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carraysx86-64calling-convention

Why is there a calling convention for arrays?

I am reading the System V Application Binary Interface, and there is one part that I cannot make sense of.

First, the document states that

No attempt has been made to specify an ABI for languages other than C

(page 10).

Later, on page 20, arrays are classified as MEMORY, POINTER etc.:

The classification of aggregate (structures and arrays) and union types works as follows:
...

The classification is then used to define the calling conventions — how the values and bounds on them are passed to and returned from functions. If I am reading the algorithm correctly, an array could be classified as INTEGER, MEMORY, or SSE.

But in the C language, arrays are always passed and returned as pointers. So why is it useful to classify arrays and in which situation does the array class matter?

Solution

  • I figured it out: if an array is part of a struct or union, it may be passed in a register.

    This C code

    #include <stdint.h>

struct somebytes {
  uint8_t bytes[8];
};

uint8_t plus(struct somebytes p) {
  return p.bytes[3]+p.bytes[5];
}

    translates to this assembly:

    mov    %rdi,%rax
shr    $0x28,%rdi
shr    $0x18,%rax
add    %edi,%eax
retq