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diff --git a/clang/www/diagnostics.html b/clang/www/diagnostics.html new file mode 100644 index 0000000..45f6907 --- /dev/null +++ b/clang/www/diagnostics.html @@ -0,0 +1,374 @@ +<!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"> + .warn { color:magenta; } + .err { color:red; } + .snip { color:darkgreen; } + .point { color:blue; } + </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. The clang command-line compiler driver uses this information +to print "point diagnostics". +(IDEs can use the information to display in-line error markup.) +Precise error location in the source 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</p> + +<p>The point (the blue "^" character) exactly shows where the problem is, even +inside of a 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> + +<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: <span class="warn">warning:</span> '.*' specified field precision is missing a matching 'int' argument + <span class="snip"> printf("%.*d");</span> + <span class="point"> ^</span> +</pre> + +<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. In the following somewhat +nonsensical example 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 +point, 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. +Range information is very useful for +cases involving precedence issues and many other cases.</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: <span class="err">error:</span> invalid operands to binary expression ('int' and 'struct A') + <span class="snip"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</span> + <span class="point"> ~~~~~~~~~~~~~~ ^ ~~~~~</span> +</pre> + +<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 +point (e.g., that this is a "binary +").</p> + +<p>Many other examples abound. In the following 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> + +<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: <span class="err">error:</span> indirection requires pointer operand ('int' invalid) + <span class="snip"> int y = *SomeA.X;</span> + <span class="point"> ^~~~~~~~</span> +</pre> + +<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. 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: <span class="err">error:</span> called object type 'int' is not a function or function pointer + <span class="snip"> (P-Q)();</span> + <span class="point"> ~~~~~^</span> +</pre> + +<p>This can be particularly bad in G++, which often emits errors + containing lowered vtable references. For example:</p> + +<pre> + $ <b>cat t.cc</b> + struct a { + virtual int bar(); + }; + + struct foo : public virtual a { + }; + + void test(foo *P) { + return P->bar() + *P; + } + $ <b>gcc-4.2 t.cc</b> + t.cc: In function 'void test(foo*)': + t.cc:9: error: no match for 'operator+' in '(((a*)P) + (*(long int*)(P->foo::<anonymous>.a::_vptr$a + -0x00000000000000020)))->a::bar() + * P' + t.cc:9: error: return-statement with a value, in function returning 'void' + $ <b>clang t.cc</b> + t.cc:9:18: <span class="err">error:</span> invalid operands to binary expression ('int' and 'foo') + <span class="snip"> return P->bar() + *P;</span> + <span class="point"> ~~~~~~~~ ^ ~~</span> +</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>The following example shows where it is important to preserve +a typedef in C. 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.</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: <span class="err">error:</span> can't convert between vector values of different size ('__m128' and 'int const *') + <span class="snip"> myvec[1]/P;</span> + <span class="point"> ~~~~~~~~^~</span> +</pre> + +<p>The following example shows where it is useful for the compiler to expose +underlying details of a typedef. If the user was somehow confused about how the +system "pid_t" typedef is defined, Clang helpfully displays it with "aka".</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: <span class="err">error:</span> member reference base type 'pid_t' (aka 'int') is not a structure or union + <span class="snip"> myvar = myvar.x;</span> + <span class="point"> ~~~~~ ^</span> +</pre> + +<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: <span class="err">error:</span> invalid operands to binary expression ('servers::Server const' and '::services::WebService const *') + <span class="snip">server += http;</span> + <span class="point">~~~~~~ ^ ~~~~</span> +</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: <span class="err">error:</span> incompatible type assigning 'vector<Real>', expected 'std::string' (aka 'class std::basic_string<char>') + <span class="snip">str = vec</span>; + <span class="point">^ ~~~</span> +</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. In the following example, Clang warns about the use of a GCC +extension that has been considered obsolete since 1993. The underlined +code should be removed, then replaced with the code below the +point line (".x =" or ".y =", respectively).</p> + +<pre> + $ <b>clang t.c</b> + t.c:5:28: <span class="warn">warning:</span> use of GNU old-style field designator extension + <span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span> + <span class="err">~~</span> <span class="point">^</span> + <span class="snip">.x = </span> + t.c:5:36: <span class="warn">warning:</span> use of GNU old-style field designator extension + <span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span> + <span class="err">~~</span> <span class="point">^</span> + <span class="snip">.y = </span> +</pre> + +<p>"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 error in the following example. Again, after describing the problem, +Clang provides the fix--add <code>template<></code>--as part of the +diagnostic.<p> + +<pre> + $ <b>clang t.cpp</b> + t.cpp:9:3: <span class="err">error:</span> template specialization requires 'template<>' + struct iterator_traits<file_iterator> { + <span class="point">^</span> + <span class="snip">template<> </span> +</pre> + +<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. The following simple example shows how +Clang helps you out by automatically printing 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.</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: <span class="err">error:</span> invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float')) + <span class="snip"> X = MYMAX(P, F);</span> + <span class="point"> ^~~~~~~~~~~</span> + t.c:76:94: note: instantiated from: + <span class="snip">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __b : __a; })</span> + <span class="point"> ~~~ ^ ~~~</span> +</pre> + +<p>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: <span class="warn">warning:</span> type specifier missing, defaults to 'int' + <span class="snip"> ILPAD();</span> + <span class="point"> ^</span> + t.c:17:17: note: instantiated from: + <span class="snip">#define ILPAD() PAD((NROW - tt.tt_row) * 10) /* 1 ms per char */</span> + <span class="point"> ^</span> + t.c:14:2: note: instantiated from: + <span class="snip"> register i; \</span> + <span class="point"> ^</span> +</pre> + +<p>In practice, we've found that Clang's treatment of macros is actually more useful in multiply nested +macros that in simple ones.</p> + +<h2>Quality of Implementation and Attention to Detail</h2> + +<p>Finally, we have put a lot of work polishing the little things, because +little things add up over time and contribute to a great user experience.</p> + +<p>The following example shows a trivial little tweak, where we tell you to put the semicolon at +the end of the line that is missing it (line 4) instead of at the beginning of +the following line (line 5). This is particularly important with fixit hints +and point diagnostics, because otherwise you don't get the important context. +</p> + +<pre> + $ <b>gcc-4.2 t.c</b> + t.c: In function 'foo': + t.c:5: error: expected ';' before '}' token + $ <b>clang t.c</b> + t.c:4:8: <span class="err">error:</span> expected ';' after expression + <span class="snip"> bar()</span> + <span class="point"> ^</span> + <span class="point"> ;</span> +</pre> + +<p>The following example shows much better error recovery than GCC. The message coming out +of GCC is completely useless for diagnosing the problem. Clang tries much harder +and produces a much more useful diagnosis of the problem.</p> + +<pre> + $ <b>gcc-4.2 t.c</b> + t.c:3: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token + $ <b>clang t.c</b> + t.c:3:1: <span class="err">error:</span> unknown type name 'foo_t' + <span class="snip">foo_t *P = 0;</span> + <span class="point">^</span> +</pre> + +<p>The following example shows that we recover from the simple case of +forgetting a ; after a struct definition much better than GCC.</p> + +<pre> + $ <b>cat t.cc</b> + template<class T> + class a {} + class temp {}; + a<temp> b; + struct b { + } + $ <b>gcc-4.2 t.cc</b> + t.cc:3: error: multiple types in one declaration + t.cc:4: error: non-template type 'a' used as a template + t.cc:4: error: invalid type in declaration before ';' token + t.cc:6: error: expected unqualified-id at end of input + $ <b>clang t.cc</b> + t.cc:2:11: <span class="err">error:</span> expected ';' after class + <span class="snip">class a {}</span> + <span class="point"> ^</span> + <span class="point"> ;</span> + t.cc:6:2: <span class="err">error:</span> expected ';' after struct + <span class="snip">}</span> + <span class="point"> ^</span> + <span class="point"> ;</span> +</pre> + +<p>While each of these details is minor, we feel that they all add up to provide +a much more polished experience.</p> + +</div> +</body> +</html> |