Question

Here is a very small source file in D:

void main()
{
}

and here objdump’s disassembly of the .o file:

Disassembly of section .text._Dmain:
0000000000000000 <_Dmain>:
void main()
   0:   55                      push   %rbp
   1:   48 8b ec                mov    %rsp,%rbp
   4:   31 c0                   xor    %eax,%eax
{
   6:   5d                      pop    %rbp
   7:   c3                      retq   

The compiler is DMD64 D Compiler v2.056 running on an x86_64 Linux machine.

What I wonder is why only 32-bit EAX is being cleared rather than the whole 64-bit RAX? I assume this is a return value, required just as in a C program even if not acknowledged in D source.

Answer 1

xor    %eax,%eax

DOES clear the entire rax in x64. Operation on the dword sized registers automatically clear the high dword of the full register.