maxjhandsome
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Releases Wiki Activity

Update page on clang diagnostics to contrast to GCC 4.9 instead of 4.2. A lot 

of the differences we identified here have been fixed by improvements in GCC.

llvm-svn: 199970
This commit is contained in:
Richard Smith 2014-01-24 03:13:34 +00:00
parent 61adb27de4
commit 0e50a883ff
1 changed files with 127 additions and 162 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

287
clang/www/diagnostics.html
View File

@ -7,10 +7,14 @@
<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; }
.loc { font-weight: bold; }
.err { color:red; font-weight: bold; }
.warn { color:magenta; font-weight: bold; }
.note { color:gray; font-weight: bold; }
.msg { font-weight: bold; }
.cmd { font-style: italic; }
.snip { }
.point { color:green; font-weight: bold; }
</style>
</head>
<body>
@ -29,10 +33,7 @@ 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.-->
Clang output to GCC 4.9's output in some cases.
</p>
<h2>Column Numbers and Caret Diagnostics</h2>
@ -41,25 +42,35 @@ that embed Clang and extract equivalent information through internal APIs.-->
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>
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
<p>The point (the green "^" 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="cmd">gcc-4.9 -fsyntax-only -Wformat format-strings.c</span>
format-strings.c: In function 'void f()':
format-strings.c:91:16: warning: field precision specifier '.*' expects a matching 'int' argument [-Wformat=]
printf("%.*d");
^
format-strings.c:91:16: warning: format '%d' expects a matching 'int' argument [-Wformat=]
$ <span class="cmd">clang -fsyntax-only format-strings.c</span>
<span class="loc">format-strings.c:91:13:</span> <span class="warn">warning:</span> <span class="msg">'.*' specified field precision is missing a matching 'int' argument</span>
<span class="snip" > printf("%.*d");</span>
<span class="point"> ^</span>
</pre>
<p>Note that modern versions of GCC have followed Clang's lead, and are
now able to give a column for a diagnostic, and include a snippet of source
text in the result. However, Clang's column number is much more accurate,
pointing at the problematic format specifier, rather than the <tt>)</tt>
character the parser had reached when the problem was detected.
Also, Clang's diagnostic is colored by default, making it easier to
distinguish from nearby text.</p>
<h2>Range Highlighting for Related Text</h2>
<p>Clang captures and accurately tracks range information for expressions,
@ -74,11 +85,14 @@ 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="cmd">gcc-4.9 -fsyntax-only t.c</span>
t.c: In function 'int f(int, int)':
t.c:7:39: error: invalid operands to binary + (have 'int' and 'struct A')
return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);
^
$ <span class="cmd">clang -fsyntax-only t.c</span>
<span class="loc">t.c:7:39:</span> <span class="err">error:</span> <span class="msg">invalid operands to binary expression ('int' and 'struct A')</span>
<span class="snip" > return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</span>
<span class="point"> ~~~~~~~~~~~~~~ ^ ~~~~~</span>
</pre>
@ -90,62 +104,24 @@ 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 *
<p>Many other examples abound. In the following example, not only do we tell you
that there is a problem with the <tt>*</tt>
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="cmd">gcc-4.9 -fsyntax-only t.c</span>
t.c:5:11: error: invalid type argument of unary '*' (have 'int')
return *SomeA.X;
^
$ <span class="cmd">clang -fsyntax-only t.c</span>
<span class="loc">t.c:5:11:</span> <span class="err">error:</span> <span class="msg">indirection requires pointer operand ('int' invalid)</span>
<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-&gt;foo::&lt;anonymous&gt;.a::_vptr$a + -0x00000000000000020)))-&gt;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
@ -156,15 +132,11 @@ 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>
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: <span class="err">error:</span> can't convert between vector values of different size ('__m128' and 'int const *')
$ <span class="cmd">clang -fsyntax-only t.c</span>
<span class="loc">t.c:15:11:</span> <span class="err">error:</span> <span class="msg">can't convert between vector values of different size ('__m128' and 'int const *')</span>
<span class="snip"> myvec[1]/P;</span>
<span class="point"> ~~~~~~~~^~</span>
</pre>
@ -174,10 +146,8 @@ 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="cmd">clang -fsyntax-only t.c</span>
<span class="loc">t.c:13:9:</span> <span class="err">error:</span> <span class="msg">member reference base type 'pid_t' (aka 'int') is not a structure or union</span>
<span class="snip"> myvar = myvar.x;</span>
<span class="point"> ~~~~~ ^</span>
</pre>
@ -202,13 +172,11 @@ void addHTTPService(servers::Server const &amp;server, ::services::WebService co
</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"):
<p>and then compile it, we see that Clang is both providing 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="cmd">clang -fsyntax-only t.cpp</span>
<span class="loc">t.cpp:9:10:</span> <span class="err">error:</span> <span class="msg">invalid operands to binary expression ('servers::Server const' and '::services::WebService const *')</span>
<span class="snip">server += http;</span>
<span class="point">~~~~~~ ^ ~~~~</span>
</pre>
@ -216,10 +184,8 @@ void addHTTPService(servers::Server const &amp;server, ::services::WebService co
<p>Naturally, type preservation extends to uses of templates, and Clang retains information about how a particular template specialization (like <code>std::vector&lt;Real&gt;</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&lt;Real&gt;', expected 'std::string' (aka 'class std::basic_string&lt;char&gt;')
$ <span class="cmd">clang -fsyntax-only t.cpp</span>
<span class="loc">t.cpp:12:7:</span> <span class="err">error:</span> <span class="msg">incompatible type assigning 'vector&lt;Real&gt;', expected 'std::string' (aka 'class std::basic_string&lt;char&gt;')</span>
<span class="snip">str = vec</span>;
<span class="point">^ ~~~</span>
</pre>
@ -237,14 +203,14 @@ 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="cmd">clang t.c</span>
<span class="loc">t.c:5:28:</span> <span class="warn">warning:</span> <span class="msg">use of GNU old-style field designator extension</span>
<span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span>
<span class="err">~~</span> <span class="point">^</span>
<span class="err">~~</span> <span class="msg"><span class="point">^</span></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="loc">t.c:5:36:</span> <span class="warn">warning:</span> <span class="msg">use of GNU old-style field designator extension</span>
<span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span>
<span class="err">~~</span> <span class="point">^</span>
<span class="err">~~</span> <span class="msg"><span class="point">^</span></span>
<span class="snip">.y = </span>
</pre>
@ -256,8 +222,8 @@ Clang provides the fix--add <code>template&lt;&gt;</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&lt;&gt;'
$ <span class="cmd">clang t.cpp</span>
<span class="loc">t.cpp:9:3:</span> <span class="err">error:</span> <span class="msg">template specialization requires 'template&lt;&gt;'</span>
struct iterator_traits&lt;file_iterator&gt; {
<span class="point">^</span>
<span class="snip">template&lt;&gt; </span>
@ -273,15 +239,15 @@ printed as an indented text tree.</p>
Default: template diff with type elision
<pre>
t.cc:4:5: <span class="note">note:</span> candidate function not viable: no known conversion from 'vector&lt;map&lt;[...], <span class="template-highlight">float</span>&gt;&gt;' to 'vector&lt;map&lt;[...], <span class="template-highlight">double</span>&gt;&gt;' for 1st argument;
<span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion from 'vector&lt;map&lt;[...], <span class="template-highlight">float</span>&gt;&gt;' to 'vector&lt;map&lt;[...], <span class="template-highlight">double</span>&gt;&gt;' for 1st argument;
</pre>
-fno-elide-type: template diff without elision
<pre>
t.cc:4:5: <span class="note">note:</span> candidate function not viable: no known conversion from 'vector&lt;map&lt;int, <span class="template-highlight">float</span>&gt;&gt;' to 'vector&lt;map&lt;int, <span class="template-highlight">double</span>&gt;&gt;' for 1st argument;
<span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion from 'vector&lt;map&lt;int, <span class="template-highlight">float</span>&gt;&gt;' to 'vector&lt;map&lt;int, <span class="template-highlight">double</span>&gt;&gt;' for 1st argument;
</pre>
-fdiagnostics-show-template-tree: template tree printing with elision
<pre>
t.cc:4:5: <span class="note">note:</span> candidate function not viable: no known conversion for 1st argument;
<span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion for 1st argument;
vector&lt;
map&lt;
[...],
@ -289,7 +255,7 @@ t.cc:4:5: <span class="note">note:</span> candidate function not viable: no know
</pre>
-fdiagnostics-show-template-tree -fno-elide-type: template tree printing with no elision
<pre>
t.cc:4:5: <span class="note">note:M</span> candidate function not viable: no known conversion for 1st argument;
<span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion for 1st argument;
vector&lt;
map&lt;
int,
@ -306,14 +272,11 @@ 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 &lt; (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="cmd">clang -fsyntax-only t.c</span>
<span class="loc">t.c:80:3:</span> <span class="err">error:</span> <span class="msg">invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float'))</span>
<span class="snip"> X = MYMAX(P, F);</span>
<span class="point"> ^~~~~~~~~~~</span>
t.c:76:94: note: expanded from:
<span class="loc">t.c:76:94:</span> <span class="note">note:</span> expanded from:
<span class="snip">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a &lt; __b ? __b : __a; })</span>
<span class="point"> ~~~ ^ ~~~</span>
</pre>
@ -322,14 +285,14 @@ and also shows how some of the other pieces work in a bigger example.</p>
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="cmd">clang -fsyntax-only t.c</span>
<span class="loc">t.c:22:2:</span> <span class="warn">warning:</span> <span class="msg">type specifier missing, defaults to 'int'</span>
<span class="snip"> ILPAD();</span>
<span class="point"> ^</span>
t.c:17:17: note: expanded from:
<span class="loc">t.c:17:17:</span> <span class="note">note:</span> expanded 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: expanded from:
<span class="loc">t.c:14:2:</span> <span class="note">note:</span> expanded from:
<span class="snip"> register i; \</span>
<span class="point"> ^</span>
</pre>
@ -342,63 +305,65 @@ macros that in simple ones.</p>
<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>
$ <span class="cmd">cat t.cc</span>
template&lt;class T&gt;
class a {}
class temp {};
a&lt;temp&gt; 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>
class a {};
struct b {}
a&lt;int&gt; c;
$ <span class="cmd">gcc-4.9 t.cc</span>
t.cc:4:8: error: invalid declarator before 'c'
a<int> c;
^
$ <span class="cmd">clang t.cc</span>
<span class="loc">t.cc:3:12:</span> <span class="err">error:</span> <span class="msg">expected ';' after struct</span>
<span class="snip" >struct b {}</span>
<span class="point"> ^</span>
<span class="point"> ;</span>
</pre>
<p>The following example shows that we diagnose and recover from a missing
<tt>typename</tt> keyword well, even in complex circumstances where GCC
cannot cope.</p>
<pre>
$ <span class="cmd">cat t.cc</span>
template&lt;class T&gt; void f(T::type) { }
struct A { };
void g()
{
A a;
f&lt;A&gt;(a);
}
$ <span class="cmd">gcc-4.9 t.cc</span>
t.cc:1:33: error: variable or field 'f' declared void
template&lt;class T&gt; void f(T::type) { }
^
t.cc: In function 'void g()':
t.cc:6:5: error: 'f' was not declared in this scope
f&lt;A&gt;(a);
^
t.cc:6:8: error: expected primary-expression before '>' token
f&lt;A&gt;(a);
^
$ <span class="cmd">clang t.cc</span>
<span class="loc">t.cc:1:26:</span> <span class="error">error:</span> <span class="msg">missing 'typename' prior to dependent type name 'T::type'</span>
<span class="snip" >template&lt;class T&gt; void f(T::type) { }</span>
<span class="point"> ^~~~~~~</span>
<span class="point"> typename </span>
<span class="loc">t.cc:6:5:</span> <span class="error">error:</span> <span class="msg">no matching function for call to 'f'</span>
<span class="snip" > f&lt;A&gt;(a);</span>
<span class="point"> ^~~~</span>
<span class="loc">t.cc:1:24:</span> <span class="note">note:</span> <span class="msg">candidate template ignored: substitution failure [with T = A]: no type named 'type' in 'A'</span>
<span class="snip" >template&lt;class T&gt; void f(T::type) { }</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>