Re: [avr-llvm-devel] Current Status?

[Borja F. <bor...@gm...>]

Haha, well in avr asm it would be something like:

in    r30, 0x3d
in    r31, 0x3e
sbiw    r30, 0x08
in    r0, 0x3f
cli
out    0x3e, r30
out    0x3f, r0
out    0x3d, r31 <--- 8 instructions to allocate 8 bytes of stack space!
std Z+1, reg1 <-- move 8 regs into the stack
std Z+2, reg2
....
std Z+8, reg8
call foo

instead of:

push reg1
push reg2
...
push reg8
call foo

You still have 8 instructions to copy the data into the stack for both
cases, but in the 2nd you've saved 8 instructions that manipulate SP.
Does it make more sense now?

2012/1/12 Weddington, Eric <Eri...@at...>

>
>
> > -----Original Message-----
> > From: Borja Ferrer [mailto:bor...@gm...]
> > Sent: Thursday, January 12, 2012 1:25 PM
> > To: Weddington, Eric
> > Cc: Sherard Dames; avr...@li...
> > Subject: Re: [avr-llvm-devel] Current Status?
> >
> > For now I've implemented it the same way as gcc, but it's something
> that
> > popped to me when i was coding it.
> > I don't see how it saves space :) what I think is that you get the
> same
> > amount of pushes and std's when moving data into the stack no matter
> what
> > method is used. The difference I see is that when using std, you have
> to
> > adjust the SP pointer which creates an additional overhead. Just to
> > clarify I'm talking about the case when you want to pass function
> > arguments before a function call, not in a function prologue which is
> a
> > different story.
> >
> > In x86 it would be something like:
> >
> > sub esp, 8
> > mov DWORD PTR [esp], reg1
> > mov DWORD PTR [esp+4], reg 2
> > call foo
> > ...
> >
> > instead of
> >
> > push reg1
> > push reg2
> > call foo
> > ...
> >
> > which saves 1 instruction, but in our case doing "sub esp, 8" means
> adding
> > 5 or 6 more instructions.
> >
>
> My x86 assembly is extremely rusty. ;-)
>
> Maybe we should work with a real test case... Do you have something that
> shows the issue?
>
> Eric
>
>
>