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extend.texi: Create Label Attributes section, move all label attributes into it and reference it.
2014-05-19 David Wohlferd <dw@LimeGreenSocks.com>
* doc/extend.texi: Create Label Attributes section,
move all label attributes into it and reference it.
From-SVN: r210621
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David Wohlferd
Jeff Law
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883755a164
@ -1,3 +1,8 @@
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2014-05-19 David Wohlferd <dw@LimeGreenSocks.com>
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* doc/extend.texi: Create Label Attributes section,
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move all label attributes into it and reference it.
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2014-05-19 Richard Earnshaw <rearnsha@arm.com>
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* arm.c (thumb1_reorg): When scanning backwards skip anything
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@ -55,6 +55,7 @@ extensions, accepted by GCC in C90 mode and in C++.
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* Mixed Declarations:: Mixing declarations and code.
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* Function Attributes:: Declaring that functions have no side effects,
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or that they can never return.
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* Label Attributes:: Specifying attributes on labels.
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* Attribute Syntax:: Formal syntax for attributes.
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* Function Prototypes:: Prototype declarations and old-style definitions.
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* C++ Comments:: C++ comments are recognized.
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@ -2181,7 +2182,8 @@ attributes are currently defined for functions on all targets:
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@code{error} and @code{warning}.
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Several other attributes are defined for functions on particular
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target systems. Other attributes, including @code{section} are
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supported for variables declarations (@pxref{Variable Attributes})
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supported for variables declarations (@pxref{Variable Attributes}),
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labels (@pxref{Label Attributes})
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and for types (@pxref{Type Attributes}).
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GCC plugins may provide their own attributes.
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@ -3617,8 +3619,8 @@ than 2.96.
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@cindex @code{hot} function attribute
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The @code{hot} attribute on a function is used to inform the compiler that
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the function is a hot spot of the compiled program. The function is
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optimized more aggressively and on many target it is placed into special
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subsection of the text section so all hot functions appears close together
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optimized more aggressively and on many targets it is placed into a special
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subsection of the text section so all hot functions appear close together,
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improving locality.
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When profile feedback is available, via @option{-fprofile-use}, hot functions
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@ -3627,23 +3629,14 @@ are automatically detected and this attribute is ignored.
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The @code{hot} attribute on functions is not implemented in GCC versions
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earlier than 4.3.
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@cindex @code{hot} label attribute
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The @code{hot} attribute on a label is used to inform the compiler that
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path following the label are more likely than paths that are not so
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annotated. This attribute is used in cases where @code{__builtin_expect}
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cannot be used, for instance with computed goto or @code{asm goto}.
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The @code{hot} attribute on labels is not implemented in GCC versions
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earlier than 4.8.
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@item cold
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@cindex @code{cold} function attribute
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The @code{cold} attribute on functions is used to inform the compiler that
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the function is unlikely to be executed. The function is optimized for
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size rather than speed and on many targets it is placed into special
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subsection of the text section so all cold functions appears close together
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size rather than speed and on many targets it is placed into a special
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subsection of the text section so all cold functions appear close together,
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improving code locality of non-cold parts of program. The paths leading
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to call of cold functions within code are marked as unlikely by the branch
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to calls of cold functions within code are marked as unlikely by the branch
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prediction mechanism. It is thus useful to mark functions used to handle
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unlikely conditions, such as @code{perror}, as cold to improve optimization
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of hot functions that do call marked functions in rare occasions.
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@ -3654,15 +3647,6 @@ are automatically detected and this attribute is ignored.
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The @code{cold} attribute on functions is not implemented in GCC versions
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earlier than 4.3.
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@cindex @code{cold} label attribute
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The @code{cold} attribute on labels is used to inform the compiler that
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the path following the label is unlikely to be executed. This attribute
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is used in cases where @code{__builtin_expect} cannot be used, for instance
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with computed goto or @code{asm goto}.
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The @code{cold} attribute on labels is not implemented in GCC versions
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earlier than 4.8.
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@item no_sanitize_address
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@itemx no_address_safety_analysis
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@cindex @code{no_sanitize_address} function attribute
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@ -4527,6 +4511,65 @@ attachment of attributes to their corresponding declarations, whereas
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@code{#pragma GCC} is of use for constructs that do not naturally form
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part of the grammar. @xref{Pragmas,,Pragmas Accepted by GCC}.
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@node Label Attributes
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@section Label Attributes
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@cindex Label Attributes
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GCC allows attributes to be set on C labels. @xref{Attribute Syntax}, for
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details of the exact syntax for using attributes. Other attributes are
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available for functions (@pxref{Function Attributes}), variables
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(@pxref{Variable Attributes}) and for types (@pxref{Type Attributes}).
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This example uses the @code{cold} label attribute to indicate the
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@code{ErrorHandling} branch is unlikely to be taken and that the
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@code{ErrorHandling} label is unused:
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@smallexample
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asm goto ("some asm" : : : : NoError);
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/* This branch (the fallthru from the asm) is less commonly used */
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ErrorHandling:
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__attribute__((cold, unused)); /* Semi-colon is required here */
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printf("error\n");
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return 0;
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NoError:
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printf("no error\n");
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return 1;
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@end smallexample
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@table @code
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@item unused
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@cindex @code{unused} label attribute
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This feature is intended for program-generated code that may contain
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unused labels, but which is compiled with @option{-Wall}. It is
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not normally appropriate to use in it human-written code, though it
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could be useful in cases where the code that jumps to the label is
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contained within an @code{#ifdef} conditional.
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@item hot
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@cindex @code{hot} label attribute
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The @code{hot} attribute on a label is used to inform the compiler that
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the path following the label is more likely than paths that are not so
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annotated. This attribute is used in cases where @code{__builtin_expect}
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cannot be used, for instance with computed goto or @code{asm goto}.
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The @code{hot} attribute on labels is not implemented in GCC versions
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earlier than 4.8.
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@item cold
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@cindex @code{cold} label attribute
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The @code{cold} attribute on labels is used to inform the compiler that
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the path following the label is unlikely to be executed. This attribute
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is used in cases where @code{__builtin_expect} cannot be used, for instance
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with computed goto or @code{asm goto}.
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The @code{cold} attribute on labels is not implemented in GCC versions
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earlier than 4.8.
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@end table
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@node Attribute Syntax
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@section Attribute Syntax
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@cindex attribute syntax
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@ -4550,6 +4593,8 @@ applying to functions. @xref{Variable Attributes}, for details of the
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semantics of attributes applying to variables. @xref{Type Attributes},
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for details of the semantics of attributes applying to structure, union
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and enumerated types.
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@xref{Label Attributes}, for details of the semantics of attributes
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applying to labels.
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An @dfn{attribute specifier} is of the form
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@code{__attribute__ ((@var{attribute-list}))}. An @dfn{attribute list}
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@ -4587,14 +4632,10 @@ with the list being a single string constant.
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An @dfn{attribute specifier list} is a sequence of one or more attribute
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specifiers, not separated by any other tokens.
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@subsubheading Label Attributes
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In GNU C, an attribute specifier list may appear after the colon following a
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label, other than a @code{case} or @code{default} label. The only
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attribute it makes sense to use after a label is @code{unused}. This
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feature is intended for program-generated code that may contain unused labels,
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but which is compiled with @option{-Wall}. It is
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not normally appropriate to use in it human-written code, though it
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could be useful in cases where the code that jumps to the label is
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contained within an @code{#ifdef} conditional. GNU C++ only permits
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label, other than a @code{case} or @code{default} label. GNU C++ only permits
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attributes on labels if the attribute specifier is immediately
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followed by a semicolon (i.e., the label applies to an empty
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statement). If the semicolon is missing, C++ label attributes are
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@ -4602,6 +4643,8 @@ ambiguous, as it is permissible for a declaration, which could begin
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with an attribute list, to be labelled in C++. Declarations cannot be
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labelled in C90 or C99, so the ambiguity does not arise there.
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@subsubheading Type Attributes
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An attribute specifier list may appear as part of a @code{struct},
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@code{union} or @code{enum} specifier. It may go either immediately
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after the @code{struct}, @code{union} or @code{enum} keyword, or after
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@ -4616,6 +4659,9 @@ defined is not complete until after the attribute specifiers.
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@c attributes could use sizeof for the structure, but the size could be
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@c changed later by "packed" attributes.
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@subsubheading All other attributes
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Otherwise, an attribute specifier appears as part of a declaration,
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counting declarations of unnamed parameters and type names, and relates
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to that declaration (which may be nested in another declaration, for
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@ -4856,7 +4902,8 @@ by an attribute specification inside double parentheses. Some
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attributes are currently defined generically for variables.
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Other attributes are defined for variables on particular target
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systems. Other attributes are available for functions
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(@pxref{Function Attributes}) and for types (@pxref{Type Attributes}).
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(@pxref{Function Attributes}), labels (@pxref{Label Attributes}) and for
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types (@pxref{Type Attributes}).
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Other front ends might define more attributes
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(@pxref{C++ Extensions,,Extensions to the C++ Language}).
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@ -5512,8 +5559,8 @@ inside double parentheses. Seven attributes are currently defined for
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types: @code{aligned}, @code{packed}, @code{transparent_union},
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@code{unused}, @code{deprecated}, @code{visibility}, and
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@code{may_alias}. Other attributes are defined for functions
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(@pxref{Function Attributes}) and for variables (@pxref{Variable
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Attributes}).
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(@pxref{Function Attributes}), labels (@pxref{Label
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Attributes}) and for variables (@pxref{Variable Attributes}).
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You may also specify any one of these attributes with @samp{__}
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preceding and following its keyword. This allows you to use these
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@ -6935,7 +6982,9 @@ GotoLabels section in an @code{asm goto} statement contains a comma-separated
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list of all C labels to which the assembler code may jump. GCC assumes that
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@code{asm} execution falls through to the next statement (if this is not the
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case, consider using the @code{__builtin_unreachable} intrinsic after the
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@code{asm} statement). The total number of input + output + goto operands has
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@code{asm} statement). Optimization of @code{asm goto} may be improved by
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using the @code{hot} and @code{cold} label attributes (@pxref{Label
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Attributes}). The total number of input + output + goto operands has
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a limit of 30.
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An @code{asm goto} statement can not have outputs (which means that the
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