vasilito
cb424d7448
verify-patch-sanity.py validates every active recipe .patch has internally- consistent hunk line counts — catching the 'malformed patch at line N' failure at commit/CI/preflight time instead of hours into a cook. This cycle hit that class three times (qtwaylandscanner, sddm, xwayland), each only discovered when cookbook tried to apply the patch. Running it across the repo found 29 latent malformed patches (validated against GNU patch: e.g. relibc/P3-sysv-ipc reproduces 'malformed patch at line 22'). They were harmless only because they sit in vendored recipes (baked, not re- applied) — but would fail on any version-bump re-derivation. --fix recounts the hunk headers (body untouched) and repaired all 29. Wired into build-preflight.sh (Phase 1.0D) and redbear-ci.yml, with a unit test (test-patch-sanity.sh). Skips archived/legacy trees and unvalidatable formats (empty placeholders, bare-@@ git hunks).
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367 lines
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<title>Clang - Expressive Diagnostics</title>
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<!--#include virtual="menu.html.incl"-->
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<div id="content">
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<!--=======================================================================-->
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<h1>Expressive Diagnostics</h1>
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<!--=======================================================================-->
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<p>In addition to being fast and functional, we aim to make Clang extremely user
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friendly. As far as a command-line compiler goes, this basically boils down to
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making the diagnostics (error and warning messages) generated by the compiler
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be as useful as possible. There are several ways that we do this. This section
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talks about the experience provided by the command line compiler, contrasting
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Clang output to GCC 4.9's output in some cases.
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</p>
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<h2>Column Numbers and Caret Diagnostics</h2>
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<p>First, all diagnostics produced by clang include full column number
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information. The clang command-line compiler driver uses this information
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to print "point diagnostics".
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(IDEs can use the information to display in-line error markup.)
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This is nice because it makes it very easy to understand exactly
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what is wrong in a particular piece of code.</p>
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<p>The point (the green "^" character) exactly shows where the problem is, even
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inside of a string. This makes it really easy to jump to the problem and
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helps when multiple instances of the same character occur on a line. (We'll
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revisit this more in following examples.)</p>
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<pre>
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$ <span class="cmd">clang -fsyntax-only format-strings.c</span>
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<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>
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<span class="snip" > printf("%.*d");</span>
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<span class="point"> ^</span>
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</pre>
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<p>Note that modern versions of GCC have followed Clang's lead, and are
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now able to give a column for a diagnostic, and include a snippet of source
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text in the result. However, Clang's column number is much more accurate,
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pointing at the problematic format specifier, rather than the <tt>)</tt>
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character the parser had reached when the problem was detected.
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Also, Clang's diagnostic is colored by default, making it easier to
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distinguish from nearby text.</p>
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<h2>Range Highlighting for Related Text</h2>
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<p>Clang captures and accurately tracks range information for expressions,
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statements, and other constructs in your program and uses this to make
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diagnostics highlight related information. In the following somewhat
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nonsensical example you can see that you don't even need to see the original source code to
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understand what is wrong based on the Clang error. Because clang prints a
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point, you know exactly <em>which</em> plus it is complaining about. The range
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information highlights the left and right side of the plus which makes it
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immediately obvious what the compiler is talking about.
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Range information is very useful for
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cases involving precedence issues and many other cases.</p>
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<pre>
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$ <span class="cmd">gcc-4.9 -fsyntax-only t.c</span>
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t.c: In function 'int f(int, int)':
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t.c:7:39: error: invalid operands to binary + (have 'int' and 'struct A')
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return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);
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^
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$ <span class="cmd">clang -fsyntax-only t.c</span>
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<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>
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<span class="snip" > return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</span>
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<span class="point"> ~~~~~~~~~~~~~~ ^ ~~~~~</span>
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</pre>
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<h2>Precision in Wording</h2>
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<p>A detail is that we have tried really hard to make the diagnostics that come
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out of clang contain exactly the pertinent information about what is wrong and
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why. In the example above, we tell you what the inferred types are for
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the left and right hand sides, and we don't repeat what is obvious from the
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point (e.g., that this is a "binary +").</p>
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<p>Many other examples abound. In the following example, not only do we tell you
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that there is a problem with the <tt>*</tt>
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and point to it, we say exactly why and tell you what the type is (in case it is
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a complicated subexpression, such as a call to an overloaded function). This
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sort of attention to detail makes it much easier to understand and fix problems
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quickly.</p>
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<pre>
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$ <span class="cmd">gcc-4.9 -fsyntax-only t.c</span>
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t.c:5:11: error: invalid type argument of unary '*' (have 'int')
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return *SomeA.X;
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^
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$ <span class="cmd">clang -fsyntax-only t.c</span>
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<span class="loc">t.c:5:11:</span> <span class="err">error:</span> <span class="msg">indirection requires pointer operand ('int' invalid)</span>
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<span class="snip" > int y = *SomeA.X;</span>
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<span class="point"> ^~~~~~~~</span>
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</pre>
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<h2>Typedef Preservation and Selective Unwrapping</h2>
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<p>Many programmers use high-level user defined types, typedefs, and other
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syntactic sugar to refer to types in their program. This is useful because they
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can abbreviate otherwise very long types and it is useful to preserve the
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typename in diagnostics. However, sometimes very simple typedefs can wrap
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trivial types and it is important to strip off the typedef to understand what
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is going on. Clang aims to handle both cases well.<p>
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<p>The following example shows where it is important to preserve
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a typedef in C.</p>
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<pre>
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$ <span class="cmd">clang -fsyntax-only t.c</span>
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<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>
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<span class="snip"> myvec[1]/P;</span>
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<span class="point"> ~~~~~~~~^~</span>
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</pre>
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<p>The following example shows where it is useful for the compiler to expose
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underlying details of a typedef. If the user was somehow confused about how the
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system "pid_t" typedef is defined, Clang helpfully displays it with "aka".</p>
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<pre>
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$ <span class="cmd">clang -fsyntax-only t.c</span>
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<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>
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<span class="snip"> myvar = myvar.x;</span>
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<span class="point"> ~~~~~ ^</span>
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</pre>
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<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:
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<blockquote>
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<pre>
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namespace services {
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struct WebService { };
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}
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namespace myapp {
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namespace servers {
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struct Server { };
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}
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}
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using namespace myapp;
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void addHTTPService(servers::Server const &server, ::services::WebService const *http) {
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server += http;
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}
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</pre>
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</blockquote>
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<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"):
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<pre>
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$ <span class="cmd">clang -fsyntax-only t.cpp</span>
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<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>
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<span class="snip">server += http;</span>
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<span class="point">~~~~~~ ^ ~~~~</span>
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</pre>
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<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>
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<pre>
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$ <span class="cmd">clang -fsyntax-only t.cpp</span>
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<span class="loc">t.cpp:12:7:</span> <span class="err">error:</span> <span class="msg">incompatible type assigning 'vector<Real>', expected 'std::string' (aka 'class std::basic_string<char>')</span>
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<span class="snip">str = vec</span>;
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<span class="point">^ ~~~</span>
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</pre>
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<h2>Fix-it Hints</h2>
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<p>"Fix-it" hints provide advice for fixing small, localized problems
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in source code. When Clang produces a diagnostic about a particular
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problem that it can work around (e.g., non-standard or redundant
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syntax, missing keywords, common mistakes, etc.), it may also provide
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specific guidance in the form of a code transformation to correct the
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problem. In the following example, Clang warns about the use of a GCC
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extension that has been considered obsolete since 1993. The underlined
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code should be removed, then replaced with the code below the
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point line (".x =" or ".y =", respectively).</p>
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<pre>
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$ <span class="cmd">clang t.c</span>
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<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>
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<span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span>
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<span class="err">~~</span> <span class="msg"><span class="point">^</span></span>
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<span class="snip">.x = </span>
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<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>
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<span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span>
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<span class="err">~~</span> <span class="msg"><span class="point">^</span></span>
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<span class="snip">.y = </span>
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</pre>
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<p>"Fix-it" hints are most useful for
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working around common user errors and misconceptions. For example, C++ users
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commonly forget the syntax for explicit specialization of class templates,
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as in the error in the following example. Again, after describing the problem,
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Clang provides the fix--add <code>template<></code>--as part of the
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diagnostic.<p>
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<pre>
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$ <span class="cmd">clang t.cpp</span>
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<span class="loc">t.cpp:9:3:</span> <span class="err">error:</span> <span class="msg">template specialization requires 'template<>'</span>
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struct iterator_traits<file_iterator> {
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<span class="point">^</span>
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<span class="snip">template<> </span>
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</pre>
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<h2>Template Type Diffing</h2>
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<p>Templates types can be long and difficult to read. More so when part of an
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error message. Instead of just printing out the type name, Clang has enough
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information to remove the common elements and highlight the differences. To
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show the template structure more clearly, the templated type can also be
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printed as an indented text tree.</p>
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Default: template diff with type elision
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<pre>
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<span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion from 'vector<map<[...], <span class="template-highlight">float</span>>>' to 'vector<map<[...], <span class="template-highlight">double</span>>>' for 1st argument;
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</pre>
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-fno-elide-type: template diff without elision
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<pre>
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<span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion from 'vector<map<int, <span class="template-highlight">float</span>>>' to 'vector<map<int, <span class="template-highlight">double</span>>>' for 1st argument;
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</pre>
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-fdiagnostics-show-template-tree: template tree printing with elision
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<pre>
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<span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion for 1st argument;
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vector<
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map<
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[...],
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[<span class="template-highlight">float</span> != <span class="template-highlight">double</span>]>>
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</pre>
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-fdiagnostics-show-template-tree -fno-elide-type: template tree printing with no elision
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<pre>
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<span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion for 1st argument;
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vector<
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map<
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int,
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[<span class="template-highlight">float</span> != <span class="template-highlight">double</span>]>>
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</pre>
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<h2>Automatic Macro Expansion</h2>
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<p>Many errors happen in macros that are sometimes deeply nested. With
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traditional compilers, you need to dig deep into the definition of the macro to
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understand how you got into trouble. The following simple example shows how
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Clang helps you out by automatically printing instantiation information and
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nested range information for diagnostics as they are instantiated through macros
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and also shows how some of the other pieces work in a bigger example.</p>
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<pre>
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$ <span class="cmd">clang -fsyntax-only t.c</span>
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<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>
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<span class="snip"> X = MYMAX(P, F);</span>
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<span class="point"> ^~~~~~~~~~~</span>
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<span class="loc">t.c:76:94:</span> <span class="note">note:</span> expanded from:
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<span class="snip">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __b : __a; })</span>
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<span class="point"> ~~~ ^ ~~~</span>
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</pre>
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<p>Here's another real world warning that occurs in the "window" Unix package (which
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implements the "wwopen" class of APIs):</p>
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<pre>
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$ <span class="cmd">clang -fsyntax-only t.c</span>
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<span class="loc">t.c:22:2:</span> <span class="warn">warning:</span> <span class="msg">type specifier missing, defaults to 'int'</span>
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<span class="snip"> ILPAD();</span>
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<span class="point"> ^</span>
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<span class="loc">t.c:17:17:</span> <span class="note">note:</span> expanded from:
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<span class="snip">#define ILPAD() PAD((NROW - tt.tt_row) * 10) /* 1 ms per char */</span>
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<span class="point"> ^</span>
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<span class="loc">t.c:14:2:</span> <span class="note">note:</span> expanded from:
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<span class="snip"> register i; \</span>
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<span class="point"> ^</span>
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</pre>
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<p>In practice, we've found that Clang's treatment of macros is actually more useful in multiply nested
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macros than in simple ones.</p>
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<h2>Quality of Implementation and Attention to Detail</h2>
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<p>Finally, we have put a lot of work polishing the little things, because
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little things add up over time and contribute to a great user experience.</p>
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<p>The following example shows that we recover from the simple case of
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forgetting a ; after a struct definition much better than GCC.</p>
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<pre>
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$ <span class="cmd">cat t.cc</span>
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template<class T>
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class a {};
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struct b {}
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a<int> c;
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$ <span class="cmd">gcc-4.9 t.cc</span>
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t.cc:4:8: error: invalid declarator before 'c'
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a<int> c;
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^
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$ <span class="cmd">clang t.cc</span>
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<span class="loc">t.cc:3:12:</span> <span class="err">error:</span> <span class="msg">expected ';' after struct</span>
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<span class="snip" >struct b {}</span>
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<span class="point"> ^</span>
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<span class="point"> ;</span>
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</pre>
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<p>The following example shows that we diagnose and recover from a missing
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<tt>typename</tt> keyword well, even in complex circumstances where GCC
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cannot cope.</p>
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<pre>
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$ <span class="cmd">cat t.cc</span>
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template<class T> void f(T::type) { }
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struct A { };
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void g()
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{
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A a;
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f<A>(a);
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}
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$ <span class="cmd">gcc-4.9 t.cc</span>
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t.cc:1:33: error: variable or field 'f' declared void
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template<class T> void f(T::type) { }
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^
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t.cc: In function 'void g()':
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t.cc:6:5: error: 'f' was not declared in this scope
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f<A>(a);
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^
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t.cc:6:8: error: expected primary-expression before '>' token
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f<A>(a);
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^
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$ <span class="cmd">clang t.cc</span>
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<span class="loc">t.cc:1:26:</span> <span class="err">error:</span> <span class="msg">missing 'typename' prior to dependent type name 'T::type'</span>
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<span class="snip" >template<class T> void f(T::type) { }</span>
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<span class="point"> ^~~~~~~</span>
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<span class="point"> typename </span>
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<span class="loc">t.cc:6:5:</span> <span class="err">error:</span> <span class="msg">no matching function for call to 'f'</span>
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<span class="snip" > f<A>(a);</span>
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<span class="point"> ^~~~</span>
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<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>
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<span class="snip" >template<class T> void f(T::type) { }</span>
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<span class="point"> ^ ~~~~</span>
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</pre>
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<p>While each of these details is minor, we feel that they all add up to provide
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a much more polished experience.</p>
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</div>
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</body>
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</html>
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