Cortex M0 Floating Point Library

[Daniel Engel <libgcc at danielengel dot com>]

Hi, 

Over the past couple of years, I have hand-assembled a new floating point library for the ARM Cortex M0 architecture.  I know the M0 is not generally regarded as a number-crunching machine, but I felt it deserved at least some of the attention that has previously been bestowed on the AVR architecture.  As this work has been incidental to my employer's line of business, they have tentatively agreed to assign the copyright and facilitate a release of this library as open source.  

I have efficient implementations of all of the integer and single-precision AEABI functions:

*  clzsi2, clzdi2, umulsidi3, mulsidi3, muldi3 (aeabi_lmul)
*  ashldi3 (aeabi_llsl), lshrdi3 (aeabi_llsr), ashrdi3 (aeabi_lasr)
*  aeabi_lcmp, aeabi_ulcmp
*  udivsi3 (aeabi_uidivmod), divsi3 (aeabi_idivmod), udivdi3 _aeabi_uldivmod), divdi3 (aeabi_ldivmod)
*  addsf3 (aeabi_fadd), subsf3 (aeabi_fsub, aeabi_frsub), mulsf3 (aeabi_fmul), divsf3 (aeabi_fdiv), fdimf
*  cmpsf2 (aeabi_fcmpun), eqsf2 (aeabi_fcmpeq), nesf2 (aeabi_fcmpne), gesf2 (aeabi_fcmpge), gtsf2, unordsf2
*  floatundisf (aeabi_ul2f),floatunsisf (aeabi_ui2f),floatdisf (aeabi_l2f),floatsisf (aeabi_i2f)
*  fixsfdi (aeabi_f2lz), fixunssfdi (aeabi_f2ulz), fixsfsi (aeabi_f2iz), fixunssfsi (aeabi_f2uiz)
*  aeabi_f2d, aeabi_d2f, aeabi_h2f, aeabi_f2h

I also have efficient implementations of several of the simpler libm functions:

*  frexpf, ldexpf, scalbnf
*  fmaxf, fminf
*  rintf, lrintf, ulrintf, llrintf, ullrintf, roundf, lroundf, ulroundf, llroundf, ullroundf
*  truncf, ceilf, floorf
*  fpclassifyf, isnormalf, isnanf, isinff, isfinitef, isposf, isnegf
*  ilogbf, logbf, modff
*  sqrtf, cbrtf
*  log2f, logf, log10f, log1p2f, log1pf, log1p10f, logXf, log1pXf
*  sinf, cosf, sincosf, sinpif, cospif, sincospif
*  tanf, cotf, tanpif, cotpif

Presently, the library comprises about 40 files with about 8000 lines of asm (unified syntax).  The test vectors weigh significantly more.  All of the floating point functions are IEEE754 compliant.  I can provide more complete performance statistics on request, but here are a few highlights: 

* Small: Less than 3kb for everything above.  Only 450 bytes for basic addsf3, subsf3, mulsf3, divsf3, and cmpsf2.
* Fast: addsf3 = 75 instruction cycles, subsf3 = 80, mulsf3 = 95, divsf3 = 260 to 360, cmpsf2 = 35.
* Correct: Simultaneous calculation of sincosf() in less than 500 instruction cycles, accurate within +/- 1 ulp, including arbitrarily large values of 'x'.
* Bonus: round10iff(x, n) (a non-standard function) correctly rounds floating point values 'x' to an integer power of 10 'n'; this function simulates conversion to a decimal string, truncation, and conversion back to binary32 without any string-handling overhead.

To date, I have only built this library as part of a user space embedded application.  I have not attempted to build or patch the GCC toolchain itself.  If accepted, I suspect there will be at least a little work to restructure it for inclusion with libgcc.  But, before proceeding with that work, I need to have some idea of direction and goal.  

The first question, then, is what might the best home for this library be?  Many of the lower level functions (e.f. clzsi2, addsf3) replace the generic implementations of libgcc.  However, the higher level functions (e.g. ldexpf, sincosf) traditionally link from libm, which I don't believe is typically distributed with gcc.  The compact nature of this library of course follows from a tight integration between higher and lower level functions.  I have considered a few strategies: 

* Add everything into the base libgcc, 
* Add everything into libm (newlib?) and rely on link order to supersede libgcc, 
* Split the implementation with some magic to ensure that libm functions only link in the presence of the correct libgcc,
* Establish an independent library specific to the Cortex M0 architecture, or
* Something else entirely...

If there is any interest in incorporating this work into GCC, please advise.  

Thanks,
Daniel Engel