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

[Weddington, E. <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