[1.] gcc -O0 assembly arch/x86/kvm/emulate.c gets compilation failure -- incorrect register restrictions
[2.] Full description of the problem/report:
I'm trying to compile this file at -O0, but gcc chokes in register allocation at the inline assembly.

In the ordinary Linux build, this file compiles with gcc at -O2, without compilation errors.

At -O0, gcc chokes with this message: 
gcc  -w -c ./emulateE.c               // (using preprocessed file)
./emulateE.c: In function `em_mul_ex':
./emulateE.c:1918:5: error: can't find a register in class `AREG' while reloading `asm'
./emulateE.c:1918:5: error: `asm' operand has impossible constraints

Explanation:
The file contains an inline asm of a kind:

    __asm__ __volatile__ ( " ..... " :

 "=m" ((ctxt)->eflags), "=&r" (_tmp), "+a" (*rax), "+d" (*rdx), "+qm"(ex) :
 "i" (11), "m" ((ctxt)->src . val), "a" (*rax), "d" (*rdx)); 

Note that "+a" in inputs already means that eax is the return value. An then "a" is used as an output constraint too.

Suggested fix:
Instead of specifying something as both in/out and out operand
    __asm__ ( "" :  "+a" (rax) : "a"  (rax)); 

it should specify only in/out:
    __asm__ ( "" :  "+a" (rax)); 

Or, alternatively in and out:
    __asm__ ( "" :  "a" (rax) : "a"  (rax));
 [3.] Keywords (i.e., modules, networking, kernel):
[4.] Kernel information
[4.1.] Kernel version (from /proc/version): cat /proc/version
Linux version 2.6.32-279.el6.i686 (mockbuild@x86-010.build.bos.redhat.com) (gcc version 4.4.6 20120305 (Red Hat 4.4.6-4) (GCC) ) #1 SMP Wed Jun 13 18:23:32 EDT 2012
[4.2.] Kernel .config file: See attached
[5.] Most recent kernel version which did not have the bug: Unknown
[6.] Output of Oops.. message (if applicable) with symbolic information N/A
     resolved (see Documentation/oops-tracing.txt)
[7.] A small shell script or example program which triggers the
     problem (if possible) // Omit -O2 from compilation:
  gcc -Wp,-MD,arch/x86/kvm/.emulate.o.d  -nostdinc -isystem /ref/gcc/4.7.1/rhel60/x86/bin/../lib/gcc/i686-pc-linux-gnu/4.7.1/include -I/local/mb/sandbox/linux-3.5.2/arch/x86/include -Iarch/x86/include/generated -Iinclude  -include /local/mb/sandbox/linux-3.5.2/include/linux/kconfig.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -m32 -msoft-float -mregparm=3 -freg-struct-return -mpreferred-stack-boundary=2 -march=i686 -maccumulate-outgoing-args -Wa,-mtune=generic32 -ffreestanding -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_AVX=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -Wframe-larger-than=2048 -Wno-unused-but-set-variable -fno-omit-frame-pointer -fno-optimize-sibling-calls -g -pg -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fconserve-stack -DCC_HAVE_ASM_GOTO -w -Ivirt/kvm -Iarch/x86/kvm  -DMODULE  -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(emulate)"  -D"KBUILD_MODNAME=KBUILD_STR(kvm)" -c -o arch/x86/kvm/emulate.o arch/x86/kvm/emulate.c
[8.] Environment
[8.1.] Software (add the output of the ver_linux script here)

Gnu C                  4.7.1
Gnu make               3.78.1
binutils               2.20.51.0.2
5.34.
util-linux             2.17.2
mount                  support
module-init-tools      3.9
e2fsprogs              1.41.12
pcmciautils            015
quota-tools            3.17.
PPP                    2.4.5
Linux C Library        2.12
Dynamic linker (ldd)   2.12
Procps                 3.2.8
Net-tools              1.60
Kbd                    1.15
oprofile               0.9.7
Sh-utils               8.4
wireless-tools         29
Modules Loaded         nfs nfsd lockd nfs_acl auth_rpcgss exportfs autofs4 sunrpc target_core_iblock target_core_file target_core_pscsi target_core_mod configfs bnx2fc cnic uio fcoe libfcoe libfc scsi_transport_fc 8021q scsi_tgt garp stp llc pcc_cpufreq cachefiles fscache ipv6 uinput hpilo hpwdt tg3 sg microcode serio_raw iTCO_wdt iTCO_vendor_support ioatdma power_meter igb dca ext3 jbd mbcache sd_mod crc_t10dif sr_mod cdrom hpsa pata_acpi ata_generic ata_piix dm_mirror dm_region_hash dm_log dm_mod

[8.2.] Processor information (from /proc/cpuinfo):
vendor_id       : GenuineIntel,cpu family      : 6,model           : 45,model name      : Intel(R) Xeon(R) CPU E5-2650 0 @ 2.00GHz