| A Few Coding Standards | -
+
| -Introduction - |
+
This document attempts to describe a few coding standards that are being used +in the LLVM source tree. Although no coding standards should be regarded as +absolute requirements to be followed in all instances, coding standards can be +useful.
+ +This document intentionally does not prescribe fixed standards for religious +issues such as brace placement and space usage. For issues like this, follow +the golden rule:
-This document intentionally does not prescribe fixed standards for religious issues such as brace placement and space usage. For issues like this, follow the golden rule: +- -+If you are adding a significant body of source to a project, feel free to use whatever style you are most comfortable with. If you are extending, enhancing, or bug fixing already implemented code, use the style that is already being used so that the source is uniform and easy to follow.+ -The ultimate goal of these guidelines is the increase readability and maintainability of our common source base. If you have suggestions for topics to be included, please mail them to Chris.+
The ultimate goal of these guidelines is the increase readability and +maintainability of our common source base. If you have suggestions for topics to +be included, please mail them to Chris.
+ +| -Mechanical Source Issues - |
| -Source Code Formatting - |
+
+
Comments are one critical part of readability and maintainability. Everyone +knows they should comment, so should you. When writing comments, write them as +English prose, which means they should use proper capitalization, punctuation, +etc. Although we all should probably +comment our code more than we do, there are a few very critical places that +documentation is very useful:
+ +Every source file should have a header on it that describes the basic +purpose of the file. If a file does not have a header, it should not be +checked into Subversion. Most source trees will probably have a standard +file header format. The standard format for the LLVM source tree looks like +this:
+ +-//===-- llvm/Instruction.h - Instruction class definition --------*- C++ -*--=// +//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// // // This file contains the declaration of the Instruction class, which is the // base class for all of the VM instructions. // //===----------------------------------------------------------------------===//+
A few things to note about this particular format: The "-*- C++ +-*-" string on the first line is there to tell Emacs that the source file +is a C++ file, not a C file (Emacs assumes .h files are C files by default). +Note that this tag is not necessary in .cpp files. The name of the file is also +on the first line, along with a very short description of the purpose of the +file. This is important when printing out code and flipping though lots of +pages.
+ +The next section in the file is a concise note that defines the license +that the file is released under. This makes it perfectly clear what terms the +source code can be distributed under and should not be modified in any way.
+ +The main body of the description does not have to be very long in most cases. +Here it's only two lines. If an algorithm is being implemented or something +tricky is going on, a reference to the paper where it is published should be +included, as well as any notes or "gotchas" in the code to watch out for.
+ ++
+
Classes are one fundamental part of a good object oriented design. As such, +a class definition should have a comment block that explains what the class is +used for... if it's not obvious. If it's so completely obvious your grandma +could figure it out, it's probably safe to leave it out. Naming classes +something sane goes a long ways towards avoiding writing documentation.
-+
Methods defined in a class (as well as any global functions) should also be +documented properly. A quick note about what it does and a description of the +borderline behaviour is all that is necessary here (unless something +particularly tricky or insidious is going on). The hope is that people can +figure out how to use your interfaces without reading the code itself... that is +the goal metric.
-Good things to talk about here are what happens when something unexpected +happens: does the method return null? Abort? Format your hard disk?
-Methods defined in a class (as well as any global functions) should also be documented properly. A quick note about what it does any a description of the borderline behaviour is all that is neccesary here (unless something particularly tricky or insideous is going on). The hope is that people can figure out how to use your interfaces without reading the code itself... that is the goal metric.+
+
In general, prefer C++ style (//) comments. They take less space, +require less typing, don't have nesting problems, etc. There are a few cases +when it is useful to use C style (/* */) comments however:
+ +To comment out a large block of code, use #if 0 and #endif. +These nest properly and are better behaved in general than C style comments.
+ ++
Immediately after the header file comment (and +include guards if working on a header file), the minimal list of #includes required by the +file should be listed. We prefer these #includes to be listed in this +order:
+
and each category should be sorted by name.
+ +The "Main Module Header" file applies to .cpp files +which implement an interface defined by a .h file. This #include +should always be included first regardless of where it lives on the file +system. By including a header file first in the .cpp files that implement the +interfaces, we ensure that the header does not have any hidden dependencies +which are not explicitly #included in the header, but should be. It is also a +form of documentation in the .cpp file to indicate where the interfaces it +implements are defined.
+ ++
Write your code to fit within 80 columns of text. This helps those of us who +like to print out code and look at your code in an xterm without resizing +it.
+ +The longer answer is that there must be some limit to the width of the code +in order to reasonably allow developers to have multiple files side-by-side in +windows on a modest display. If you are going to pick a width limit, it is +somewhat arbitrary but you might as well pick something standard. Going with +90 columns (for example) instead of 80 columns wouldn't add any significant +value and would be detrimental to printing out code. Also many other projects +have standardized on 80 columns, so some people have already configured their +editors for it (vs something else, like 90 columns).
+ +This is one of many contentious issues in coding standards, but it is not up +for debate.
+ +In all cases, prefer spaces to tabs in source files. People have different +preferred indentation levels, and different styles of indentation that they +like; this is fine. What isn't fine is that different editors/viewers expand +tabs out to different tab stops. This can cause your code to look completely +unreadable, and it is not worth dealing with.
+ +As always, follow the Golden Rule above: follow the +style of existing code if you are modifying and extending it. If you like four +spaces of indentation, DO NOT do that in the middle of a chunk of code +with two spaces of indentation. Also, do not reindent a whole source file: it +makes for incredible diffs that are absolutely worthless.
+ +Okay, in your first year of programming you were told that indentation is +important. If you didn't believe and internalize this then, now is the time. +Just do it.
+ +If your code has compiler warnings in it, something is wrong — you +aren't casting values correctly, your have "questionable" constructs in your +code, or you are doing something legitimately wrong. Compiler warnings can +cover up legitimate errors in output and make dealing with a translation unit +difficult.
+ +It is not possible to prevent all warnings from all compilers, nor is it +desirable. Instead, pick a standard compiler (like gcc) that provides +a good thorough set of warnings, and stick to it. At least in the case of +gcc, it is possible to work around any spurious errors by changing the +syntax of the code slightly. For example, a warning that annoys me occurs when +I write code like this:
+ +
+if (V = getValue()) {
+ ...
+}
+
+gcc will warn me that I probably want to use the == +operator, and that I probably mistyped it. In most cases, I haven't, and I +really don't want the spurious errors. To fix this particular problem, I +rewrite the code like this:
+
+if ((V = getValue())) {
+ ...
+}
+
+which shuts gcc up. Any gcc warning that annoys you can +be fixed by massaging the code appropriately.
+ +These are the gcc warnings that I prefer to enable:
+ ++-Wall -Winline -W -Wwrite-strings -Wno-unused ++
+
In almost all cases, it is possible and within reason to write completely +portable code. If there are cases where it isn't possible to write portable +code, isolate it behind a well defined (and well documented) interface.
-As always, follow the Golden Rule above: follow the style of existing code if your are modifying and extending it. If you like four spaces of indentation, DO NOT do that in the middle of a chunk of code with two spaces of indentation. Also, do not reindent a whole source file: it make for incredible diffs that are absolutely worthless.+
In practice, this means that you shouldn't assume much about the host +compiler, and Visual Studio tends to be the lowest common denominator. +If advanced features are used, they should only be an implementation detail of +a library which has a simple exposed API, and preferably be buried in +libSystem.
+In an effort to reduce code and executable size, LLVM does not use RTTI +(e.g. dynamic_cast<>) or exceptions. These two language features +violate the general C++ principle of "you only pay for what you use", +causing executable bloat even if exceptions are never used in the code base, or +if RTTI is never used for a class. Because of this, we turn them off globally +in the code.
+ +That said, LLVM does make extensive use of a hand-rolled form of RTTI that +use templates like isa<>, +cast<>, and dyn_cast<>. This form of RTTI is +opt-in and can be added to any class. It is also substantially more efficient +than dynamic_cast<>.
+ +In C++, the class and struct keywords can be used almost +interchangeably. The only difference is when they are used to declare a class: +class makes all members private by default while struct makes +all members public by default.
+ +Unfortunately, not all compilers follow the rules and some will generate +different symbols based on whether class or struct was used to +declare the symbol. This can lead to problems at link time.
+ +So, the rule for LLVM is to always use the class keyword, unless +all members are public and the type is a C++ +POD type, in +which case struct is allowed.
-Okay, your first year of programming you were told that indentation is important. If you didn't believe and internalize this then, now is the time. Just do it.+
| -Compiler Issues - |
C++ doesn't do too well in the modularity department. There is no real +encapsulation or data hiding (unless you use expensive protocol classes), but it +is what we have to work with. When you write a public header file (in the LLVM +source tree, they live in the top level "include" directory), you are +defining a module of functionality.
+ +Ideally, modules should be completely independent of each other, and their +header files should only #include the absolute minimum number of +headers possible. A module is not just a class, a function, or a +namespace: it's +a collection of these that defines an interface. This interface may be +several functions, classes, or data structures, but the important issue is how +they work together.
+ +In general, a module should be implemented by one or more .cpp +files. Each of these .cpp files should include the header that defines +their interface first. This ensures that all of the dependences of the module +header have been properly added to the module header itself, and are not +implicit. System headers should be included after user headers for a +translation unit.
+ +#include hurts compile time performance. Don't do it unless you +have to, especially in header files.
+ +But wait! Sometimes you need to have the definition of a class to use it, or +to inherit from it. In these cases go ahead and #include that header +file. Be aware however that there are many cases where you don't need to have +the full definition of a class. If you are using a pointer or reference to a +class, you don't need the header file. If you are simply returning a class +instance from a prototyped function or method, you don't need it. In fact, for +most cases, you simply don't need the definition of a class. And not +#include'ing speeds up compilation.
+ +It is easy to try to go too overboard on this recommendation, however. You +must include all of the header files that you are using — you can +include them either directly or indirectly (through another header file). To +make sure that you don't accidentally forget to include a header file in your +module header, make sure to include your module header first in the +implementation file (as mentioned above). This way there won't be any hidden +dependencies that you'll find out about later.
+ +Many modules have a complex implementation that causes them to use more than +one implementation (.cpp) file. It is often tempting to put the +internal communication interface (helper classes, extra functions, etc) in the +public module header file. Don't do this!
+ +If you really need to do something like this, put a private header file in +the same directory as the source files, and include it locally. This ensures +that your private interface remains private and undisturbed by outsiders.
-If your code has compiler warnings in it, something is wrong: you aren't casting values correctly, your have "questionable" constructs in your code, or you are doing something legitimately wrong. Compiler warnings can cover up legitimate errors in output and make dealing with a translation unit difficult.+
Note however, that it's okay to put extra implementation methods in a public +class itself. Just make them private (or protected) and all is well.
+ ++
When reading code, keep in mind how much state and how many previous +decisions have to be remembered by the reader to understand a block of code. +Aim to reduce indentation where possible when it doesn't make it more difficult +to understand the code. One great way to do this is by making use of early +exits and the continue keyword in long loops. As an example of using +an early exit from a function, consider this "bad" code:
+ +
- if (V = getValue()) {
- ..
+Value *DoSomething(Instruction *I) {
+ if (!isa<TerminatorInst>(I) &&
+ I->hasOneUse() && SomeOtherThing(I)) {
+ ... some long code ....
}
-+ + return 0; +} + +
This code has several problems if the body of the 'if' is large. +When you're looking at the top of the function, it isn't immediately clear that +this only does interesting things with non-terminator instructions, and +only applies to things with the other predicates. Second, it is relatively +difficult to describe (in comments) why these predicates are important because +the if statement makes it difficult to lay out the comments. Third, +when you're deep within the body of the code, it is indented an extra level. +Finally, when reading the top of the function, it isn't clear what the result is +if the predicate isn't true; you have to read to the end of the function to know +that it returns null.
+ +It is much preferred to format the code like this:
+ +
+Value *DoSomething(Instruction *I) {
+ // Terminators never need 'something' done to them because ...
+ if (isa<TerminatorInst>(I))
+ return 0;
+
+ // We conservatively avoid transforming instructions with multiple uses
+ // because goats like cheese.
+ if (!I->hasOneUse())
+ return 0;
+
+ // This is really just here for example.
+ if (!SomeOtherThing(I))
+ return 0;
+
+ ... some long code ....
+}
+
++
This fixes these problems. A similar problem frequently happens in for +loops. A silly example is something like this:
+
- if ((V = getValue())) {
- ..
+ for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
+ if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) {
+ Value *LHS = BO->getOperand(0);
+ Value *RHS = BO->getOperand(1);
+ if (LHS != RHS) {
+ ...
+ }
+ }
}
-+ +
When you have very, very small loops, this sort of structure is fine. But if +it exceeds more than 10-15 lines, it becomes difficult for people to read and +understand at a glance. The problem with this sort of code is that it gets very +nested very quickly. Meaning that the reader of the code has to keep a lot of +context in their brain to remember what is going immediately on in the loop, +because they don't know if/when the if conditions will have elses etc. +It is strongly preferred to structure the loop like this:
-...which shuts gcc up. Any gcc warning that annoys you can be fixed by massaging the code appropriately.+
+ for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
+ BinaryOperator *BO = dyn_cast<BinaryOperator>(II);
+ if (!BO) continue;
+
+ Value *LHS = BO->getOperand(0);
+ Value *RHS = BO->getOperand(1);
+ if (LHS == RHS) continue;
+
+ ...
+ }
+
++
This has all the benefits of using early exits for functions: it reduces +nesting of the loop, it makes it easier to describe why the conditions are true, +and it makes it obvious to the reader that there is no else coming up +that they have to push context into their brain for. If a loop is large, this +can be a big understandability win.
++
+
For similar reasons above (reduction of indentation and easier reading), +please do not use 'else' or 'else if' after something that +interrupts control flow — like return, break, +continue, goto, etc. For example, this is bad:
-Just like most of the rules in this document, this isn't a hard and fast requirement. Exceptions are used in the Parser, because it simplifies error reporting significantly, and the LLVM parser is not at all in the critical path.+
+ case 'J': {
+ if (Signed) {
+ Type = Context.getsigjmp_bufType();
+ if (Type.isNull()) {
+ Error = ASTContext::GE_Missing_sigjmp_buf;
+ return QualType();
+ } else {
+ break;
+ }
+ } else {
+ Type = Context.getjmp_bufType();
+ if (Type.isNull()) {
+ Error = ASTContext::GE_Missing_jmp_buf;
+ return QualType();
+ } else {
+ break;
+ }
+ }
+ }
+ }
+
++
It is better to write it like this:
-Other features, such as templates (without partial specialization) can be used freely. The general goal is to have clear, consise, performant code... if a technique assists with that then use it.+
+ case 'J':
+ if (Signed) {
+ Type = Context.getsigjmp_bufType();
+ if (Type.isNull()) {
+ Error = ASTContext::GE_Missing_sigjmp_buf;
+ return QualType();
+ }
+ } else {
+ Type = Context.getjmp_bufType();
+ if (Type.isNull()) {
+ Error = ASTContext::GE_Missing_jmp_buf;
+ return QualType();
+ }
+ }
+ break;
+
+Or better yet (in this case) as:
+ +
+ case 'J':
+ if (Signed)
+ Type = Context.getsigjmp_bufType();
+ else
+ Type = Context.getjmp_bufType();
+
+ if (Type.isNull()) {
+ Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
+ ASTContext::GE_Missing_jmp_buf;
+ return QualType();
+ }
+ break;
+
+The idea is to reduce indentation and the amount of code you have to keep +track of when reading the code.
+ ++
+
It is very common to write small loops that just compute a boolean value. +There are a number of ways that people commonly write these, but an example of +this sort of thing is:
+ +
+ bool FoundFoo = false;
+ for (unsigned i = 0, e = BarList.size(); i != e; ++i)
+ if (BarList[i]->isFoo()) {
+ FoundFoo = true;
+ break;
+ }
+
+ if (FoundFoo) {
+ ...
+ }
+
+This sort of code is awkward to write, and is almost always a bad sign. +Instead of this sort of loop, we strongly prefer to use a predicate function +(which may be static) that uses +early exits to compute the predicate. We prefer +the code to be structured like this:
+
+/// ListContainsFoo - Return true if the specified list has an element that is
+/// a foo.
+static bool ListContainsFoo(const std::vector<Bar*> &List) {
+ for (unsigned i = 0, e = List.size(); i != e; ++i)
+ if (List[i]->isFoo())
+ return true;
+ return false;
+}
+...
+
+ if (ListContainsFoo(BarList)) {
+ ...
+ }
+
+| -Style Issues - |
There are many reasons for doing this: it reduces indentation and factors out +code which can often be shared by other code that checks for the same predicate. +More importantly, it forces you to pick a name for the function, and +forces you to write a comment for it. In this silly example, this doesn't add +much value. However, if the condition is complex, this can make it a lot easier +for the reader to understand the code that queries for this predicate. Instead +of being faced with the in-line details of how we check to see if the BarList +contains a foo, we can trust the function name and continue reading with better +locality.
+ +| -The High Level Issues - |
Poorly-chosen names can mislead the reader and cause bugs. We cannot stress +enough how important it is to use descriptive names. Pick names that +match the semantics and role of the underlying entities, within reason. Avoid +abbreviations unless they are well known. After picking a good name, make sure +to use consistent capitalization for the name, as inconsistency requires clients +to either memorize the APIs or to look it up to find the exact spelling.
+ +In general, names should be in camel case (e.g. TextFileReader +and isLValue()). Different kinds of declarations have different +rules:
+ +Type names (including classes, structs, enums, typedefs, etc) + should be nouns and start with an upper-case letter (e.g. + TextFileReader).
Function names should be verb phrases (as they represent + actions), and command-like function should be imperative. The name should + be camel case, and start with a lower case letter (e.g. openFile() + or isFoo()).
Enum declarations (e.g. enum Foo {...}) are types, so + they should follow the naming conventions for types. A common use for enums + is as a discriminator for a union, or an indicator of a subclass. When an + enum is used for something like this, it should have a Kind suffix + (e.g. ValueKind).
Enumerators (e.g. enum { Foo, Bar }) and public member + variables should start with an upper-case letter, just like types. + Unless the enumerators are defined in their own small namespace or inside a + class, enumerators should have a prefix corresponding to the enum + declaration name. For example, enum ValueKind { ... }; may contain + enumerators like VK_Argument, VK_BasicBlock, etc. + Enumerators that are just convenience constants are exempt from the + requirement for a prefix. For instance:
+ +
+enum {
+ MaxSize = 42,
+ Density = 12
+};
+
++
As an exception, classes that mimic STL classes can have member names in +STL's style of lower-case words separated by underscores (e.g. begin(), +push_back(), and empty()).
-Modules should be completely independent of each other, except for their dependence. A module is not just a class, a function, or a namespace: it's a collection of these that defines an interface. This interface may be several functions, classes or data structures, but the important issue is how they work together.+
Here are some examples of good and bad names:
- +
+class VehicleMaker {
+ ...
+ Factory<Tire> F; // Bad -- abbreviation and non-descriptive.
+ Factory<Tire> Factory; // Better.
+ Factory<Tire> TireFactory; // Even better -- if VehicleMaker has more than one
+ // kind of factories.
+};
+
+Vehicle MakeVehicle(VehicleType Type) {
+ VehicleMaker M; // Might be OK if having a short life-span.
+ Tire tmp1 = M.makeTire(); // Bad -- 'tmp1' provides no information.
+ Light headlight = M.makeLight("head"); // Good -- descriptive.
+ ...
+}
+
++
+
+
Use the "assert" macro to its fullest. Check all of your +preconditions and assumptions, you never know when a bug (not necessarily even +yours) might be caught early by an assertion, which reduces debugging time +dramatically. The "<cassert>" header file is probably already +included by the header files you are using, so it doesn't cost anything to use +it.
-It is easy to try to go too overboard on this recommendation, however. You must include all of the header files that you are using, either directly or indirectly (through another header file). To make sure that you don't accidently forget to include a header file in your module header, make sure to include your module header first in the implementation file (as mentioned above). This way there won't be any hidden dependencies that you'll find out about later...+
To further assist with debugging, make sure to put some kind of error message +in the assertion statement, which is printed if the assertion is tripped. This +helps the poor debugger make sense of why an assertion is being made and +enforced, and hopefully what to do about it. Here is one complete example:
+
+inline Value *getOperand(unsigned i) {
+ assert(i < Operands.size() && "getOperand() out of range!");
+ return Operands[i];
+}
+
+Here are more examples:
-Many modules have a complex implementation that causes them to use more than one implementation (.cpp) file. It is often tempting to put the internal communication interface (helper classes, extra functions, etc) in the public module header file. Don't do this. :)+
+assert(Ty->isPointerType() && "Can't allocate a non pointer type!"); -If you really need to do something like this, put a private header file in the same directory as the source files, and include it locally. This ensures that your private interface remains private and undisturbed by outsiders.+assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!"); -Note however, that it's okay to put extra implementation methods a public class itself... just make them private (or protected), and all is well.
+assert(idx < getNumSuccessors() && "Successor # out of range!"); +assert(V1.getType() == V2.getType() && "Constant types must be identical!"); - -
| -The Low Level Issues - |
You get the idea.
- -Please be aware that, when adding assert statements, not all compilers are aware of +the semantics of the assert. In some places, asserts are used to indicate a piece of +code that should not be reached. These are typically of the form:
-Use the "assert" function to its fullest. Check all of your preconditions and assumptions, you never know when a bug (not neccesarily even yours) might be caught early by an assertion, which reduces debugging time dramatically. The "assert.h" header file is probably already included by the header files you are using, so it doesn't cost anything to use it.+
+assert(0 && "Some helpful error message"); ++
+
When used in a function that returns a value, they should be followed with a return +statement and a comment indicating that this line is never reached. This will prevent +a compiler which is unable to deduce that the assert statement never returns from +generating a warning.
+
- inline Value *getOperand(unsigned i) {
- assert(i < Operands.size() && "getOperand() out of range!");
- return Operands[i];
- }
+assert(0 && "Some helpful error message");
+// Not reached
+return 0;
+Another issue is that values used only by assertions will produce an "unused +value" warning when assertions are disabled. For example, this code will +warn:
+- assert(Ty->isPointerType() && "Can't allocate a non pointer type!"); +unsigned Size = V.size(); +assert(Size > 42 && "Vector smaller than it should be"); - assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!"); +bool NewToSet = Myset.insert(Value); +assert(NewToSet && "The value shouldn't be in the set yet"); ++
These are two interesting different cases. In the first case, the call to +V.size() is only useful for the assert, and we don't want it executed when +assertions are disabled. Code like this should move the call into the assert +itself. In the second case, the side effects of the call must happen whether +the assert is enabled or not. In this case, the value should be cast to void to +disable the warning. To be specific, it is preferred to write the code like +this:
- assert(V1.getType() == V2.getType() && "Constant types must be identical!"); ++assert(V.size() > 42 && "Vector smaller than it should be"); + +bool NewToSet = Myset.insert(Value); (void)NewToSet; +assert(NewToSet && "The value shouldn't be in the set yet"); ++
-You get the idea...
+
In LLVM, we prefer to explicitly prefix all identifiers from the standard +namespace with an "std::" prefix, rather than rely on +"using namespace std;".
+ +In header files, adding a 'using namespace XXX' directive pollutes +the namespace of any source file that #includes the header. This is +clearly a bad thing.
+ +In implementation files (e.g. .cpp files), the rule is more of a stylistic +rule, but is still important. Basically, using explicit namespace prefixes +makes the code clearer, because it is immediately obvious what facilities +are being used and where they are coming from. And more portable, because +namespace clashes cannot occur between LLVM code and other namespaces. The +portability rule is important because different standard library implementations +expose different symbols (potentially ones they shouldn't), and future revisions +to the C++ standard will add more symbols to the std namespace. As +such, we never use 'using namespace std;' in LLVM.
+ +The exception to the general rule (i.e. it's not an exception for +the std namespace) is for implementation files. For example, all of +the code in the LLVM project implements code that lives in the 'llvm' namespace. +As such, it is ok, and actually clearer, for the .cpp files to have a +'using namespace llvm;' directive at the top, after the +#includes. This reduces indentation in the body of the file for source +editors that indent based on braces, and keeps the conceptual context cleaner. +The general form of this rule is that any .cpp file that implements +code in any namespace may use that namespace (and its parents'), but should not +use any others.
+ ++
+
If a class is defined in a header file and has a v-table (either it has +virtual methods or it derives from classes with virtual methods), it must +always have at least one out-of-line virtual method in the class. Without +this, the compiler will copy the vtable and RTTI into every .o file +that #includes the header, bloating .o file sizes and +increasing link times.
++
Because C++ doesn't have a standard "foreach" loop (though it can be +emulated with macros and may be coming in C++'0x) we end up writing a lot of +loops that manually iterate from begin to end on a variety of containers or +through other data structures. One common mistake is to write a loop in this +style:
+ +- cout << endl; - cout << "\n" << flush; + BasicBlock *BB = ... + for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) + ... use I ...+
The problem with this construct is that it evaluates "BB->end()" +every time through the loop. Instead of writing the loop like this, we strongly +prefer loops to be written so that they evaluate it once before the loop starts. +A convenient way to do this is like so:
+ ++ BasicBlock *BB = ... + for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) + ... use I ... ++
The observant may quickly point out that these two loops may have different +semantics: if the container (a basic block in this case) is being mutated, then +"BB->end()" may change its value every time through the loop and the +second loop may not in fact be correct. If you actually do depend on this +behavior, please write the loop in the first form and add a comment indicating +that you did it intentionally.
+ +Why do we prefer the second form (when correct)? Writing the loop in the +first form has two problems. First it may be less efficient than evaluating it +at the start of the loop. In this case, the cost is probably minor — a +few extra loads every time through the loop. However, if the base expression is +more complex, then the cost can rise quickly. I've seen loops where the end +expression was actually something like: "SomeMap[x]->end()" and map +lookups really aren't cheap. By writing it in the second form consistently, you +eliminate the issue entirely and don't even have to think about it.
+ +The second (even bigger) issue is that writing the loop in the first form +hints to the reader that the loop is mutating the container (a fact that a +comment would handily confirm!). If you write the loop in the second form, it +is immediately obvious without even looking at the body of the loop that the +container isn't being modified, which makes it easier to read the code and +understand what it does.
+ +While the second form of the loop is a few extra keystrokes, we do strongly +prefer it.
+ +The use of #include <iostream> in library files is +hereby forbidden. The primary reason for doing this is to +support clients using LLVM libraries as part of larger systems. In particular, +we statically link LLVM into some dynamic libraries. Even if LLVM isn't used, +the static constructors are run whenever an application starts up that uses the +dynamic library. There are two problems with this:
+ +Note that using the other stream headers (<sstream> for +example) is not problematic in this regard — +just <iostream>. However, raw_ostream provides various +APIs that are better performing for almost every use than std::ostream +style APIs. Therefore new code should always +use raw_ostream for writing, or +the llvm::MemoryBuffer API for reading files.
-Most of the time, you probably have no reason to flush the output stream, so it's better to use a literal "\n".+
+
+
LLVM includes a lightweight, simple, and efficient stream implementation +in llvm/Support/raw_ostream.h, which provides all of the common +features of std::ostream. All new code should use raw_ostream +instead of ostream.
+Unlike std::ostream, raw_ostream is not a template and can +be forward declared as class raw_ostream. Public headers should +generally not include the raw_ostream header, but use forward +declarations and constant references to raw_ostream instances.
+The std::endl modifier, when used with iostreams outputs a +newline to the output stream specified. In addition to doing this, however, it +also flushes the output stream. In other words, these are equivalent:
+ ++std::cout << std::endl; +std::cout << '\n' << std::flush; ++
Most of the time, you probably have no reason to flush the output stream, so +it's better to use a literal '\n'.
+ +| -Writing Iterators - |
+
This section describes preferred low-level formatting guidelines along with +reasoning on why we prefer them.
+ +We prefer to put a space before an open parenthesis only in control flow +statements, but not in normal function call expressions and function-like +macros. For example, this is good:
+ +