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Unformatted text preview: Inline Assembly Code 9 T ODAY , FEW PROGRAMMERS USE ASSEMBLY LANGUAGE . Higher-level languages such as C and C++ run on nearly all architectures and yield higher productivity when writing and maintaining code. For occasions when programmers need to use assembly instructions in their programs, the GNU Compiler Collection permits programmers to add architecture-dependent assembly language instructions to their programs. GCCs inline assembly statements should not be used indiscriminately.Assembly language instructions are architecture-dependent, so, for example, programs using x86 instructions cannot be compiled on PowerPC computers.To use them, youll require a facility in the assembly language for your architecture. However, inline assembly statements permit you to access hardware directly and can also yield faster code. An asm instruction allows you to insert assembly instructions into C and C++ programs. For example, this instruction asm (fsin : =t (answer) : 0 (angle)); is an x86-specific way of coding this C statement: 1 answer = sin (angle); 1.The expression sin (angle) is usually implemented as a function call into the math library, but if you specify the -O1 or higher optimization flag, GCC is smart enough to replace the function call with a single fsin assembly instruction. 190 Chapter 9 Inline Assembly Code Observe that unlike ordinary assembly code instructions, asm statements permit you to specify input and output operands using C syntax. To read more about the x86 instruction set, which we will use in this chapter, see http://developer.intel.com/design/pentiumii/manuals/ and http://www.x86-64.org/documentation . 9.1 When to Use Assembly Code Although asm statements can be abused, they allow your programs to access the computer hardware directly, and they can produce programs that execute quickly. You can use them when writing operating system code that directly needs to interact with hardware. For example, /usr/include/asm/io.h contains assembly instructions to access input/output ports directly.The Linux source code file /usr/src/linux/arch/i386/kernel/process.s provides another example, using hlt in idle loop code. See other Linux source code files in /usr/src/linux/arch/ and /usr/src/linux/drivers/ . Assembly instructions can also speed the innermost loop of computer programs. For example, if the majority of a programs running time is computing the sine and cosine of the same angles, this innermost loop could be recoded using the fsincos x86 instruction. 2 See, for example, /usr/include/bits/mathinline.h , which wraps up into macros some inline assembly sequences that speed transcendental function computation. You should use inline assembly to speed up code only as a last resort. Current com- pilers are quite sophisticated and know a lot about the details of the processors for which they generate code.Therefore, compilers can often choose code sequences that may seem unintuitive or roundabout but that actually execute faster than other...
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- Spring '09