AVR libc device defines ?

Here's a newbie question. I'm doing the the example program that's
listed in the gcc AVR libc documentation. I'm using the STK500 dev.
board with a ATMEGA8515 and AVR Studio. The example provides a file
iocompat.h that defines some device specific stuff. Here's a exerpt:

#if defined(__AVR_AT90S2313__)
#  define OC1 PB3
#  define OCR OCR1
#  define DDROC DDRB
#  define TIMER1_OVF_vect TIMER1_OVF1_vect
#elif defined(__AVR_AT90S2333__) || defined(__AVR_AT90S4433__)
#  define OC1 PB1
#  define DDROC DDRB
#  define OCR OCR1
#elif defined(__AVR_AT90S4414__) || defined(__AVR_AT90S8515__) || \
      defined(__AVR_AT90S4434__) || defined(__AVR_AT90S8535__) || \
      defined(__AVR_ATmega163__) || defined(__AVR_ATmega8515__) || \
      defined(__AVR_ATmega8535__) || \
      defined(__AVR_ATmega164P__) || defined(__AVR_ATmega324P__) || \
      defined(__AVR_ATmega644__) || defined(__AVR_ATmega644P__)

My question, where are these device constants defined? For example, I
think (__AVR_AT90S8515__)
is defined somewhere for my project but where. I've searched all the
include headers that come with gcc and can't find it. Can a makefile
define this somehow? Does AVR studio do it somehow by knowing I'm using
a 8515? Is it getting built by some algorithm somewhere and that's why
I can't find this string? 

I'm baffled. Any insight is appreciated.

On 2006-11-19, JoeyB <joseph.burgel@gm.com> wrote:

> My question, where are these device constants defined? For example, I
> think (__AVR_AT90S8515__)

They're not in any source file.  They're provided by the compiler.

You can examine this like so:

    touch foo.S
    avr-gcc -dM -E -mmcu=atmega8 foo.S

(substitute the name of your compiler for "avr-gcc" if it's different).
The "-dM" instructs it to print out the things which are defined; the
"-E" tells it to stop after preprocessing.

You'll see several dozen predefined constants, including the ones you're
interested in (for example, -mmcu=atmega8 sets __AVR_ENHANCED__,
__AVR_ATmega8__, __AVR_ARCH__, etc.)

> Can a makefile define this somehow?

In fact, it could (you can explicitly set things on the command line as
well with the -D option), but in this case that probably isn't the
mechanism being used.

> Does AVR studio do it somehow by knowing I'm using a 8515?

There's probably an -mmcu= or equivalent option being passed from your
development environment if you picked the target part there, yes.