From: Bernhard Rosenkraenzer <bero@arklinux.org>
To: Avi Kivity <avi@qumranet.com>
Subject: Re: 2.6.19-rc5-mm1 fails to compile with gcc 4.2
Date: Sun, 12 Nov 2006 14:36:15 +0100
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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
}


> 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.
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