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

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

Ahh I see, well any function call that needs 8 bytes passed through the
stack will do.

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

>
> Well, then, I'm not sure what to tell you. I agree it seems stupid, if
> the compiler generates it that way.
>
> Another reason why I suggested a real test case is that way I can check
> what different gcc versions are doing, and specifically what gcc trunk
> is doing now. There's been a lot of new work going on with the avr port
> in gcc. I'd like to be able to do some comparisons.
>
> Eric
>
> > -----Original Message-----
> > From: Borja Ferrer [mailto:bor...@gm...]
> > Sent: Thursday, January 12, 2012 2:16 PM
> > To: Weddington, Eric
> > Cc: Sherard Dames; avr...@li...
> > Subject: Re: [avr-llvm-devel] Current Status?
> >
> > 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
> >
> >
> >
> >
>
>