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.
What's after the code?
The 0x76 call is an inline call function, the ret returns it. The
stuff before it is setting up the registers for the call and what
comes after
0x0000000000000076 <+54>: call 0x7b
<_D3sse5addtoFAiAiZv+59>
0x000000000000007b <+59>: mov rsp,rbp
0x000000000000007e <+62>: pop rbp
0x000000000000007f <+63>: ret
As you can see, the call is calling the function right below it,
but when it returns it depends on what is on the stack as to
where the function returns(since ip is being popped into rbp).
To me, and this is a guess, this looks like some type of table of
functions being called(the ret function is being redirected to
somewhere other than to the place that it was being called from).
So there is much more going on than meets the eye. It would be
easier to understand if you stepped through the code to see where
the ret is headed.
