/*
* The below program demonstrates a defeciency with gcc's constraint
* syntax for 64-bit code: it's not possible to directly constrain an
* operand to a register in the range r8 through r15.
*
* Furthermore, it is not possible to use '%rbp' in a constraint.
*
* It has been shown that gcc can assign a local variable to a
* particular register
* (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16331) but that does
* not properly work when %rbp is being used.
*
* The code below executes a 'vmrun' instruction. The intent of the
* inline assembly is to indicate that all the named registers can be
* clobbered by the 'vmrun' instruction.
*
* For simplicity, I've only set '_rbp' to zero to show that this
* local variable is assigned to '%rbp'.
*
* Here is a snippet of the code that is generated:
*
*
* 32: 48 8b 2d 00 00 00 00 mov 0(%rip),%rbp # origRSP
* 39: 48 8b 05 00 00 00 00 mov 0(%rip),%rax # &guestVMCB
* 40: 31 ed xor %ebp,%ebp # zero _rbp
* 42: 0f 01 d8 vmrun
* 45: 48 8b 3d 00 00 00 00 mov 0(%rip),%rdi # 4c
<test_function+0x3c>
* 4c: 48 8d 75 f8 lea 0xfffffffffffffff8(%rbp),%rsi # origRSP
- 8
*
*
* origRSP is allocated to '%rbp', and '_rsp' is allocated to the same
* register. Even though '_rsp' is set to zero and 'vmrun' can
* clobber all the named registers, the compiler still uses '%rbp' as
* if it were not modified at address 0x4c.
*
* Also, to add to the unusual behavior, removing the function
* 'SetupVMCB' causes the issue to vanish. SetupVMCB is unused.
*
* Using built-in specs.
* Target: x86_64-redhat-linux
* Configured with: ../configure --prefix=/usr \
* --mandir=/usr/share/man \
* --infodir=/usr/share/info \
* --enable-shared --enable-threads=posix \
* --enable-checking=release \
* --with-system-zlib --enable-__cxa_atexit \
* --disable-libunwind-exceptions --enable-libgcj-multifile \
* --enable-languages=c,c++,objc,obj-c++,java,fortran,ada \
* --enable-java-awt=gtk --disable-dssi --enable-plugin \
* --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre \
* --with-cpu=generic --host=x86_64-redhat-linux
* Thread model: posix
* gcc version 4.1.2 20071124 (Red Hat 4.1.2-42)
*
*
* To build:
*
* gcc -O2 -c -o main.o main.c
* objdump --disassemble main.o
*
*/
typedef unsigned long uint64;
typedef struct {
unsigned char pad[256];
uint64 rsp;
} VMCB;
void failure(uint64 a, uint64 b);
void SetupVMCB(uint64 rip, unsigned cpl);
uint64 guestVMCB;
volatile VMCB mappedGuestVMCB;
static __attribute__((always_inline)) void
VMRUN(uint64 vmcb)
{
register unsigned long _rbx asm("rbx");
register unsigned long _rcx asm("rcx");
register unsigned long _rdx asm("rdx");
register unsigned long _rbp asm("rbp") = 0;
register unsigned long _rsi asm("rsi");
register unsigned long _rdi asm("rdi");
register unsigned long _r8 asm("r8");
register unsigned long _r9 asm("r9");
register unsigned long _r10 asm("r10");
register unsigned long _r11 asm("r11");
register unsigned long _r12 asm("r12");
register unsigned long _r13 asm("r13");
register unsigned long _r14 asm("r14");
register unsigned long _r15 asm("r15");
__asm__ __volatile__("vmrun"
: "+c" (_rcx), "+d" (_rdx), "+b" (_rbx), "+r" (_rbp),
"+S" (_rsi), "+D" (_rdi), "+r" (_r8), "+r" (_r9),
"+r" (_r10), "+r" (_r11), "+r" (_r12), "+r" (_r13),
"+r" (_r14), "+r" (_r15)
: "a" (vmcb)
: "memory");
}
int
test_function(void)
{
uint64 origRSP;
origRSP = mappedGuestVMCB.rsp;
VMRUN(guestVMCB);
{
uint64 _act = (mappedGuestVMCB.rsp);
uint64 _exp = (origRSP - sizeof(unsigned long));
if (_exp != _act) {
failure(_act, _exp);
}
}
return 0;
}
void
SetupVMCB(uint64 rip, unsigned cpl)
{
}
--
Summary: gcc ignores use of %rbp via assembly, generates bad code
Product: gcc
Version: 4.1.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: thutt at vmware dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38925
