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

[Weddington, E. <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
> 
> 
> 
>