Re: [avr-gcc-list] May avr-libc use functions from libgcc?

[Weddington, Eric]

> -----Original Message-----
> From: address@hidden
> [mailto:address@hidden On
> Behalf Of Georg-Johann Lay
> Sent: Sunday, October 07, 2012 2:26 AM
> To: address@hidden
> Subject: [avr-gcc-list] May avr-libc use functions from libgcc?
> 
> Is is legitimate that assembler functions in avr-libc call functions
> implemented in libgcc directly?
> 
> This would make some assumptions, e.g. on naming conventions.
> 
> Calling directly can lead so smaller and faster code because some
> functions in libgcc implement a special ABI.
> 
> For example __udivmodqi4 (unsigned 8-bit quotient and remainder) does
> not clobber X or Z so that the caller can hold values in these registers
> across the call which reduces register pressure in the caller.
> 
> Are such calls "allowed" in avr-libc provided
> 
> - The special interface is documented in libgcc sources and mentions
>    that avr-libc makes use of the special interface
> 
> - The functions with their special interface are documented, e.g. in
>    the GCC wiki
> 
> The question is for code that is not yet part of avr-libc, namely the
> upcoming fixed-point support; it's not about optimizing code that's
> already there.
> 
> Sean said that CORDIC shows bad rounding for asin, for example.
> 
> I played around with a home-brew asin approach that has no rounding
> issues.  The performance of the implementation can be improved if
> knowledge of libgcc is used.

I'm ok with this using knowledge of libgcc in avr-libc, if the benefits are 
faster, and especially smaller, code.

I'd like to hear from other project members...

Eric