Manuel LÃpez-IbÃÃez - RFC: Wextra digest (fixing PR7651) (original) (raw)

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• From: "Manuel LÃpez-IbÃÃez"
• To: gcc
• Cc: "Gabriel Dos Reis"
• Date: Thu, 11 Jan 2007 01:56:20 +0000
• Subject: RFC: Wextra digest (fixing PR7651)

The goal is to fix PR7651 and convert Wextra into a super-option, that is an -W* option that just enables other options but it doesn't emit warnings by itself (other super-options are Wall and Wunused).

This is a summary of the current status of Wextra for mainline to the best of my knowledge. I welcome comments on how to group and name the new -W* options that will take over the warnings produced by Wextra.

In Java, Wextra warns for unreachable bytecode. (maybe this should be warned by -Wunreachable-code or by a new option -Wunreachable-bytecode)

In C++, it warns for* Subscripting an array which has been declared register.* Taking the address of a variable which has been declared register.* A base class is not initialized in a derived class' copy constructor.

• A non-static reference or non-static const member appears in a class without constructors.

• Ambiguous virtual bases (virtual base inaccessible due to ambiguity). (There is also an unconditional warning for direct base inaccessible due to ambiguity)

• An enumerator and a non-enumerator both appear in a conditional expression. (There is also an unconditional warning for two different enumeral types used in a conditional expression).

Only for C, it enables:

• -Wmissing-parameter-type : A function parameter is declared without a type specifier in K&R-style functions.

• -Wold-style-declaration : Storage-class specifiers like static are not the first things in a declaration.

For both C and C++:* A function can return either with or without a value.

• An expression-statement or the left-hand side of a comma expression contains no side effects. For example, an expression such as x[i,j]. This is also warned by Wunused-value. In addition, Wextra enables Wunused-value but this is not documented (and -Wunused-value is already enabled by -Wall).

• If -Wunused or -Wall is given, it enables -Wunused-parameter. (manual could be more explicit here)

* -Wuninitialized is enabled if -Ox is given. (not documented)

• A pointer is compared against integer zero with <, <=, >, or >=. This is a pedwarn and it can also be enabled by using -pedantic. If the pointer is the rightmost operator, there is no warning for Wextra (surely a bug).

* -Wclobbered : A variable might be changed by longjmp or vfork.* -Wempty-body : An empty body occurs in an if or else statement.

• -Wsign-compare : A comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned.

• -Wmissing-field-initializers : An aggregate has an initializer which does not initialize all members.

• -Woverride-init : An initialized field without side effects is overridden when using designated initializers.

• An unsigned value is compared against zero with < or >=. Walways-true claims to warn for this but it doesn't. There is also an unconditional warning for expressions that are always true or false due to the range of types.

In ./gcc/config/sh/symbian.c:158 there is a warning enabled by Wextra with the following code (notice the OPT_Wattributes) :

/* We ignore the dllimport attribute for inline member functions. This differs from MSVC behavior which treats it like GNUC 'extern inline' extension. */ else if (TREE_CODE (decl) == FUNCTION_DECL && DECL_INLINE (decl)) { if (extra_warnings) warning (OPT_Wattributes, "inline function %q+D is declared as " "dllimport: attribute ignored", decl); return false; }

Finally, the manual page claims that Wextra warns for any of several floating-point events that often indicate errors, such as overflow, underflow, loss of precision, etc. I wasn't able to find any instance of this. I am fairly sure that Wextra doesn't do such thing.

Cheers,Manuel.

• Follow-Ups:
  • Re: RFC: Wextra digest (fixing PR7651)
    * From: Ian Lance Taylor