Re: Code size issues on FP-emulation on libgcc compared to LLVM's compiler_rt

[Joel Sherrill <joel dot sherrill at oarcorp dot com>]

On 6/30/2015 1:42 PM, H.J. Lu wrote:
> On Tue, Jun 30, 2015 at 10:44 AM, Joseph Myers <joseph@codesourcery.com> wrote:
> > On Tue, 30 Jun 2015, Zinovy Nis wrote:
> >
> > > It works properly but I noticed that code the size for many arithmetic
> > > functions is much more larger than for soft-fp emulation provided by
> > > LLVM's compiler_rt library.
> >
> > Code size is discussed in the paper about soft-fp in the 2006 Summit
> > proceedings.  In general, soft-fp is optimized for speed (having
> > completely separate "if" cases for addition and subtraction, both of which
> > appear in both __addsf3 and __subsf3, for example) and for generality
> > (it's used in the Linux kernel for floating-point emulation, where you
> > want to follow the same choice of NaN etc. as a particular processor even
> > where IEEE semantics don't specify a particular choice, and it has
> > optional support for floating-point exceptions and rounding modes - note
> > that __float128 support in libgcc uses the hardware exceptions and
> > rounding modes when available), not for size (and generality takes
> > precedence over speed where they conflict, hence being a bit slower than
> > ieeelib even after the 2006 optimization work).
> >
> > soft-fp is expected to be used on 32-bit and 64-bit systems for which a
> > few kB code size is insignificant.
>
> Size is very important for IA MCU.  Would it be acceptable to update
> soft-fp to optimize for size with
>
> #ifdef __OPTIMIZE_SIZE__
> #else
> #endif

I don't dispute that optimizing for size would probably be wise
on any x86 CPU that needs soft-float. They are all relatively slow.
But posted measurements of size and speed impact of doing this
would be appreciated.

Unless I slept through it getting removed, the rtems targets
never dropped the lower end i386 to keep legacy hardware supported.

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