener
I think the standard contains enough information on the usage of different INTENT attributes. Section "5.1.2.7 INTENT attribute" of Final Committee Draft of Fortran 2003 standard (PDF, 5 MB) is a good starting point.
As the others have said, you don't have to know how the compiler works, as long as it works correctly. Fortran >=90 allows you to specify the purpose of an argument: input, output or both, and then the compiler takes care of passing the argument. As long as the compiler is correctly implemented so that both sides (caller and callee) use the same mechanism, why should the programmer care? So the Fortran standard tries not to limit how compiler writers implement their compilers, allowing them to design and optimize as they wish.
In terms of programming style, decide the purpose of your procedure arguments, and declare them with the appropriate intent. Then, for example, if you accidentally try to modify an intent(in) argument in a procedure, the compiler will issue an error message, preventing you from making this mistake. It might even do this before emitting object code, so the actual argument passing mechanism has nothing to do with enforcing this requirement of the language.
C is a lower-level language and the programmer needs to choose between passing by value or by reference for various reasons. But for problems such as scientific programming this is an unnecessary aspect to have to control. It can be essential in embedded or device driver programming.
If there is a reason that you must control of the argument passing mechanism, such as interfacing to another language, Fortran 2003 provides the ISO C Binding (which is already widely implemented). With this you can match the C method of argument passing.
I'm not sure I understand your question, but one way of seeing things is that as long as one writes standard-conforming code, one does not need to be concerned with how argument passing is implemented. The compiler makes sure (well, barring compiler bugs, of course) that arguments are passed in a way that conforms to the standard.
Now, if you play tricks e.g. with C interop or such, say by comparing argument addresses you might be able to determine whether the compiler uses pass by reference, copy-in/copy-out or something else in some particular case. But generally speaking, at that point you're outside the scope of the standard and all bets are off.
Related
Does compiler make optimzation on data structures when input size is small ?
unordered_set<int>TmpSet;
TmpSet.insert(1);
TmpSet.insert(2);
TmpSet.insert(3);
...
...
Since since is small using hashing would be not required we can simply store this in 3variables. Do optimization like this happen ? If yes who is responsible for them ?
edit: Replaced set with unordered_set as former doesn't do hasing.
Possible in theory to totally replace whole data structures with a different implementation. (As long as escape analysis can show that a reference to it couldn't be passed to separately-compiled code).
But in practice what you've written is a call to a constructor, and then three calls to template functions which aren't simple. That's all the compiler sees, not the high-level semantics of a set.
Unless it's going to optimize stuff away, real compilers aren't going to use a different implementation which would have different object representations.
If you want to micro-optimize like this, in C++ you should do it yourself, e.g. with a std::bitset<32> and set bits to indicate set membership.
It's far too complex a problem for a compiler to reliably do a good job. And besides, there could be serious quality-of-implementation issues if the compiler starts inventing different data-structures that have different speed/space tradeoffs, and it guesses wrong and uses one that doesn't suit the use-case well.
Programmers have a reasonable expectation that their compiled code will work somewhat like what they wrote, at least on a large scale. Including calling those standard header template functions with their implementation of the functions.
Maybe there's some scope for a C++ implementation adapting to the use-case, but I that would need much debate and probably a special mechanism to allow it in practice. Perhaps name-recognition of std:: names could be enough, making those into builtins instead of header implementations, but currently the implementation strategy is to write those functions in C++ in .h headers. e.g. in libstdc++ or libc++.
Type traits are cool and I've used them since they originated in boost a few years ago. However, when you look at their implementation (check out "How is_base_of works?" StackOverflow thread).
Why won't compiler help here? For example, if you want to check if some class is base of another, compiler already knows that, why can't it tell us? This would make things like concepts so much easier to implement and use. You could use language constructs right there.
I am not sure, but I am guessing that it would increase general performance. It is like asking compiler for help, instead of C++ language.
I suspect that the primary reason will sound something like "we need to maintain backwards compatibility" and I agree, but why won't the compiler be more active in generating generic templated code?
Actually... some do.
The thing is that if something can be implemented in pure C++ code, there is no reason, other than simplifying the code, to hard-wire them in the compiler. It is then a matter of trade-off, is the value brought by the code simplification worth the hard-wiring ?
This depends on several points:
correctness (some times software may only partially emulate the trait)
complexity of the code ~~ maintenance burden
performance
...
Once all those points have been weighted, then you can determine whether it's more advantageous to put things in the library or the compiler; and the more likely situation is that you will end up with a mixed strategy: a couple intrinsics in the compiler used as building blocks to provide the required interface in the library.
Note that the maintenance burden is much more significant in a compiler: any C++ developer sufficiently acquainted with the language can delve into a library implementation, whereas the compiler code is a black-box. Therefore, it'll be much easier to debug and patch the library than the compiler, so there is incentive not to put things in the compiler unless you have a very good reason to.
It's hard to give an objective answer here, but I suspect the following.
The code using language quirks to find out this stuff has often already been written (Boost, etc).
The compiler does not have to be changed to implement this if it can be done with language quirks (which saves a lot of time in writing, compiling, debugging and testing).
It's basically a "don't fix what isn't broken" mentality.
Compiler help for type traits has always been a design goal. TR1 formally introduced type traits, and included a section that described acceptable incorrect results in some cases to enable writing the type traits in straight C++. When those type traits were added to C++11 (with some name changes that don't affect their implementation) the allowance for incorrect results was removed, effectively requiring compiler help to implement some of them. And even for those that can be implemented in straight C++, compiler writers prefer intrinsics to complicated templates so that you don't have to put a drip pan under your computer to catch the slag as the overworked compiler causes the computer to melt down.
Does it allow the compiler to inline it, knowing that only functions in the same class can access it? Or is it only for the programmer's convenience?
The compiler can (but is not required to) optimize as you suggest, but that's not the point. The point of access modifiers is to catch certain classes (no pun intended) of programming errors at compile time. Private functions are functions that, if someone called them from outside the class, that would be a bug, and you want to know about it as early as possible.
(Any time you ask the question "could the compiler make optimizations based on this information available to it", the answer is "yes, unless there's a specific rule in the standard that says it's not allowed to" (such as the rules for volatile, whose entire purpose is to inhibit optimizations). However, compilers do not necessarily bother optimizing based on any given piece of information. There is, after all, no requirement for compilers to do any optimization in the first place! How clever your compiler is, nowadays, largely depends on how long you are willing to let it run; MSVC's whole-program PGO mode is capable of inlining through virtual method dispatch -- it guesses the most likely target, and falls back to a regular virtual call at runtime if the guess was wrong -- but slows down compiles by at least a factor of two.)
The access specifiers are a part of C++ mechanism to implement OOP principles of Encapsulation and Abstraction and not optimization for compilers.
Some intelligent compiler can possibly implement some optimization through it but it not enforced to do so by the C++ Standard. The purpose of access specifiers is not Optimization but to facilitate language constructs for principles supported by the C++ language.
I've few questions about C++ compilers
Are C++ compilers required to be one-pass compiler? Does the Standard talk about it anywhere?
In particular, is GCC one-pass compiler? If it is, then why does it generate the following error twice in this example (though the template argument is different in each error message)?
error: declaration of ‘adder<T> item’ shadows a parameter
error: declaration of ‘adder<char [21]> item’ shadows a parameter
A more general question
What are the advantages and disadvantages of one-pass compiler and multi-pass compiler?
Useful links:
A List of C/C++ compilers (wikipedia)
An incomplete list of C++ compilers (Bjarne Stroustrup's site)
The standard sets no requirements what so ever with regards to
how a compiler is implemented. But what do you mean by
"one-pass"? Most compilers today do only read the input file
once. They create an in memory representation (often in the
form of some sort of parse tree), and may make multiple passes
over that. And almost certainly make multiple passes over parts
of it. The compiler must make a "pass" over the internal
representation of a template each time it is instantiated, for
example; there's no way of avoiding that. G++ also makes
a "pass" over the template when it is defined, before any
instantiation, and reports some errors then. (The standard
committee expressedly designed templates to allow a maximum of
error detection at the point of definition. This is the
motivation behind the requirement for typename in certain
places, for example.) Even without templates, a compiler will
generally have to make two passes over a class definition if
there are functions defined in it.
With regards to the more general question, again, I think you'd
have to define exactly what you mean by "one-pass". I don't
know of any compiler today which reads the source file several
times, but almost all will visit some or all of the nodes in the
parse tree more than once. Is this one-pass or multi-pass? The
distinction was more significant in the past, when memory wasn't
sufficient to maintain much of the source code in an internal
representation. Languages like Pascal and, to a lesser degree
C, were sometimes designed to be easy to implement with a single
pass compiler, since a single pass compiler would be
significantly faster. Today, this issue is largely irrelevant,
and modern languages, including C++, tend to ignore it; where
C++ seems to conform to the needs of a one-pass compiler, it's
largely for reasons of C compatibility, and where
C compatibility is not an issue (e.g. in a class definition), it
often makes order of declaration irrelevant.
From what I know, 30 years ago it was important for a compiler to be one-pass, because reads and writes to disk (or magnetic tape) were very slow and there was not enough memory to hold whole code (thanks James Kanze). Also, a single-pass is a requirement for scripting/interactive languages.
Nowdays compilers are usually not one-pass, there are several intermediate representations (e.g Abstract Syntax Tree or Static Single Assignment Form) that the code is transformed into and then analised/optimised.
Some elements in C++ cannot be solved without some intermediate steps, e.g. in a class you can reference members which are defined only later in the class body. Also, all templates need to be somehow remembered for further access during instantiation.
What does not happen usually, is that the source code is not parsed several times --- there is no need for that. So you should not experience same syntactic error being reported several times.
No, I would be surprised if you found a heavily used C++ single pass compiler.
No, it does multiple passes and even different optimizations based on the flags you pass it.
Advantages (single-pass): fast! Since all the source only needs to be examined once the compilation phase (and thus beginning of execution) can happen very quickly. It is also a model that is attractive because it makes the compiler easy to understand and often times "easier" to implement. (I worked on a single pass Pascal compiler once, but don't encounter them often, whereas single pass interpreters are common)
Disadvantages (sinlge-pass): Optimization, semantic/syntactic analysis. Sometimes a single code look lets things through that are easily caught by simple mechanisms in multiple passes. (kind of why we have things like JSLint)
Advantages (multi-pass): optimizations, semantic/syntactic analysis. Even pseudo interpreted languages like "JRuby" go through a pipeline compilation process to get to java/jvm bytecode before execution, you could consider this multi-pass and the multiple looks at the varying representations (and consequently the resulting optimizations) of code can make it very fast.
Disadvantages (multi-pass): complexity, sometimes time (depending on if AOT/JIT is being used as your compilation method)
Also, single-pass is pretty common in academia to help learn the aspects of compiler design.
Walter Bright, the developer of the first C++ compiler, has stated that he believes it is not possible to compile C++ without at least 3 passes. And, yes, that means 3 full text-transforming passes over the source, not just traversals through an internal tree representation. See his Dr. Dobb's magazine article, "Why is C++ compilation so slow?" So any hope of finding a true one-pass compiler seems doomed. (I think this was part of the motivation Bright had to develop D, his C++ alternative.)
The compiler only needs to look at the sources once top down, but that does not mean that it does not have to process the parsed contents more than once. In particular with templates, it has to instantiate the templated code with the type, and that cannot happen until the template is used (or explicitly instantiated by the user), which is the reason for your duplicate errors:
When the template is defined, the compiler detects an error and at that point the type has not been substituted. When the actual instantiation occurs it substitutes the template arguments and processes the result, which is what triggers the second error. Note that if the template was specialized after the first definition, and before the instantiation, for that particular type, the second error need not occur.
I'm fully aware that the return value of std::type_info::name() is implementation-defined.
From the C++ standard (ISO/IEC 14882:2003 §18.5.1.7):
Returns: an implementation-defined NTBS.
My question is: why? Wouldn't this member function be much more useful if the standard dictated what the return value should be?
Basically, if an implementation decides that they can't or doesn't want to support RTTI, they can just return "";. If the standard forced it to return something, they'd possibly kill any ability to have a compliant compiler for an environment where the resources for RTTI don't exist or want to be disabled (a microchip, for example.)
And let's not forget we don't want to force an ABI/name-mangling scheme on any compilers.
This follows the C++ philosophy "You don't pay for things you don't need."
Where we're talking about vendors returning different strings, I think it's just a "we do it this way, you change" "no, we do it this way, YOU change" thing between the compiler vendors. Even the Standards committee doesn't want to annoy the compiler teams, and creating some neutral, new standard that isn't employed by any vendor tends to mean finding something that's nonsensical anyway.
Why aren't they all the obvious namespaces::class::functions etc. already? Some current implementations may have historically found it convenient to have it match the linker-required mangled names, been paranoid (or had paranoid clients) re memory usage etc..