linux-kernel - Re: 2.6.19-rc5-mm1 fails to compile with gcc 4.2

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Date:	Sun, 12 Nov 2006 15:43:25 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Bernhard Rosenkraenzer <bero@...linux.org>
CC:	linux-kernel@...r.kernel.org, akpm@...l.org
Subject: Re: 2.6.19-rc5-mm1 fails to compile with gcc 4.2

Bernhard Rosenkraenzer wrote:
> On Sunday, 12. November 2006 10:24, Avi Kivity wrote:
>   
>>>> Or better yet, preprocessed source and full gcc command line (as seen on
>>>> 'make V=1').
>>>>         
>
> gcc -m32 -Wp,-MD,drivers/kvm/.kvm_main.o.d  -nostdinc -isystem /usr/lib/gcc/i586-ark-linux/4.2.0/include -D__KERNEL__ -Iinclude  -include 
> include/linux/autoconf.h -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -O2 -pipe -msoft-float -mpreferred-stack-boundary=2  -march=i686 -mtune=pentium3 -maccumulate-outgoing-args -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -Iinclude/asm-i386/mach-default -fomit-frame-pointer  -fno-stack-protector -Wdeclaration-after-statement -Wno-pointer-sign   -DMODULE -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(kvm_main)"  -D"KBUILD_MODNAME=KBUILD_STR(kvm)" -c -o 
> drivers/kvm/.tmp_kvm_main.o drivers/kvm/kvm_main.c
> drivers/kvm/kvm_main.c: In function 'kvm_dev_ioctl_run':
> drivers/kvm/kvm_main.c:153: error: 'asm' operand has impossible constraints
> drivers/kvm/kvm_main.c:158: error: 'asm' operand has impossible constraints
>
>   
>>> It does look like a gcc bug -- -O0 makes it go away.
>>> Details at http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29808
>>>       
>> That's a different bug, gcc generates code that the assembler can't
>> handle.  Might be an assembler bug.
>>     
>
> It's the same thing, the code is taken from kvm_main.c:
>
> static void load_fs(u16 sel)
> {
>         asm ("mov %0, %%fs" : : "g"(sel));	<--- line 153
> }
>
> static void load_gs(u16 sel)
> {
>         asm ("mov %0, %%gs" : : "g"(sel));	<--- line 158
> }
>
>   

'sel' is a variable, so gcc can't provide it as an immediate operand.  
Specifying it as "rm" instead of "g" would have been better, but can't 
have any real influence.

>   
>> Can you compile it with -S and post the generated assembly?
>>     
>
> It can't generate assembly with asm() constructs it perceives as invalid -- -S 
> produces the same error.
>   

Well, for the code you posted in in the gcc bug, it probaby generated 
something like

    mov $0, %fs

which is indeed invalid assembly.  But the kvm miscompile is something 
else (running out of registers or something like that).


-- 
error compiling committee.c: too many arguments to function

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