Re: [avr-gcc-list] Union as parameter

[Dean Hall]

Larry,

Sounds like you're passing the union by value.  If you are, can you  
instead pass it by reference (which would occupy 2 bytes on the  
stack)?  If you're worried about modifying the data in the function,  
you can shield it with a const declaration.  More details here:  
http://www.embedded.com/shared/printableArticle.jhtml?articleID=9900322

I compiled to assembly a dummy program that used your union.  I  
didn't see anything out of the ordinary (but I'm not an expert in gcc  
prologue code, so take that statement with a modicum of sodium  
chloride).  One thing you might consider is to make the types in the  
bits struct all uint32_t.  This way you explicitly tell the compiler  
to pack them all into a 4 byte container.  I did this in my dummy  
program and didn't see a difference, but I didn't see a much of a  
"riot" in the first place.  Maybe my dummy program didn't exercise a  
certain case of bit manipulation that your program does.

!!Dean

On Aug 24, 2005, at 07:15, address@hidden wrote:

> ------------------------------
>
> Message: 8
> Date: Wed, 24 Aug 2005 00:19:08 -0700
> From: "Larry Barello" <address@hidden>
> Subject: [avr-gcc-list] Union as parameter
> To: "AVR GCC List" <address@hidden>
> Message-ID: <address@hidden>
> Content-Type: text/plain;    charset="us-ascii"
>
> I tried using this typedef, below, as a parameter to a function.   
> When I do,
> it gets stuck onto the stack frame.  The resulting code is a riot  
> since
> there is all sorts of set up and teardown, the value gets stuck  
> onto the
> stack, the routine called, the variable pulled off the stack into  
> registers
> where it is acted upon.