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

From: Georg-Johann Lay
Subject: [avr-gcc-list] May avr-libc use functions from libgcc?
Date: Sun, 07 Oct 2012 10:25:45 +0200
User-agent: Thunderbird (Windows/20100228)

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.


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