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diff --git a/www/diagnostics.html b/www/diagnostics.html new file mode 100644 index 0000000..5be4db2 --- /dev/null +++ b/www/diagnostics.html @@ -0,0 +1,279 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" + "http://www.w3.org/TR/html4/strict.dtd"> +<html> +<head> + <META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /> + <title>Clang - Expressive Diagnostics</title> + <link type="text/css" rel="stylesheet" href="menu.css" /> + <link type="text/css" rel="stylesheet" href="content.css" /> + <style type="text/css"> +</style> +</head> +<body> + +<!--#include virtual="menu.html.incl"--> + +<div id="content"> + + +<!--=======================================================================--> +<h1>Expressive Diagnostics</h1> +<!--=======================================================================--> + +<p>In addition to being fast and functional, we aim to make Clang extremely user +friendly. As far as a command-line compiler goes, this basically boils down to +making the diagnostics (error and warning messages) generated by the compiler +be as useful as possible. There are several ways that we do this. This section +talks about the experience provided by the command line compiler, contrasting +Clang output to GCC 4.2's output in several examples. +<!-- +Other clients +that embed Clang and extract equivalent information through internal APIs.--> +</p> + +<h2>Column Numbers and Caret Diagnostics</h2> + +<p>First, all diagnostics produced by clang include full column number +information, and use this to print "caret diagnostics". This is a feature +provided by many commercial compilers, but is generally missing from open source +compilers. This is nice because it makes it very easy to understand exactly +what is wrong in a particular piece of code, an example is:</p> + +<pre> + $ <b>gcc-4.2 -fsyntax-only -Wformat format-strings.c</b> + format-strings.c:91: warning: too few arguments for format + $ <b>clang -fsyntax-only format-strings.c</b> + format-strings.c:91:13: warning: '.*' specified field precision is missing a matching 'int' argument + <font color="darkgreen"> printf("%.*d");</font> + <font color="blue"> ^</font> +</pre> + +<p>The caret (the blue "^" character) exactly shows where the problem is, even +inside of the string. This makes it really easy to jump to the problem and +helps when multiple instances of the same character occur on a line. We'll +revisit this more in following examples.</p> + +<h2>Range Highlighting for Related Text</h2> + +<p>Clang captures and accurately tracks range information for expressions, +statements, and other constructs in your program and uses this to make +diagnostics highlight related information. For example, here's a somewhat +nonsensical example to illustrate this:</p> + +<pre> + $ <b>gcc-4.2 -fsyntax-only t.c</b> + t.c:7: error: invalid operands to binary + (have 'int' and 'struct A') + $ <b>clang -fsyntax-only t.c</b> + t.c:7:39: error: invalid operands to binary expression ('int' and 'struct A') + <font color="darkgreen"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</font> + <font color="blue"> ~~~~~~~~~~~~~~ ^ ~~~~~</font> +</pre> + +<p>Here you can see that you don't even need to see the original source code to +understand what is wrong based on the Clang error: Because clang prints a +caret, you know exactly <em>which</em> plus it is complaining about. The range +information highlights the left and right side of the plus which makes it +immediately obvious what the compiler is talking about, which is very useful for +cases involving precedence issues and many other cases.</p> + +<h2>Precision in Wording</h2> + +<p>A detail is that we have tried really hard to make the diagnostics that come +out of clang contain exactly the pertinent information about what is wrong and +why. In the example above, we tell you what the inferred types are for +the left and right hand sides, and we don't repeat what is obvious from the +caret (that this is a "binary +"). Many other examples abound, here is a simple +one:</p> + +<pre> + $ <b>gcc-4.2 -fsyntax-only t.c</b> + t.c:5: error: invalid type argument of 'unary *' + $ <b>clang -fsyntax-only t.c</b> + t.c:5:11: error: indirection requires pointer operand ('int' invalid) + <font color="darkgreen"> int y = *SomeA.X;</font> + <font color="blue"> ^~~~~~~~</font> +</pre> + +<p>In this example, not only do we tell you that there is a problem with the * +and point to it, we say exactly why and tell you what the type is (in case it is +a complicated subexpression, such as a call to an overloaded function). This +sort of attention to detail makes it much easier to understand and fix problems +quickly.</p> + +<h2>No Pretty Printing of Expressions in Diagnostics</h2> + +<p>Since Clang has range highlighting, it never needs to pretty print your code +back out to you. This is particularly bad in G++ (which often emits errors +containing lowered vtable references), but even GCC can produce +inscrutible error messages in some cases when it tries to do this. In this +example P and Q have type "int*":</p> + +<pre> + $ <b>gcc-4.2 -fsyntax-only t.c</b> + #'exact_div_expr' not supported by pp_c_expression#'t.c:12: error: called object is not a function + $ <b>clang -fsyntax-only t.c</b> + t.c:12:8: error: called object type 'int' is not a function or function pointer + <font color="darkgreen"> (P-Q)();</font> + <font color="blue"> ~~~~~^</font> +</pre> + + +<h2>Typedef Preservation and Selective Unwrapping</h2> + +<p>Many programmers use high-level user defined types, typedefs, and other +syntactic sugar to refer to types in their program. This is useful because they +can abbreviate otherwise very long types and it is useful to preserve the +typename in diagnostics. However, sometimes very simple typedefs can wrap +trivial types and it is important to strip off the typedef to understand what +is going on. Clang aims to handle both cases well.<p> + +<p>For example, here is an example that shows where it is important to preserve +a typedef in C:</p> + +<pre> + $ <b>gcc-4.2 -fsyntax-only t.c</b> + t.c:15: error: invalid operands to binary / (have 'float __vector__' and 'const int *') + $ <b>clang -fsyntax-only t.c</b> + t.c:15:11: error: can't convert between vector values of different size ('__m128' and 'int const *') + <font color="darkgreen"> myvec[1]/P;</font> + <font color="blue"> ~~~~~~~~^~</font> +</pre> + +<p>Here the type printed by GCC isn't even valid, but if the error were about a +very long and complicated type (as often happens in C++) the error message would +be ugly just because it was long and hard to read. Here's an example where it +is useful for the compiler to expose underlying details of a typedef:</p> + +<pre> + $ <b>gcc-4.2 -fsyntax-only t.c</b> + t.c:13: error: request for member 'x' in something not a structure or union + $ <b>clang -fsyntax-only t.c</b> + t.c:13:9: error: member reference base type 'pid_t' (aka 'int') is not a structure or union + <font color="darkgreen"> myvar = myvar.x;</font> + <font color="blue"> ~~~~~ ^</font> +</pre> + +<p>If the user was somehow confused about how the system "pid_t" typedef is +defined, Clang helpfully displays it with "aka".</p> + +<p>In C++, type preservation includes retaining any qualification written into type names. For example, if we take a small snippet of code such as: + +<blockquote> +<pre> +namespace services { + struct WebService { }; +} +namespace myapp { + namespace servers { + struct Server { }; + } +} + +using namespace myapp; +void addHTTPService(servers::Server const &server, ::services::WebService const *http) { + server += http; +} +</pre> +</blockquote> + +<p>and then compile it, we see that Clang is both providing more accurate information and is retaining the types as written by the user (e.g., "servers::Server", "::services::WebService"): + +<pre> + $ <b>g++-4.2 -fsyntax-only t.cpp</b> + t.cpp:9: error: no match for 'operator+=' in 'server += http' + $ <b>clang -fsyntax-only t.cpp</b> + t.cpp:9:10: error: invalid operands to binary expression ('servers::Server const' and '::services::WebService const *') + <font color="darkgreen">server += http;</font> + <font color="blue">~~~~~~ ^ ~~~~</font> +</pre> + +<p>Naturally, type preservation extends to uses of templates, and Clang retains information about how a particular template specialization (like <code>std::vector<Real></code>) was spelled within the source code. For example:</p> + +<pre> + $ <b>g++-4.2 -fsyntax-only t.cpp</b> + t.cpp:12: error: no match for 'operator=' in 'str = vec' + $ <b>clang -fsyntax-only t.cpp</b> + t.cpp:12:7: error: incompatible type assigning 'vector<Real>', expected 'std::string' (aka 'class std::basic_string<char>') + <font color="darkgreen">str = vec</font>; + <font color="blue">^ ~~~</font> +</pre> + +<h2>Fix-it Hints</h2> + +<p>"Fix-it" hints provide advice for fixing small, localized problems +in source code. When Clang produces a diagnostic about a particular +problem that it can work around (e.g., non-standard or redundant +syntax, missing keywords, common mistakes, etc.), it may also provide +specific guidance in the form of a code transformation to correct the +problem. For example, here Clang warns about the use of a GCC +extension that has been considered obsolete since 1993:</p> + +<pre> + $ <b>clang t.c</b> + t.c:5:28: warning: use of GNU old-style field designator extension + <font color="darkgreen">struct point origin = { x: 0.0, y: 0.0 };</font> + <font color="red">~~</font> <font color="blue">^</font> + <font color="darkgreen">.x = </font> + t.c:5:36: warning: use of GNU old-style field designator extension + <font color="darkgreen">struct point origin = { x: 0.0, y: 0.0 };</font> + <font color="red">~~</font> <font color="blue">^</font> + <font color="darkgreen">.y = </font> +</pre> + +<p>The underlined code should be removed, then replaced with the code below the caret line (".x =" or ".y =", respectively). "Fix-it" hints are most useful for working around common user errors and misconceptions. For example, C++ users commonly forget the syntax for explicit specialization of class templates, as in the following error:</p> + +<pre> + $ <b>clang t.cpp</b> + t.cpp:9:3: error: template specialization requires 'template<>' + struct iterator_traits<file_iterator> { + <font color="blue">^</font> + <font color="darkgreen">template<> </font> +</pre> + +<p>Again, after describing the problem, Clang provides the fix--add <code>template<></code>--as part of the diagnostic.<p> + +<h2>Automatic Macro Expansion</h2> + +<p>Many errors happen in macros that are sometimes deeply nested. With +traditional compilers, you need to dig deep into the definition of the macro to +understand how you got into trouble. Here's a simple example that shows how +Clang helps you out:</p> + +<pre> + $ <b>gcc-4.2 -fsyntax-only t.c</b> + t.c: In function 'test': + t.c:80: error: invalid operands to binary < (have 'struct mystruct' and 'float') + $ <b>clang -fsyntax-only t.c</b> + t.c:80:3: error: invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float')) + <font color="darkgreen"> X = MYMAX(P, F);</font> + <font color="blue"> ^~~~~~~~~~~</font> + t.c:76:94: note: instantiated from: + <font color="darkgreen">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __b : __a; })</font> + <font color="blue"> ~~~ ^ ~~~</font> +</pre> + +<p>This shows how clang automatically prints instantiation information and +nested range information for diagnostics as they are instantiated through macros +and also shows how some of the other pieces work in a bigger example. Here's +another real world warning that occurs in the "window" Unix package (which +implements the "wwopen" class of APIs):</p> + +<pre> + $ <b>clang -fsyntax-only t.c</b> + t.c:22:2: warning: type specifier missing, defaults to 'int' + <font color="darkgreen"> ILPAD();</font> + <font color="blue"> ^</font> + t.c:17:17: note: instantiated from: + <font color="darkgreen">#define ILPAD() PAD((NROW - tt.tt_row) * 10) /* 1 ms per char */</font> + <font color="blue"> ^</font> + t.c:14:2: note: instantiated from: + <font color="darkgreen"> register i; \</font> + <font color="blue"> ^</font> +</pre> + +<p>In practice, we've found that this is actually more useful in multiply nested +macros that in simple ones.</p> + +</div> +</body> +</html> |
