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Re: [avr-gcc-list] avr-gcc: Add --with-avrlibc configure option for AVR-

From: Weddington, Eric
Subject: Re: [avr-gcc-list] avr-gcc: Add --with-avrlibc configure option for AVR-Libc cooperation?
Date: Mon, 27 Aug 2012 18:20:18 +0000

> -----Original Message-----
> From: address@hidden [mailto:avr-
> address@hidden On Behalf Of Georg-
> Johann Lay
> Sent: Sunday, August 26, 2012 4:10 AM
> To: address@hidden
> Subject: [avr-gcc-list] avr-gcc: Add --with-avrlibc configure option for AVR-
> Libc cooperation?
> Hi,
> for historical reasons, AVR-Libc implements functions that
> GCC expects to live in libgcc, namely float functions like
> __fixsfsi.
> Currently, avr-gcc is not configurable to accommodate that
> situation which leads to performance problems if the float
> support functions from libgcc are used.
> This happens at least in the following situations:
> * The user does not specify -lm.  -lm should only be needed
>    if function from math.h are used, not for language core
>    features like int i = (int) float.
> * The application is LTO compiled, i.e. linked with -flto.
>    The plugin machinery passes lgcc -lc lgcc through to the
>    linker by means of -plugin-opt=-pass-through=-lgcc etc.
>    so that -lgcc is linked prior to -lm.
> * The user uses fixed <-> float conversion routines from
>    libgcc.  These routines refer float functions, and the linker
>    resolves these function in libgcc if they are there.
> * avr-g++ is used as linker driver.

Yes, we definitely want to make sure all the above are working.

> In order to fix all of these cases, at least two changes
> are needed in the compiler.
> 1) Remove respective functions from libgcc
> 2) libm.a is not a "math library", it is a "libgcc supplement".
>     Thus, -lm has to be treated like libgcc.

Joerg Wunsch and I have talked for years about the idea of integrating libm 
into libc, so that way there was only one library to deal with. Would this be a 
good point to do that as well? Would that make all the linker flag stuff easier 
to deal with?

Eric Weddington

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