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Using the asm suffix


As a followup to my update to the inline asm docs, I'm cleaning up the docs for 'Asm Labels.' The changes I want to make are pretty straight-forward (attached; comments welcome). But then I came across this line of code (from https://github.com/rschmukler/cs537-p5/blob/master/xv6/proc.h#L38):

    extern struct proc *proc asm("%gs:4");

This x86 code says that 'proc' is located at an offset of 4 bytes from the gs register.

There isn't any description of using asm like this in the current Asm Labels docs. But 'gs:4' isn't really a label. There also isn't any description of it in the Explicit Reg Vars section. But 'gs:4' isn't really a register either. So apparently using asm like this this isn't documented anywhere.

Which makes me wonder: Is this not doc'ed because using 'asm' like this isn't supported? Or is there a supported feature here that needs to be doc'ed?

dw
Index: extend.texi
===================================================================
--- extend.texi	(revision 226751)
+++ extend.texi	(working copy)
@@ -8367,8 +8367,14 @@
 
 You can specify the name to be used in the assembler code for a C
 function or variable by writing the @code{asm} (or @code{__asm__})
-keyword after the declarator as follows:
+keyword after the declarator.
+It is up to you to make sure that the assembler names you choose do not
+conflict with any other assembler symbols.
 
+@subsubheading Assembler names for data:
+
+This sample shows how to specify the assembler name for data:
+
 @smallexample
 int foo asm ("myfoo") = 2;
 @end smallexample
@@ -8379,33 +8385,30 @@
 @samp{_foo}.
 
 On systems where an underscore is normally prepended to the name of a C
-function or variable, this feature allows you to define names for the
+variable, this feature allows you to define names for the
 linker that do not start with an underscore.
 
 It does not make sense to use this feature with a non-static local
 variable since such variables do not have assembler names.  If you are
 trying to put the variable in a particular register, see @ref{Explicit
-Reg Vars}.  GCC presently accepts such code with a warning, but will
-probably be changed to issue an error, rather than a warning, in the
-future.
+Reg Vars}.
 
-You cannot use @code{asm} in this way in a function @emph{definition}; but
-you can get the same effect by writing a declaration for the function
-before its definition and putting @code{asm} there, like this:
+@subsubheading Assembler names for functions:
 
+To specify the assember name for functions, write a declaration for the 
+function before its definition and put @code{asm} there, like this:
+
 @smallexample
-extern func () asm ("FUNC");
-
-func (x, y)
-     int x, y;
-/* @r{@dots{}} */
+extern int func (int x, int y) asm ("MYFUNC");
+     
+int func (int x, int y)
+@{
+   /* @r{@dots{}} */
 @end smallexample
 
-It is up to you to make sure that the assembler names you choose do not
-conflict with any other assembler symbols.  Also, you must not use a
-register name; that would produce completely invalid assembler code.  GCC
-does not as yet have the ability to store static variables in registers.
-Perhaps that will be added.
+@noindent
+This specifies that the name to be used for the function @code{func} in
+the assembler code should be @code{MYFUNC}.
 
 @node Explicit Reg Vars
 @subsection Variables in Specified Registers

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