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Compiler generation of embedded assembly language functions

The bodies of all the __asm functions in a translation unit are assembled as if they are concatenated into a single file that is then passed to the ARM assembler. The order of __asm functions in the assembly language file that is passed to the assembler is guaranteed to be the same order as in the source file, except for functions that are generated using a template instantiation.


This means that it is possible for control to pass from one __asm function to another by falling off the end of the first function into the next __asm function in the file, if the return instruction is omitted.

When you invoke armcc, the object file produced by the assembler is combined with the object file of the compiler by a partial link that produces a single object file.

The compiler generates an AREA directive for each __asm function, as in Example 42:

#include <cstddef>
struct X
    int x,y;
    void addto_y(int);
__asm void X::addto_y(int)
    LDR      r2, [r0, #__cpp(offsetof(X, y))]
    ADD      r1, r2, r1
    STR      r1, [r0, #__cpp(offsetof(X, y))]
    BX       lr

For this function, the compiler generates:

    AREA ||.emb_text||, CODE, READONLY
    EXPORT |_ZN1X7addto_yEi|
#line num "file"
|_ZN1X7addto_yEi| PROC
    LDR r2, [r0, #4]
    ADD r1, r2, r1
    STR r1, [r0, #4]
    BX lr

The use of offsetof must be inside __cpp() because it is the normal offsetof macro from the cstddef header file.

Ordinary __asm functions are put in an ELF section with the name .emb_text. That is, embedded assembly functions are never inlined. However, implicitly instantiated template functions and out-of-line copies of inline functions are placed in an area with a name that is derived from the name of the function, and an extra attribute that marks them as common. This ensures that the special semantics of these kinds of functions are maintained.


Because of the special naming of the area for out-of-line copies of inline functions and template functions, these functions are not in the order of definition, but in an arbitrary order. Therefore, do not assume that code execution falls out of an inline or template function and into another __asm function.