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

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

Eric one more thing, you mentioned that some work has been done lately in
gcc's backend, can you tell me what are the main features added and if
there are any code quality improvements.

2012/1/13 Borja Ferrer <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
>> >
>> >
>> >
>> >
>>
>>
>