[Bug c/26371] New: dead variable attributes

[Bug c/26371] New: dead variable attributes

[acahalan at gmail dot com]

A large project may have a configurable build to allow optional features. Fully
eliminating a feature may require #ifdef all over the place, making the code
ugly and unmaintainable. Example: you want an OS kernel to support an embedded
systems build that lacks the normal user ID handling.

If members of a struct could be marked as being useless, then the compiler's
dead code elimination could often take the place of adding #ifdef all over the
place.

Useful attributes would be something like this:

a. readas(x)   When used as an rvalue, you get x.
b. nomem(x)    Does not take up space. Address reads as x.
c. writeas(x)  When used as an lvalue, store x.
d. nowrite     Don't bother storing to this location.

(you'd never use more than one of the last three)


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           Summary: dead variable attributes
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: acahalan at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26371

[Bug c/26371] dead variable attributes

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-02-20 02:02 -------
Can you show an example how to use these variables?  I don't understand why you
cannot use if(TARGET_WANTS_XXX)
instead of #if TARGET_WANTS_XXX.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26371

[Bug c/26371] dead variable attributes

[acahalan at gmail dot com]

------- Comment #2 from acahalan at gmail dot com  2006-02-20 03:18 -------
For variables that are commonly used in a large project, you sure wouldn't want
to be putting conditional code all over the place if you could avoid it. The
choice could be something like:

a. put #if TARGET_WANTS_XXX in 12345 locations
b. put if(TARGET_WANTS_XXX) in 12345 locations
c. just live with lots of bloat
d. make use of this proposed feature in 1 location


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26371

[Bug c/26371] dead variable attributes

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2006-02-21 10:14 -------
So you want

typedef struct {
  int bar;
  void *foobar
#if !NEEDED_BY_ARCH
  __attribute__((readas(0)))
#endif
  ;
} Foo;

so Foo.foobar is NULL for !NEEDED_BY_ARCH?  Why not do the proper thing
and provide accessors like f.i. int the LK:

#define foo_foo(x) (x).bar
#if !NEEDED_BY_ARCH
#define foo_foobar(x) NULL
#else
#define foo_foobar(x) (x).foobar
#endif

no need to invent funny attributes for this.  And #defines are properly
localized in some header.  OO makes you happy.


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rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26371