On 12/4/18 2:04 PM, Sandra Loosemore wrote:
On 12/4/18 9:26 AM, Martin Sebor wrote:
Thanks for the comments. Attached is an updated patch with
the typos fixed. I've left the rest as is.
Well, I still think a number of points I commented on before need to be
clarified in the text. If I'm confused, I think other readers will be,
too, and users shouldn't have to search for discussion on gcc-patches to
figure out what the manual means.
+Unless specified otherwise, function attributes that apply to function
+parameters of pointer types also apply to the implicit C++ @code{this}
+argument in non-static member functions, and to parameters of reference
+type. In attributes that refer to function parameters by their position
+in the argument list the first parameter is at position one. In C++
+non-static member functions position one refers to the implicit
@code{this}
+pointer. The same restrictions and effects apply to function attributes
+used with ordinary functions or C++ member functions.
Like this paragraph above. Users may read that first sentence and think
they can attach function attributes to parameter declarations. I'd
suggest rewriting this as
"function attributes that specify properties of the function's
parameters, like @code{nonnull}, also apply...."
or something like that.
I've adjusted the words to hopefully make this clearer. I've added
a new term: /positional argument/ and used it to avoid the confusion.
There is unfortunately no good example to use here because the this
pointer is not a valid argument to any such attribute. All but
nonnull trigger a warning when applied to this. It's only important
to discuss because position 1 refers to it and the remaining arguments
in a member function start at position 2.
(I also noticed that the text referred to "attribute declarations"
so I changed those to "attribute specifications.")
+Unless specified otherwise, function attributes that apply to variables
+or function parameters of pointer types also apply to the implicit C++
+@code {this} argument in non-static member functions, and to
arguments of
+reference type.
+
And similarly here, but this is even more confusing. What function
attributes apply to variables, and not just parameters? Given this is
the variable attribute section, I think it would be more useful to say
something about how you attach a variable attribute to the "this"
parameter, rather than talking about function attributes.
This didn't come out quite right. The intent is to make it clear
that variable attributes that apply to pointers can also be used
with references (to pointers or to other types), and ditto for
structs and classes (see the questions below). Perhaps adding
the clarification isn't helpful here so I've removed it.
+The keyword @code{__attribute__} allows you to specify various special
+properties of types. Some type attributes apply only to structure and
+union types, and in C++, also class types, while others can apply to
+any type defined via a @code{typedef} declaration. Unless otherwise
+specified, the same restrictions and effects apply to attributes
regardless
+of whether a type is a trivial structure or a C++ class with
user-defined
+constructors, destructors, or a copy assignment.
And here I would really prefer to use standard terminology than trying
to inaccurately summarize what the terminology means. E.g.
"...whether or not a class is trivial (in the C++11 sense) or POD (in
C++98)."
This doesn't say what we want to say (nor is it accurate).
Here are the user's questions again:
The documentation should clarify how it handles structs/
classes/unions and references. Does it threat references
like pointers? Does it only allow PODs/trivial types to be
returned, or does it invoke the copy constructor, when it
is used again?
They were about const/pure but the same questions could be asked
about other attributes as well. The simple answer I'm trying to
give is that it doesn't matter: (unless the manual says otherwise)
references [to pointers] are treated the same as pointers, and
there is no difference between functions that take arguments or
return classes with user-defined ctors and plain old C structs,
or between attributes applied to such types. It doesn't help
to use C++ standard terms when they are subtly or substantially
different between C++ revisions, and then try to draw
a distinction between those different terms, when they don't
matter.
Martin
gcc/ChangeLog:
* doc/extend.texi (Function Attributes): Clarify C++ aspects.
(Variable Attributes): Same.
(Type Attributes): Same.
Index: gcc/doc/extend.texi
===================================================================
--- gcc/doc/extend.texi (revision 266799)
+++ gcc/doc/extend.texi (working copy)
@@ -2332,12 +2332,13 @@ the enclosing block.
@cindex @code{volatile} applied to function
@cindex @code{const} applied to function
-In GNU C, you can use function attributes to declare certain things
-about functions called in your program which help the compiler
-optimize calls and check your code more carefully. For example, you
-can use attributes to declare that a function never returns
-(@code{noreturn}), returns a value depending only on its arguments
-(@code{pure}), or has @code{printf}-style arguments (@code{format}).
+In GNU C and C++, you can use function attributes to specify certain
+function properties that may help the compiler optimize calls or
+check code more carefully for correctness. For example, you
+can use attributes to specify that a function never returns
+(@code{noreturn}), returns a value depending only on the values of
+its arguments (@code{const}), or has @code{printf}-style arguments
+(@code{format}).
You can also use attributes to control memory placement, code
generation options or call/return conventions within the function
@@ -2344,19 +2345,34 @@ generation options or call/return conventions with
being annotated. Many of these attributes are target-specific. For
example, many targets support attributes for defining interrupt
handler functions, which typically must follow special register usage
-and return conventions.
+and return conventions. Such attributes are described in the subsection
+for each target. However, a considerable number of attributes are
+supported by most, if not all targets. Those are described in
+the @ref{Common Function Attributes} section.
Function attributes are introduced by the @code{__attribute__} keyword
-on a declaration, followed by an attribute specification inside double
-parentheses. You can specify multiple attributes in a declaration by
-separating them by commas within the double parentheses or by
-immediately following an attribute declaration with another attribute
-declaration. @xref{Attribute Syntax}, for the exact rules on attribute
-syntax and placement. Compatible attribute specifications on distinct
-declarations of the same function are merged. An attribute specification
-that is not compatible with attributes already applied to a declaration
-of the same function is ignored with a warning.
+in the declaration of a function, followed by an attribute specification
+enclosed in double parentheses. You can specify multiple attributes in
+a declaration by separating them by commas within the double parentheses
+or by immediately following one attribute specification with another.
+@xref{Attribute Syntax}, for the exact rules on attribute syntax and
+placement. Compatible attribute specifications on distinct declarations
+of the same function are merged. An attribute specification that is not
+compatible with attributes already applied to a declaration of the same
+function is ignored with a warning.
+Some function attributes take one or more arguments that refer to
+the function's parameters by their positions within the function parameter
+list. Such attribute arguments are referred to as @dfn{positional arguments}.
+Unless specified otherwise, positional arguments that specify properties
+of pointer types can also specify the same properties for the implicit C++
+@code{this} argument in non-static member functions, and, to parameters of
+reference to a pointer type. For ordinary functions, position one refers
+to the first parameter on the list. In C++ non-static member functions,
+position one refers to the implicit @code{this} pointer. The same
+restrictions and effects apply to function attributes used with ordinary
+functions or C++ member functions.
+
GCC also supports attributes on
variable declarations (@pxref{Variable Attributes}),
labels (@pxref{Label Attributes}),
@@ -6127,13 +6143,13 @@ when this attribute is present.
@cindex attribute of variables
@cindex variable attributes
-The keyword @code{__attribute__} allows you to specify special
-attributes of variables or structure fields. This keyword is followed
-by an attribute specification inside double parentheses. Some
-attributes are currently defined generically for variables.
-Other attributes are defined for variables on particular target
-systems. Other attributes are available for functions
-(@pxref{Function Attributes}), labels (@pxref{Label Attributes}),
+The keyword @code{__attribute__} allows you to specify special properties
+of variables, function parameters, or structure, union, and, in C++, class
+members. This @code{__attribute__} keyword is followed by an attribute
+specification enclosed in double parentheses. Some attributes are currently
+defined generically for variables. Other attributes are defined for
+variables on particular target systems. Other attributes are available
+for functions (@pxref{Function Attributes}), labels (@pxref{Label Attributes}),
enumerators (@pxref{Enumerator Attributes}), statements
(@pxref{Statement Attributes}), and for types (@pxref{Type Attributes}).
Other front ends might define more attributes
@@ -7086,17 +7102,21 @@ placed in either the @code{.bss_below100} section
@cindex attribute of types
@cindex type attributes
-The keyword @code{__attribute__} allows you to specify special
-attributes of types. Some type attributes apply only to @code{struct}
-and @code{union} types, while others can apply to any type defined
-via a @code{typedef} declaration. Other attributes are defined for
-functions (@pxref{Function Attributes}), labels (@pxref{Label
-Attributes}), enumerators (@pxref{Enumerator Attributes}),
-statements (@pxref{Statement Attributes}), and for
-variables (@pxref{Variable Attributes}).
+The keyword @code{__attribute__} allows you to specify various special
+properties of types. Some type attributes apply only to structure and
+union types, and in C++, also class types, while others can apply to
+any type defined via a @code{typedef} declaration. Unless otherwise
+specified, the same restrictions and effects apply to attributes regardless
+of whether a type is a trivial structure or a C++ class with user-defined
+constructors, destructors, or a copy assignment.
+Other attributes are defined for functions (@pxref{Function Attributes}),
+labels (@pxref{Label Attributes}), enumerators (@pxref{Enumerator
+Attributes}), statements (@pxref{Statement Attributes}), and for variables
+(@pxref{Variable Attributes}).
+
The @code{__attribute__} keyword is followed by an attribute specification
-inside double parentheses.
+enclosed in double parentheses.
You may specify type attributes in an enum, struct or union type
declaration or definition by placing them immediately after the
