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Concept
Inline assembler
Résumé
In computer programming, an inline assembler is a feature of some compilers that allows low-level code written in assembly language to be embedded within a program, among code that otherwise has been compiled from a higher-level language such as C or Ada.
The embedding of assembly language code is usually done for one of these reasons:
Optimization: Programmers can use assembly language code to implement the most performance-sensitive parts of their program's algorithms, code that is apt to be more efficient than what might otherwise be generated by the compiler.
Access to processor specific instructions: Most 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. Examples of specialized instructions are found in the SPARC VIS, Intel MMX and SSE, and Motorola Altivec instruction sets.
Access to special calling conventions not yet supported by the compiler.
System calls and interrupts: High-level languages rarely have a direct facility to make arbitrary system calls, so assembly code is used. Direct interrupts are even more rarely supplied.
To emit special directives for the linker or assembler, for example to change sectioning, macros, or to make symbol aliases.
On the other hand, inline assembler poses a direct problem for the compiler itself as it complicates the analysis of what is done to each variable, a key part of register allocation. This means the performance might actually decrease. Inline assembler also complicates future porting and maintenance of a program.
Alternative facilities are often provided as a way to simplify the work for both the compiler and the programmer. Intrinsic functions for special instructions are provided by most compilers and C-function wrappers for arbitrary system calls are available on every Unix platform.
Source officielle
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