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

From: Gabriel Dos Reis
Subject: Re: [avr-gcc-list] May avr-libc use functions from libgcc?
Date: Sun, 7 Oct 2012 17:20:50 -0500

On Sun, Oct 7, 2012 at 12:56 PM, Weddington, Eric
<address@hidden> wrote:
>> -----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.

That makes sense to me.

-- Gaby

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