[cfe-commits] r67318 - /cfe/trunk/www/diagnostics.html
Chris Lattner
sabre at nondot.org
Thu Mar 19 11:52:18 PDT 2009
Author: lattner
Date: Thu Mar 19 13:52:17 2009
New Revision: 67318
URL: http://llvm.org/viewvc/llvm-project?rev=67318&view=rev
Log:
split diagnostics examples out to its own file.
Added:
cfe/trunk/www/diagnostics.html
Added: cfe/trunk/www/diagnostics.html
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/www/diagnostics.html?rev=67318&view=auto
==============================================================================
--- cfe/trunk/www/diagnostics.html (added)
+++ cfe/trunk/www/diagnostics.html Thu Mar 19 13:52:17 2009
@@ -0,0 +1,213 @@
+<!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>
+
+<h2>Fix-it Hints</h2>
+
+<p>simple example + template<> example</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>
+
+<h2>C++ Fun Examples</h2>
+
+<p>...</p>
+
+
+</div>
+</body>
+</html>
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