Re: [PATCH] document that alias and target must have the same type

[Martin Sebor <msebor@gmail.com>]

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On 2/4/20 6:05 PM, Martin Sebor wrote:
> GCC diagnoses declarations of function aliases whose type doesn't
> match that of the target (ditto for attribute weakref).  It doesn't
> yet diagnose such incompatbilities for variable aliases but that's
> just an oversight that I will try to remember to correct in GCC 11.
> The attached patch updates the manual to make it clear that
> aliases must have the same type as their targets, or the behavior
> is undefined (and may be diagnosed).

On further review I noticed a few problems with the documentation
of attribute weakref.  The manual says that

   Without a target, given as an argument to weakref or to alias,
   weakref is equivalent to weak.

and

   At present, a declaration to which weakref is attached can only
   be static.

However, GCC accepts the following declaration:

   extern int x (void) __attribute__ ((weakref));

so the second sentence isn't correct without qualification (unlike
weakref, a weak declaration must be external).

Another documentation problem is with the example in the manual that
says that this declaration:

   static int x() __attribute__ ((weakref ("y")));

   /* is equivalent to... */

   static int x() __attribute__ ((weak, weakref, alias ("y")));

but GCC rejects the latter with

   error: weak declaration of ‘x’ must be public

Changing the latter from static to extern changes the error to

   error: ‘weakref’ symbol ‘x’ must have static linkage

So clearly two declarations with the two sets of attributes are not
equivalent, either extern or static.

I've fixed up these documentation problems in the attached revision
of the original patch.  I also mention that besides their types
having to match, the alias must have the same size and alignment
as the target.

Martin

PS I also noted that when the function is then also used GCC issues:

   warning: ‘weakref’ attribute should be accompanied with an ‘alias’ 
attribute

This matches what the manual says in "Without arguments, it should
be accompanied by an alias attribute naming the target symbol."

But when the weakref function is not used there is no warning.
That seems like an unfortunate side-effect of the choice of issuing
the warning a little too late (waning from the front-end it would
make it consistent regardless of whether the function was used, and
it would highlight the omission even in translation units that
define the weakref without using it).

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diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index ec99c38a607..3634ce1c423 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -2557,8 +2557,11 @@ __attribute__ ((access (write_only, 1, 2), access (read_write, 3))) int fgets (c
 
 @item alias ("@var{target}")
 @cindex @code{alias} function attribute
-The @code{alias} attribute causes the declaration to be emitted as an
-alias for another symbol, which must be specified.  For instance,
+The @code{alias} attribute causes the declaration to be emitted as an alias
+for another symbol, which must have been previously declared with the same
+type, and for variables same size and alignment.  Declaring an alias with
+a different type than the target is undefined and may be diagnosed.  For
+instance, an alias for another symbol, which must be specified.  For instance,
 
 @smallexample
 void __f () @{ /* @r{Do something.} */; @}
@@ -3919,31 +3922,41 @@ results in warning on line 5.
 
 @item weak
 @cindex @code{weak} function attribute
-The @code{weak} attribute causes the declaration to be emitted as a weak
-symbol rather than a global.  This is primarily useful in defining
-library functions that can be overridden in user code, though it can
-also be used with non-function declarations.  Weak symbols are supported
-for ELF targets, and also for a.out targets when using the GNU assembler
-and linker.
+The @code{weak} attribute causes a declaration of an external symbol
+to be emitted as a weak symbol rather than a global.  This is primarily
+useful in defining library functions that can be overridden in user code,
+though it can also be used with non-function declarations.  The overriding
+symbol must have the same type, and for variables size, and alignment as
+the weak symbol.  Weak symbols are supported for ELF targets, and also for
+a.out targets when using the GNU assembler and linker.
 
 @item weakref
 @itemx weakref ("@var{target}")
 @cindex @code{weakref} function attribute
 The @code{weakref} attribute marks a declaration as a weak reference.
 Without arguments, it should be accompanied by an @code{alias} attribute
-naming the target symbol.  Optionally, the @var{target} may be given as
-an argument to @code{weakref} itself.  In either case, @code{weakref}
-implicitly marks the declaration as @code{weak}.  Without a
-@var{target}, given as an argument to @code{weakref} or to @code{alias},
-@code{weakref} is equivalent to @code{weak}.
+naming the target symbol.  Alernatively, @var{target} may be given as
+an argument to @code{weakref} itself, naming the target definition of
+the alias.  The the @var{target} must have the same type, and for
+variables also the same size and alignment as the declaration.  In either
+form of the declaration @code{weakref} implicitly marks the declaration
+as @code{weak}.  Without a @var{target} given as an argument to
+@code{weakref} or to @code{alias}, @code{weakref} is equivalent to
+@code{weak} (in that case the declaration may be @code{extern}).
 
 @smallexample
-static int x() __attribute__ ((weakref ("y")));
+/* Given the declaration: */
+extern int y (void);
+
+/* the following... */
+static int x (void) __attribute__ ((weakref ("y")));
+
 /* is equivalent to... */
-static int x() __attribute__ ((weak, weakref, alias ("y")));
-/* and to... */
-static int x() __attribute__ ((weakref));
-static int x() __attribute__ ((alias ("y")));
+static int x (void) __attribute__ ((weakref, alias ("y")));
+
+/* or, alternatively, to... */
+static int x (void) __attribute__ ((weakref));
+static int x (void) __attribute__ ((alias ("y")));
 @end smallexample
 
 A weak reference is an alias that does not by itself require a
@@ -3956,10 +3969,10 @@ symbol, not necessarily in the same translation unit.
 The effect is equivalent to moving all references to the alias to a
 separate translation unit, renaming the alias to the aliased symbol,
 declaring it as weak, compiling the two separate translation units and
-performing a link with relocatable output (ie: @code{ld -r}) on them.
+performing a link with relocatable output (i.e.@: @code{ld -r}) on them.
 
-At present, a declaration to which @code{weakref} is attached can
-only be @code{static}.
+A declaration to which @code{weakref} is attached and that is associated
+with a named @code{target} must be @code{static}.
 
 @end table