A Few Coding Standards

Comments are one critical part of readability and maintainability. Everyone -knows they should comment, so should you. Although we all should probably +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:

File Headers -

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 CVS. Most source trees will probably have a standard +

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++ -*-===//
-// 
+//
 //                     The LLVM Compiler Infrastructure
 //
-// This file was developed by the LLVM research group and is distributed under
-// the University of Illinois Open Source License. See LICENSE.TXT for details.
-// 
+// 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
@@ -154,9 +173,9 @@ 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.
+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
@@ -175,9 +194,9 @@ something sane goes a long ways towards avoiding writing documentation.
 Method information
 
 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
+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 insideous is going on).  The hope is that people can
+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.
 
@@ -238,7 +257,7 @@ order:
   System #includes
 
 
-... and each catagory should be sorted by name.
+... and each category should be sorted by name.
 
 The "Main Module Header" file applies to .cpp file
 which implement an interface defined by a .h file.  This #include
@@ -262,6 +281,18 @@ implements are defined.
 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 is not up
+for debate.
+

@@ -272,7 +303,7 @@ it.

In all cases, prefer spaces to tabs in source files. People have different -prefered indentation levels, and different styles of indentation that they +preferred indentation levels, and different styles of indentation that they like... this is fine. What isn't 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.

@@ -367,9 +398,8 @@ code, isolate it behind a well defined (and well documented) interface.

In practice, this means that you shouldn't assume much about the host compiler, including its support for "high tech" features like partial -specialization of templates. In fact, Visual C++ 6 could be an important target -for our work in the future, and we don't want to have to rewrite all of our code -to support it.

+specialization of templates. If these features are used, they should only be +an implementation detail of a library which has a simple exposed API.

@@ -389,7 +419,8 @@ 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, in which case struct is allowed.

+all members are public and the type is a C++ "POD" type, in which case +struct is allowed.

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 +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 -accidently forget to include a header file in your module header, make sure to +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...

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 c'tors are run whenever an application start up that uses the dynamic -library. There are two problems with this:

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:

The time to run the static c'tors impacts startup time of - applications—a critical time for gui apps.
The static c'tors cause the app to pull many extra pages of memory off the - disk: both the code for the static c'tors in each .o file and the small - amount of data that gets touched. In addition, touched/dirty pages put - more pressure on the VM system on low-memory machines.

+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:

+ +

+  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:

+ +

+  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 from 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":

+ +

+  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 this something like:

+ +

+  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.

+ +

Old Way	New Way
#include <iostream>	#include "llvm/Support/Streams.h"
DEBUG(std::cerr << ...); -DEBUG(dump(std::cerr));	DOUT << ...; -dump(DOUT);
std::cerr << "Hello world\n";	llvm::cerr << "Hello world\n";
std::cout << "Hello world\n";	llvm::cout << "Hello world\n";
std::cin >> Var;	llvm::cin >> Var;
std::ostream	llvm::OStream
std::istream	llvm::IStream
std::stringstream	llvm::StringStream
void print(std::ostream &Out); -// ... -print(std::cerr);	void print(std::ostream &Out); -void print(std::ostream Out) { if (Out) print(Out) } -// ... -print(llvm::cerr); - - N.B. The second `print` method is called by the `print` -expression. It prevents the execution of the first `print` method if the -stream is `cnull`.

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)) {
+    ...
+  }
+

+ +

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.

Use the "assert" function to its fullest. Check all of your -preconditions and assumptions, you never know when a bug (not neccesarily even +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 @@ -619,6 +828,29 @@ assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!"

You get the idea...

Please be aware when adding assert statements that 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:

+ +

+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.

+ +

+assert(0 && "Some helpful error message");
+// Not reached
+return 0;
+

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 @@ -667,16 +899,223 @@ 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:

+ +

+  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 c'tors are run whenever an application start up that uses the dynamic +library. There are two problems with this:

+ +

The time to run the static c'tors impacts startup time of + applications—a critical time for GUI apps.
The static c'tors cause the app to pull many extra pages of memory off the + disk: both the code for the static c'tors in each .o file and the + small amount of data that gets touched. In addition, touched/dirty pages + put more pressure on the VM system on low-memory machines.

+ +

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, so you should just use it for new +code.

+ +

New code should always +use raw_ostream for writing, or +the llvm::MemoryBuffer API for reading files.

+ +

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'.

+ +

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.

+ +

This section describes preferred low-level formatting guidelines along with +reasoning on why we prefer them.

+ +

We prefer to put a space before a parentheses only in control flow +statements, but not in normal function call expressions and function-like +macros. For example, this is good:

+ +

+  if (x) ...
+  for (i = 0; i != 100; ++i) ...
+  while (llvm_rocks) ...
+
+  somefunc(42);
+  assert(3 != 4 && "laws of math are failing me");
+  
+  a = foo(42, 92) + bar(x);
+

+ +

... and this is bad:

+ +

+  if(x) ...
+  for(i = 0; i != 100; ++i) ...
+  while(llvm_rocks) ...
+
+  somefunc (42);
+  assert (3 != 4 && "laws of math are failing me");
+  
+  a = foo (42, 92) + bar (x);
+

+ +

The reason for doing this is not completely arbitrary. This style makes + control flow operators stand out more, and makes expressions flow better. The + function call operator binds very tightly as a postfix operator. Putting + a space after a function name (as in the last example) makes it appear that + the code might bind the arguments of the left-hand-side of a binary operator + with the argument list of a function and the name of the right side. More + specifically, it is easy to misread the "a" example as:

+ +

+  a = foo ((42, 92) + bar) (x);
+

+ +

... when skimming through the code. By avoiding a space in a function, we +avoid this misinterpretation.

+ +

@@ -696,27 +1135,178 @@ get in the habit of always using preincrement, and you won't have a problem.

- Avoid std::endl + Namespace Indentation

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:

+In general, we strive to reduce indentation where ever possible. This is useful +because we want code to fit into 80 columns without +wrapping horribly, but also because it makes it easier to understand the code. +Namespaces are a funny thing: they are often large, and we often desire to put +lots of stuff into them (so they can be large). Other times they are tiny, +because they just hold an enum or something similar. In order to balance this, +we use different approaches for small versus large namespaces. +

+ +

+If a namespace definition is small and easily fits on a screen (say, +less than 35 lines of code), then you should indent its body. Here's an +example: +

-std::cout << std::endl;
-std::cout << '\n' << std::flush;
+namespace llvm {
+  namespace X86 {
+    /// RelocationType - An enum for the x86 relocation codes. Note that
+    /// the terminology here doesn't follow x86 convention - word means
+    /// 32-bit and dword means 64-bit.
+    enum RelocationType {
+      /// reloc_pcrel_word - PC relative relocation, add the relocated value to
+      /// the value already in memory, after we adjust it for where the PC is.
+      reloc_pcrel_word = 0,
+
+      /// reloc_picrel_word - PIC base relative relocation, add the relocated
+      /// value to the value already in memory, after we adjust it for where the
+      /// PIC base is.
+      reloc_picrel_word = 1,
+      
+      /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
+      /// add the relocated value to the value already in memory.
+      reloc_absolute_word = 2,
+      reloc_absolute_dword = 3
+    };
+  }
+}

Most of the time, you probably have no reason to flush the output stream, so -it's better to use a literal '\n'.

Since the body is small, indenting adds value because it makes it very clear +where the namespace starts and ends, and it is easy to take the whole thing in +in one "gulp" when reading the code. If the blob of code in the namespace is +larger (as it typically is in a header in the llvm or clang namespaces), do not +indent the code, and add a comment indicating what namespace is being closed. +For example:

+ +

+namespace llvm {
+namespace knowledge {
+
+/// Grokable - This class represents things that Smith can have an intimate
+/// understanding of and contains the data associated with it.
+class Grokable {
+...
+public:
+  explicit Grokable() { ... }
+  virtual ~Grokable() = 0;
+  
+  ...
+
+};
+
+} // end namespace knowledge
+} // end namespace llvm
+

+ +

Because the class is large, we don't expect that the reader can easily +understand the entire concept in a glance, and the end of the file (where the +namespaces end) may be a long ways away from the place they open. As such, +indenting the contents of the namespace doesn't add any value, and detracts from +the readability of the class. In these cases it is best to not indent +the contents of the namespace.

+ +

+ Anonymous Namespaces +

+ +

After talking about namespaces in general, you may be wondering about +anonymous namespaces in particular. +Anonymous namespaces are a great language feature that tells the C++ compiler +that the contents of the namespace are only visible within the current +translation unit, allowing more aggressive optimization and eliminating the +possibility of symbol name collisions. Anonymous namespaces are to C++ as +"static" is to C functions and global variables. While "static" is available +in C++, anonymous namespaces are more general: they can make entire classes +private to a file.

+ +

The problem with anonymous namespaces is that they naturally want to +encourage indentation of their body, and they reduce locality of reference: if +you see a random function definition in a C++ file, it is easy to see if it is +marked static, but seeing if it is in an anonymous namespace requires scanning +a big chunk of the file.

+ +

Because of this, we have a simple guideline: make anonymous namespaces as +small as possible, and only use them for class declarations. For example, this +is good:

+ +

+namespace {
+  class StringSort {
+  ...
+  public:
+    StringSort(...)
+    bool operator<(const char *RHS) const;
+  };
+} // end anonymous namespace
+
+static void Helper() { 
+  ... 
+}
+
+bool StringSort::operator<(const char *RHS) const {
+  ...
+}
+
+

+ +

This is bad:

+ + +

+namespace {
+class StringSort {
+...
+public:
+  StringSort(...)
+  bool operator<(const char *RHS) const;
+};
+
+void Helper() { 
+  ... 
+}
+
+bool StringSort::operator<(const char *RHS) const {
+  ...
+}
+
+} // end anonymous namespace
+
+

+ + +

This is bad specifically because if you're looking at "Helper" in the middle +of a large C++ file, that you have no immediate way to tell if it is local to +the file. When it is marked static explicitly, this is immediately obvious. +Also, there is no reason to enclose the definition of "operator<" in the +namespace just because it was declared there. +

+ +

@@ -731,15 +1321,12 @@ sources. Two particularly important books for our work are:

Effective -C++ by Scott Meyers. There is an online version of the book (only some -chapters though) available as well. Also +
Effective +C++ by Scott Meyers. Also interesting and useful are "More Effective C++" and "Effective STL" by the same author.
Large-Scale C++ -Software Design by John Lakos
Large-Scale C++ Software Design by John Lakos

@@ -753,9 +1340,9 @@ something.

+ src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS">

+ src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"> Chris Lattner
LLVM Compiler Infrastructure