Inline assembler
In computer programming, the inline assembler is a feature of some compilers that allows very low level code written in assembly to be embedded in a high level language like C or Ada. This embedding is usually done for one of three reasons:
- Optimization
- When assembly language is inlined for optimization, the most performance-sensitive parts of an algorithm are replaced by hand-written assembly. This allows the programmer to use the full extent of their ingenuity, without being limited by a compiler's higher-level constructs.
- Access to processor specific instructions
- Some processors offer special instructions, such as Compare and Swap and Test and Set — instructions which may be used to construct semaphores or other synchronization and locking primitives. Nearly every modern processor has these or similar instructions, as they are necessary to implement multitasking. To name a few, specialized instructions are found in the SPARC VIS, Intel MMX and SSE, and Motorola Altivec instruction sets.
- System calls
- High-level languages rarely have a direct facility to make system calls, so assembly code is used.
Syntax in language standards
The ISO C++ standard and ISO C standards (annex J) specify a conditionally-supported syntax for inline assembler:
An asm declaration has the form
asm-definition:
asm ( string-literal ) ;
The asm declaration is conditionally-supported; its meaning is implementation- defined.
Example of a system call
Calling an operating system directly is generally impossible in the presence of protected memory. The OS runs at a more privileged level (kernel mode) than the user (user mode); a (software) interrupt is used to make requests to the operating system. This is rarely a feature in a higher-level language, and so wrapper functions for system calls are written using inline assembler.
The following C code are samples including a system call wrapper in AT&T assembler syntax with the GNU Assembler. They are normally written with the aid of macros; the full code is included for clarity.
The format of basic inline assembly is very much straightforward. Its basic form is
asm("assembly code");
Example:
asm("movl %ecx, %eax"); /* moves the contents of ecx to eax */
OR
__asm__("movb %bh, (%eax)"); /* moves the byte from bh to the memory pointed by eax */
Both asm and __asm__ are valid. __asm__ can be used if the keyword asm conflicts with something in your program.
extern int errno;
int funcname(int arg1, int *arg2, int arg3)
{
int res;
__asm__ volatile(
"int $0x80" /* make the request to the OS */
: "=a" (res), /* return result in eax ("a") */
"+b" (arg1), /* pass arg1 in ebx ("b") */
"+c" (arg2), /* pass arg2 in ecx ("c") */
"+d" (arg3) /* pass arg3 in edx ("d") */
: "a" (128) /* pass system call number in eax ("a") */
: "memory", "cc"); /* announce to the compiler that the memory and condition codes have been modified */
/* The operating system will return a negative value on error;
* wrappers return -1 on error and set the errno global variable */
if (-125 <= res && res < 0) {
errno = -res;
res = -1;
}
return res;
}
Example of optimization and processor-specific instructions
This example of inline assembly is from the D programming language and computes the tangent of x using the x86's FPU instructions. This is faster than using the floating-point operations that would be emitted by the compiler, and it allows the programmer to make use of the fldpi instruction, which loads the closest approximation of pi possible on the x86 architecture.
// Compute the tangent of x
real tan(real x)
{
asm
{
fld x[EBP] ; // load x
fxam ; // test for oddball values
fstsw AX ;
sahf ;
jc trigerr ; // x is NAN, infinity, or empty
// 387's can handle denormals
SC18: fptan ;
fstp ST(0) ; // dump X, which is always 1
fstsw AX ;
sahf ;
jnp Lret ; // C2 = 1 (x is out of range)
// Do argument reduction to bring x into range
fldpi ;
fxch ;
SC17: fprem1 ;
fstsw AX ;
sahf ;
jp SC17 ;
fstp ST(1) ; // remove pi from stack
jmp SC18 ;
}
trigerr:
return real.nan;
Lret:
;
}