linux-kernel - Re: [patch 2.6.19-rc6] Stop gcc 4.1.0 optimizing wait_hpet

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Message-ID: <23790.1164954762@kao2.melbourne.sgi.com>
Date:	Fri, 01 Dec 2006 17:32:42 +1100
From:	Keith Owens <kaos@....com.au>
To:	Willy Tarreau <w@....eu>
cc:	Andrew Morton <akpm@...l.org>, Jakub Jelinek <jakub@...hat.com>,
	Nicholas Miell <nmiell@...cast.net>,
	linux-kernel@...r.kernel.org, davem@...emloft.net, ak@...e.de
Subject: Re: [patch 2.6.19-rc6] Stop gcc 4.1.0 optimizing wait_hpet_tick away 

Willy Tarreau (on Fri, 1 Dec 2006 06:26:53 +0100) wrote:
>On Fri, Dec 01, 2006 at 04:14:04PM +1100, Keith Owens wrote:
>> SuSE's SLES10 ships with gcc 4.1.0.  There is nothing to stop a
>> distributor from backporting the bug fix from gcc 4.1.1 to 4.1.0, but
>> this patch would not allow the fixed compiler to build the kernel.
>
>Then maybe replace #error with #warning ? It's too dangerous to let people
>build their kernel with a known broken compiler without being informed.

Agreed.

>I think this shows the limit of backports to known broken versions.
>Providing a full update to 4.1.1 would certainly be cleaner for all
>customers than backporting 4.1.1 to 4.1.0 and calling it 4.1.0.

Agreed, but Enterprise customers expect bug fixes, not wholesale
replacements of critical programs.

>Another solution would be to be able to check gcc for known bugs in the
>makefile, just like we check it for specific options. But I don't know
>how we can check gcc for bad code, especially in cross-compile environments

It is doable, but it is as ugly as hell.  Note the lack of a
signed-off-by :)

---
 arch/i386/kernel/Makefile |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

Index: linux/arch/i386/kernel/Makefile
===================================================================
--- linux.orig/arch/i386/kernel/Makefile
+++ linux/arch/i386/kernel/Makefile
@@ -83,3 +83,19 @@ $(obj)/vsyscall-syms.o: $(src)/vsyscall.
 k8-y                      += ../../x86_64/kernel/k8.o
 stacktrace-y		  += ../../x86_64/kernel/stacktrace.o
 
+# Some versions of gcc generate invalid code for hpet_timer, depending
+# on other config options.  Make sure that the generated code is valid.
+# Invalid versions of gcc generate a tight loop in wait_hpet_tick, with
+# no 'cmp' instructions.  Extract the generated object code for
+# wait_hpet_tick, down to the next function then check that the code
+# contains at least one comparison.
+
+ifeq ($(CONFIG_HPET_TIMER),y)
+$(obj)/built-in.o: $(obj)/.tmp_check_gcc_bug
+
+$(obj)/.tmp_check_gcc_bug: $(obj)/time_hpet.o
+	$(Q)[ -n "`$(OBJDUMP) -Sr $< | grep -A40 '<wait_hpet_tick>:' | sed -e '1d; />:$$/,$$d;' | grep -w cmp`" ] || \
+		(echo gcc volatile bug detected, fix your gcc; exit 1)
+	$(Q)touch $@
+
+endif

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