Re: [PATCH v4 5/5] Add documentation for musttail attribute

[Andi Kleen <ak@linux.intel.com>]

On Sat, Feb 03, 2024 at 09:35:43PM -0700, Sandra Loosemore wrote:
> On 2/2/24 02:09, Andi Kleen wrote:
> > gcc/ChangeLog:
> > 
> > 	* doc/extend.texi: Document [[musttail]]
> > ---
> >   gcc/doc/extend.texi | 16 ++++++++++++++++
> >   1 file changed, 16 insertions(+)
> > 
> > diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> > index 142e41ab8fbf..866f6c4a9fed 100644
> > --- a/gcc/doc/extend.texi
> > +++ b/gcc/doc/extend.texi
> > @@ -9875,6 +9875,22 @@ foo (int x, int y)
> >   @code{y} is not actually incremented and the compiler can but does not
> >   have to optimize it to just @code{return 42 + 42;}.
> > +@cindex @code{musttail} statement attribute
> > +@item musttail
> > +
> > +The @code{gnu::musttail} or @code{clang::musttail} attribute
> > +can be applied to a return statement that returns the value
> > +of a call to indicate that the call must be a tail call
> > +that does not allocate extra stack space.
> 
> It took me about 3 attempts to parse this.  :-S  I think this might be a
> little better:
> 
> ...can be applied to a @code{return} statement with a return-value
> expression that is a function call.  It asserts that the call must be a tail
> call that does not allocate extra stack space.
> 
> > +
> > +@smallexample
> > +[[gnu::musttail]] return foo();
> > +@end smallexample
> > +
> > +If the compiler cannot generate a tail call it will generate
> 
> s/will generate/generates/
> 
> I'm a big fan of writing in the present tense.  ;-)
> 
> > +an error. Tail calls generally require enabling optimization.
> > +On some targets they may not be supported.
> > +
> >   @end table
> >   @node Attribute Syntax
> 
> In addition to these changes, at the beginning of this section we have
> 
> @node Statement Attributes
> @section Statement Attributes
> @cindex Statement Attributes
> 
> GCC allows attributes to be set on null statements.  @xref{Attribute
> Syntax},
> for details of the exact syntax for using attributes. [...]
> 
> Well, we now have an attribute that goes on a non-null statement, so we have
> to fix this.  The documentation for the other statement attributes is

FWIW we always had, they just were ignored (with a warning)


Thanks Sandra. I applied the changes. Diff appeneded for reference.


diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 866f6c4a9fed..fe1ee245ed69 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -9818,7 +9818,7 @@ same manner as the @code{deprecated} attribute.
 @section Statement Attributes
 @cindex Statement Attributes
 
-GCC allows attributes to be set on null statements.  @xref{Attribute Syntax},
+GCC allows attributes to be set on statements.  @xref{Attribute Syntax},
 for details of the exact syntax for using attributes.  Other attributes are
 available for functions (@pxref{Function Attributes}), variables
 (@pxref{Variable Attributes}), labels (@pxref{Label Attributes}), enumerators
@@ -9879,15 +9879,15 @@ have to optimize it to just @code{return 42 + 42;}.
 @item musttail
 
 The @code{gnu::musttail} or @code{clang::musttail} attribute
-can be applied to a return statement that returns the value
-of a call to indicate that the call must be a tail call
-that does not allocate extra stack space.
+can be applied to a @code{return} statement with a return-value expression
+that is a function call.  It asserts that the call must be a tail call that
+does not allocate extra stack space.
 
 @smallexample
 [[gnu::musttail]] return foo();
 @end smallexample
 
-If the compiler cannot generate a tail call it will generate
+If the compiler cannot generate a tail call it generates
 an error. Tail calls generally require enabling optimization.
 On some targets they may not be supported.
 
@@ -10014,7 +10014,9 @@ the constant expression, if present.
 
 @subsubheading Statement Attributes
 In GNU C, an attribute specifier list may appear as part of a null
-statement.  The attribute goes before the semicolon.
+statement. The attribute goes before the semicolon.
+Some attributes in new style syntax are also supported
+on non-null statements.
 
 @subsubheading Type Attributes