dmd asm output

[nazriel]

On Monday, 1 April 2013 at 01:54:10 UTC, John Colvin wrote:
> I've been learning assembler a bit and I decided to have a look 
> at what dmd spits out. I tried a simple function with arrays to 
> see what vectorization gets done
>
> void addto(int[] a, int[] b) {
>     a[] += b[];
> }
>
> dmd -O -release -inline -noboundscheck -gc -c test.d
>
> disassembled with gdb:
> _D3sse5addtoFAiAiZv:
> 0x0000000000000040 <+0>:      push   rbp
> 0x0000000000000041 <+1>:      mov    rbp,rsp
> 0x0000000000000044 <+4>:      sub    rsp,0x30
> 0x0000000000000048 <+8>:      mov    QWORD PTR [rbp-0x20],rdi
> 0x000000000000004c <+12>:    mov    QWORD PTR [rbp-0x18],rsi
> 0x0000000000000050 <+16>:    mov    QWORD PTR [rbp-0x10],rdx
> 0x0000000000000054 <+20>:    mov    QWORD PTR [rbp-0x8],rcx
> 0x0000000000000058 <+24>:    mov    rcx,QWORD PTR [rbp-0x18]
> 0x000000000000005c <+28>:    mov    rax,QWORD PTR [rbp-0x20]
> 0x0000000000000060 <+32>:    mov    rdx,rax
> 0x0000000000000063 <+35>:    mov    QWORD PTR [rbp-0x28],rdx
> 0x0000000000000067 <+39>:    mov    rdx,QWORD PTR [rbp-0x8]
> 0x000000000000006b <+43>:    mov    rdi,QWORD PTR [rbp-0x10]
> 0x000000000000006f <+47>:     mov    rsi,rdx
> 0x0000000000000072 <+50>:    mov    rdx,QWORD PTR [rbp-0x28]
> 0x0000000000000076 <+54>:    call   0x7b 
> <_D3sse5addtoFAiAiZv+59>
> 0x000000000000007b <+59>:    mov    rsp,rbp
> 0x000000000000007e <+62>:    pop    rbp
> 0x000000000000007f <+63>:     ret
>
> This looks nothing like what I expected. At first I thought 
> maybe it was due to a crazy calling convention, but adding 
> extern(C) changed nothing.
>
> Can anyone explain what on earth is going on here? All that 
> moving things on and off the stack, a call to the next line 
> (strange) and then we're done bar the cleanup?  I feel i must 
> be missing something.

It just looks like wrong snippet. Probably GDB isn't best 
assembly level debugger.

.text._D4test5addtoFAiAiZAi:08000044                 public 
_D4test5addtoFAiAiZAi
.text._D4test5addtoFAiAiZAi:08000044 _D4test5addtoFAiAiZAi proc 
near
.text._D4test5addtoFAiAiZAi:08000044
.text._D4test5addtoFAiAiZAi:08000044 arg_0           = dword ptr  
8
.text._D4test5addtoFAiAiZAi:08000044 arg_8           = dword ptr  
10h
.text._D4test5addtoFAiAiZAi:08000044 arg_C           = dword ptr  
14h
.text._D4test5addtoFAiAiZAi:08000044
.text._D4test5addtoFAiAiZAi:08000044                 push    ebp
.text._D4test5addtoFAiAiZAi:08000045                 mov     ebp, 
esp
.text._D4test5addtoFAiAiZAi:08000047                 push    
dword ptr [esp+0Ch]
.text._D4test5addtoFAiAiZAi:0800004B                 push    
[ebp+arg_0]
.text._D4test5addtoFAiAiZAi:0800004E                 push    
[ebp+arg_C]
.text._D4test5addtoFAiAiZAi:08000051                 push    
[ebp+arg_8]
.text._D4test5addtoFAiAiZAi:08000054                 call    
_arraySliceSliceAddass_i
.text._D4test5addtoFAiAiZAi:08000059                 add     esp, 
10h
.text._D4test5addtoFAiAiZAi:0800005C                 pop     ebp
.text._D4test5addtoFAiAiZAi:0800005D                 retn    10h
.text._D4test5addtoFAiAiZAi:0800005D _D4test5addtoFAiAiZAi endp

Pardon 32bits, my IDA free doesn't handle 64bit too well.
The only difference is the fact that arguments here are passed on 
stack instead of rdi, rsi etc like it takes place on System V 
AMD64 calling convention