[Bug c/20319] -fkeep-static-consts with -O asserted doesn't keep consts

["gary at intrepid dot com" <gcc-bugzilla@gcc.gnu.org>]

------- Additional Comments From gary at intrepid dot com  2005-03-04 20:24 -------
"Emit variables declared @code{static const} when optimization isn't turned 
on, even if the variables aren't referenced."

How odd.  I should've checked the docs, but this brief explanation in the
help line made a lot of sense to me.

  -fkeep-static-consts        Emit static const variables even if they are not
                              used

Also, since the constant *was kept* when I compiled _without_ optimization,
the idea of passing a switch telling the compiler to do something that
it seemed to already be doing by default didn't make sense to me.

The comments inside toplev.c say the following:

/* Nonzero means that we should emit static const variables
   regardless of whether or not optimization is turned on.  */ 
                            
int flag_keep_static_consts = 1;


which makes sense to me.  Note that this is the default setting.

So ... coming back to the if statement in toplev.c:

              else if (TREE_READONLY (decl) && !TREE_PUBLIC (decl)
                       && (optimize || !flag_keep_static_consts
                           || DECL_ARTIFICIAL (decl)))
                needed = 0;

At present,

If optimize is set, the unused static constant will always be eliminated
and -fkeep-static-consts will have no effect, and there is no way to override
this behavior (except for __attribute__ ((used))). 

This agrees with the documentation, but disagrees with the
comment in the code, and with the spirit of the help message.  It also
doesn't make sense to me that -fkeep-static-consts would be ignored. In
fact, it seems to me that -fkeep-static-consts should override the
optimization setting.  Otherwise, only -fno-keep-static-consts has
any effect and that is only when optimization is not enabled.

I think the behavior of -fkeep-static-consts should be modified so that
setting this switch overrides the optimization level.  Thus, the
documentation would need to be changed as well.  Otherwise, the internal
comments and the help line are wrong.

Regarding the "used" attibute, I thought that I'd tried that, and it
didn't work.  But upon retesting, it does seem to have the desired
effect of keeping the static const around (maybe I specified the "unused"
attribute by mistake in my test).

Note that the "used" attribute is described only under "function attributes"
and not under "variable attributes" in the documentation:

used
This attribute, attached to a function, means that code must be emitted for 
the function even if it appears that the function is not referenced. This is 
useful, for example, when the function is referenced only in inline assembly. 

(see: http://gcc.gnu.org/onlinedocs/gcc-3.4.3/gcc/Function-
Attributes.html#Function-Attributes)

It would be helpful if the documentation were updated to describe the
behavior of the `used' attribute when apllied to variables.


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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20319