linux-kernel - Re: huge gcc 4.1.{0,1} _

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Message-ID: <20080502141132.GN2255@devserv.devel.redhat.com>
Date:	Fri, 2 May 2008 10:11:32 -0400
From:	Jakub Jelinek <jakub@...hat.com>
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	Alistair John Strachan <alistair@...zero.co.uk>,
	Chris Knadle <Chris.Knadle@...edump.us>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Adrian Bunk <bunk@...nel.org>, venkatesh.pallipadi@...el.com,
	davem@...emloft.net, trini@...nel.crashing.org, mingo@...e.hu,
	tglx@...utronix.de, hpa@...or.com, linux-kernel@...r.kernel.org,
	suresh.b.siddha@...el.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: huge gcc 4.1.{0,1} __weak problem

On Fri, May 02, 2008 at 03:57:08PM +0200, Sam Ravnborg wrote:
> OK, can anyone confirm that this fails to build which a
> buggy gcc:
> 
> 
> void __attribute__((weak)) func(void)
> {
>     /* no code */
> }
> 
> int main()
> {
>     func();
>     return 0;
> }

Of course it doesn't fail to build.  With buggy gcc it will be optimized
to
void __attribute__((weak)) func (void)
{
}

int main ()
{
  return 0;
}

(similarly how all recent gccs optimize this without the weak attribute)
while non-buggy gcc keeps the func call.
So, you either need to grep the assembly (that's what e.g. the GCC testcase
does), or you can e.g. use a runtime testcase:
extern void abort (void);
void __attribute__((weak)) func (void) { }
int main () { func (); abort (); }
in one compilation unit and
extern void exit (int);
void func (void) { exit (0); }
in another one.  I doubt a runtime testcase is acceptable though for the
kernel, as the cross compiler used to build the kernel might not be able to
create userland executables (missing C library, etc.).

	Jakub
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