I'm starting with assembler under Linux. I have saved the following code as testasm.c
and compiled it with: gcc testasm.c -otestasm
The compiler replies: "impossible constraint in ‘asm’".
#include <stdio.h>
int main(void)
{
int foo=10,bar=15;
__asm__ __volatile__ ("addl %%ebx,%%eax"
: "=eax"(foo)
: "eax"(foo), "ebx"(bar)
: "eax"
);
printf("foo = %d", foo);
return 0;
}
How can I resolve this problem?
(I've copied the example from here.)
Debian Lenny, kernel 2.6.26-2-amd64
gcc version 4.3.2 (Debian 4.3.2-1.1)
Resolution:
See the accepted answer - it seems the 'modified' clause is not supported any more.
__asm__ __volatile__ ("addl %%ebx,%%eax" : "=a"(foo) : "a"(foo), "b"(bar));
seems to work. I believe that the syntax for register constraints changed at some point, but it's not terribly well documented. I find it easier to write raw assembly and avoid the hassle.
The constraints are single letters (possibly with extra decorations), and you can specify several alternatives (i.e., an inmediate operand or register is "ir"). So the constraint "eax" means constraints "e" (signed 32-bit integer constant), "a" (register eax), or "x" (any SSE register). That is a bit different that what OP meant... and output to an "e" clearly doesn't make any sense. Also, if some operand (in this case an input and an output) must be the same as another, you refer to it by a number constraint. There is no need to say eax will be clobbered, it is an output. You can refer to the arguments in the inline code by %0, %1, ..., no need to use explicit register names. So the correct version for the code as intended by OP would be:
#include <stdio.h>
int main(void)
{
int foo=10, bar=15;
__asm__ __volatile__ (
"addl %2, %0"
: "=a" (foo)
: "0" (foo), "b" (bar)
);
printf("foo = %d", foo);
return 0;
}
A better solution would be to allow %2 to be anything, and %0 a register (as x86 allows, but you'd have to check your machine manual):
#include <stdio.h>
int main(void)
{
int foo=10, bar=15;
__asm__ __volatile__ (
"addl %2, %0"
: "=r" (foo)
: "0" (foo), "g" (bar)
);
printf("foo = %d", foo);
return 0;
}
If one wants to use multiline, then this will also work..
__asm__ __volatile__ (
"addl %%ebx,%%eax; \
addl %%eax, %%eax;"
: "=a"(foo)
: "a"(foo), "b"(bar)
);
'\' should be added for the compiler to accept a multiline string (the instructions).
Related
I need to modified an random number generator using rdrand(only),
It implemented in c code as follows.
uint64_t _rdrand(void)
{
uint64_t r;
__asm__ volatile("rdrand %0\n\t" : "=r"(r));
return r;
}
Now i need to modify such that it returns only if carry flag is set. (According to rdrand documentation).I think it can be implimented by jc instruction,but don't know how to use inside __asm__ volatile.please help me.
I am trying to make my own read system call for a 64bit linux system. but it keeps telling me I have a bad type. Is the compiler trying to indirectly address buf? I have a feeling I messed up in my input constraints. I just need the address of buf at %2.
error:
test.c: Assembler messages:
test.c:28: Error: operand type mismatch for `movq'
static int myread(int fd, char *buf, int size) {
register int bytes;
asm(
"movq $0, %%rax\n"
"movq %1, %%rdi\n"
"movq %2, %%rsi\n"
"movq %3, %%rdx\n"
"syscall\n"
"movq %%rax, %0"
: "=r" (bytes)
: "m" (fd), "m" (buf), "m" (size)
: "%rax", "%rdi", "%rsi", "%rdx"
);
return bytes;
}
As Mystical said, the mismatch error comes from the fact that you are using movq (which is for 64bit values) on 32bit integers (like fd and size).
But beyond that, this code is really inefficient and subtly (but dangerously) flawed. Maybe something more like this:
static int myread(int fd, char *buf, int size) {
register int bytes;
asm(
"syscall"
: "=a" (bytes)
: "D" (fd), "S" (buf), "d" (size), "0" (0)
: "rcx", "r11", "memory", "cc"
);
return bytes;
}
To understand this, check out the machine constraints for i386.
Note that syscalls clobber the rcx and r11 registers. Failing to advise the compiler that you are changing these values can lead to very strange problems. And the problems won't happen on the syscall, but a hundred lines downstream.
I'm also going to make a pitch for NOT using inline asm. I'm not sure why you don't want to just use the system calls, but you are just setting yourself up for grief.
I have an ARM cross compiler in Ubuntu(arm-linux-gnueabi-gcc) and the default archtecture is ARMv7. However, I want to compile an ARMv5 binary. I do this by giving the compiler the -march=armv5te option.
So far, so good. Since my ARM system uses BusyBox, I have to compile my binary statically linked. So I give gcc the -static option.
However, I have a problem with libc.a which the linker links to my ARMv5 binary. This file is compiled with the ARMv7 architecture option. So, even if I cross-compile my ARM binary with ARMv5, I can't run it on my BusyBox based ARMv5 box.
How can I solve this problem?
Where can I get the ARMv5 libc.a static library, and how can I link it?
Thank you in advance.
You have two choices,
Get the right compiler.
Write your own 'C' Library.
Get the right compiler.
You are always safest to have a compiler match your system. This applies to x86 Linux and various distributions. You are lucky if different compilers work. It is more difficult when you cross-compile as often the compiler will not be automatically synced. Try to run a program on a 1999 x86 Mandrake Linux compiled on your 2014 Ubuntu system.
As well as instruction compatibility (which you have identified), there are ABI and OS dependencies. Specifically, the armv7 is most likely hardfloat (has floating point FPU and register call convention) and you need a softfloat (emulated FPU). The specific glibc (or ucLibc) has specific calls and expectations of the Linux OS. For instance, the way threads works has changed over time.
Write your own
You can always use -fno-builtin and -ffreestanding as well as -static. Then you can not use any libc functions, but you can program them your self.
There are external source, like Mark Martinec's snprintf and building blocks like write() which is easy to implement,
#define _SYS_IOCTL_H 1
#include <linux/unistd.h>
#include <linux/ioctl.h>
static inline int write(int fd, void *buf, int len)
{
int rval;
asm volatile ("mov r0, %1\n\t"
"mov r1, %2\n\t"
"mov r2, %3\n\t"
"mov r7, %4\n\t"
"swi #0\n\t"
"mov %0, r0\n\t"
: "=r" (rval)
: "r" (fd),
"r" (buf),
"r" (len),
"Ir" (__NR_write)
: "r0", "r1", "r2", "r7");
return rval;
}
static inline void exit(int status)
{
asm volatile ("mov r0, %0\n\t"
"mov r7, %1\n\t"
"swi #0\n\t"
: : "r" (status),
"Ir" (__NR_exit)
: "r0", "r7");
}
You have to add your own start-up machinery taken care of by the 'C' library,
/* Called from assembler startup. */
int main (int argc, char*argv[])
{
write(STDOUT, "Hello world\n", sizeof("Hello world\n"));
return 0;
}
/* Wrapper for main return code. */
void __attribute__ ((unused)) estart (int argc, char*argv[])
{
int rval = main(argc,argv);
exit(rval);
}
/* Setup arguments for estart [like main()]. */
void __attribute__ ((naked)) _start (void)
{
asm(" sub lr, lr, lr\n" /* Clear the link register. */
" ldr r0, [sp]\n" /* Get argc... */
" add r1, sp, #4\n" /* ... and argv ... */
" b estart\n" /* Let's go! */
);
}
If this is too daunting, because you need to implement a lot of functionality, then you can try and get various library source and rebuild them with -fno-builtin and make sure that the libraries do not get linked with the Ubuntu libraries, which are incompatible.
Projects like crosstool-ng can allow you to build a correct compiler (maybe with more advanced code generation) that suits the armv5 system exactly. This may seem like a pain, but the alternatives above aren't easy either.
I want to write a 64 bit integer to a particular memory location.
sample C++ code would look like this:
extern char* base;
extern uint64_t data;
((uint64_t *)base)[1] = data;
Now, here is my attempt to write the above as inline assembly:
uint64_t addr = (uint64_t)base + 8;
asm volatile (
"movq %0, (%1)\n\t"
:: "r" (data), "r"(addr) : "memory"
);
The above works in a small test program but in my application, I suspect that something here is off.
Do I need to specify any output operands or any other constraints in the above?
Thanks!
Just say:
asm("mov %1, %0\n\t" : "=m"(*(uint64_t*)(base + 8)) : "r"(data) : "memory");
The tricky thing is that when using the "m" constraint, you're (possibly counterintuitively so) not giving an address but instead what in C looks like the "value" of the variable you want to change.
That's why, in this case, the weird pointer-cast-dereference. The compiler for this makes sure to put the address of the operand in.
in do_IRQ you can find the following code!
#ifdef CONFIG_DEBUG_STACKOVERFLOW
/* Debugging check for stack overflow: is there less than 1KB free? */
{
long esp;
__asm__ __volatile__("andl %%esp,%0" :
"=r" (esp) : "0" (THREAD_SIZE - 1));
if (unlikely(esp < (sizeof(struct thread_info) + STACK_WARN))) {
printk("do_IRQ: stack overflow: %ld\n",
esp - sizeof(struct thread_info));
dump_stack();
}
}
#endif
i did't understand the meaning of this asm assembly
asm _volatile_("andl %%esp,%0" :
"=r" (esp) : "0" (THREAD_SIZE - 1));
THREAD_SIZE - 1 means what?
I remeber the symbol in the parenthesis should be the C variable like the esp in the output part， but in the input part it looks like a integer but not a C symbol， can some noe help
The "0" constraint means: use the same constraints as the 0th operands (http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html#ss6.1, and 6.1.3 Matching(Digit) constraints).
Basically, this snippet takes THREAD_SIZE - 1 as an input register, and output an anded value in the same register. This register is referenced as the esp variable in the source code.