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Date:      Tue, 22 May 2012 14:46:01 +0200
From:      Hartmut Brandt <hartmut.brandt@dlr.de>
To:        Bruce Evans <brde@optusnet.com.au>
Cc:        svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org, Hartmut Brandt <harti@FreeBSD.org>
Subject:   Re: svn commit: r235777 - head/sys/kern
Message-ID:  <alpine.BSF.2.00.1205221359070.7484@KNOP-BEAGLE.kn.op.dlr.de>
In-Reply-To: <20120522212307.V1971@besplex.bde.org>
References:  <201205220723.q4M7Ng2I091715@svn.freebsd.org> <20120522212307.V1971@besplex.bde.org>

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On Tue, 22 May 2012, Bruce Evans wrote:

BE>On Tue, 22 May 2012, Hartmut Brandt wrote:
BE>
BE>> Log:
BE>>  Make dumptid non-static. It is used by libkvm to detect whether
BE>>  this is a VNET-kernel or not. gcc used to put the static symbol into
BE>>  the symbol table, clang does not. This fixes the 'netstat: no namelist'
BE>>  error seen on clang+VNET systems.
BE>> 
BE>> Modified:
BE>>  head/sys/kern/kern_shutdown.c
BE>
BE>That would be a bug in clang if it were done for static symbols generally,
BE>but here the bug seems to be that the symbol is not declared as __used.

I don't get this. Why should a symbol declared static be in the symbol 
table (except for debugging purposes) ? It has internal linkage and so has 
a meaning only in the given file. What is the linker supposed to do with 
several static symbols with the same name from several object files? If 
several files declared static dumptids, which one would kldsym be supposed 
to return?

harti

BE>
BE>gcc does the same for a file containing only "static int x;", but it
BE>is apparently confused by dumptid being initialized non-statically,
BE>although the initialization has no side effects.  If dumptid were a
BE>local variable, then clang would probably warn about the variable being
BE>unused, but gcc-4.2.1 never detects such unused variables (thus code
BE>that compiles with gcc -Wunused -Werror often fails with clang).  Here
BE>the initialization is to curthread->td_tid, so it isn't clear if the
BE>compiler can tell if it has no side effects.  curthread() is actually
BE>__curthread().  __curthread() is now declared as __pure2, but that
BE>never worked for me with older compilers (its result wasn't cached).
BE>If the compilers can tell that the expression has no side effects,
BE>then it is another bug that they don't warn about it having no effect
BE>when it is only assigned to the apparently-unused variable dumptid.
BE>
BE>> Modified: head/sys/kern/kern_shutdown.c
BE>> ==============================================================================
BE>> --- head/sys/kern/kern_shutdown.c	Tue May 22 07:04:23 2012
BE>> (r235776)
BE>> +++ head/sys/kern/kern_shutdown.c	Tue May 22 07:23:41 2012
BE>> (r235777)
BE>> @@ -151,7 +151,7 @@ static struct dumperinfo dumper;	/* our
BE>> 
BE>> /* Context information for dump-debuggers. */
BE>> static struct pcb dumppcb;		/* Registers. */
BE>> -static lwpid_t dumptid;			/* Thread ID. */
BE>> +lwpid_t dumptid;			/* Thread ID. */
BE>> 
BE>> static void poweroff_wait(void *, int);
BE>> static void shutdown_halt(void *junk, int howto);
BE>
BE>Now there are 3 bugs instead of 1:
BE>- the variable is declared (implicit) extern instead of static
BE>- the extern declaration is in a section for static declaration
BE>- the variable is still not declared as __used.  If the compiler did
BE>  a more extensive usage analysis, that looked at all object files but
BE>  not at the libkvm API, then it should remove this variable anyway
BE>  when it is not declared as __used.
BE>
BE>Bruce
BE>
BE>



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