[Bug c/31689] New: Support defining attributes at a scope level

["mkgnu at mkgnu dot net" <gcc-bugzilla at gcc dot gnu dot org>]

Support defining attributes at a scope level.

I'd like to request implementing the feature of defining attributes at a scope
level. For example, allow a syntax such as the following, possibly as a GNU
extension:

if (something)
__attribute__ ((section("bar"))) {
   /* body of if-statement */
}


...which will place the body of the if-statement in a separate memory region.


The motivation behind this request is the need to place uncommonly used code in
a separate memory region for better cache locality. For example, given a piece
of C code such as the following:

if (something) {
/* body */
}

... we need to place the entire body in a separate memory region.

1 -- Attempting to use extended inline assembly to implement this is
problematic since the general assumption is that outbound/inbound branches are
not allowed in inline assembly. As Jim Wilson also notes, using Labels as
values (e.g. &&label) confuses the CFG, effectively disabling some
optimizations, which means that performance may be lost when doing this
(http://lists.gnu.org/archive/html/bug-binutils/2007-04/msg00120.html).
Additionally, trying to do things behind the compilers back (in inline
assembly)  will not work in the long-term.

2 -- Using the section attribute to place an entire function in a separate
region is also unsuitable. For example a definition such as:

    extern void foobar (void) __attribute__ ((section ("bar")));

puts the entire function `foobar' in the `bar' section, not just the body of an
if statement inside the function. Additionally, putting the body of the
if-statement in a separate function and defining that function to be in a
separate section is not adequate. If the body of the if-statement contains
return instructions or gotos to other parts of the function, won't permit
returning to the function caller or other parts of the original function
(http://gcc.gnu.org/ml/gcc-help/2007-04/msg00282.html).



Discussions about this topic are listed in:

http://gcc.gnu.org/ml/gcc-help/2007-04/msg00250.html
http://lists.gnu.org/archive/html/bug-binutils/2007-04/msg00107.html


-- 
           Summary: Support defining attributes at a scope level
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mkgnu at mkgnu dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31689