floating point library on cortex-m0plus

Question

I am working on a project that uses dynamic relocations, it works fine for the Cortex-M4, but I am having some problems with the Cortex-M0+.

The problems are occuring with the symbols of the functions of floating point. This cores do not have floating point unit.

So I was trying to understand the difference between the codes generated of the two cores (M4 and M0+).

The code is this:

#include <stdint.h>
#include <math.h>           // <fastmath.h>

float a, b, c; //, d, e;

void ldMain(void)
{
    a = 1.100000f + a;
    b = 1.100000f - b;
    c = 1.100000f * c;
    //d = 1.100000f / d;
}

The commands to compile and linking are these:

arm-none-eabi-gcc.exe -c TESTE.c -o TESTE.o0 -mthumb -mcpu=cortex-m0plus -O0 -mlong-calls -mword-relocations -mabi=atpcs -mfloat-abi=soft -mcaller-super-interworking

arm-none-eabi-ld.exe -o TESTE.o TESTE.o0 --relocatable --strip-all --discard-all --embedded-relocs

The symbols that are generated are (get with arm-none-eabi-readelf):

Relocation section '.rel.text' at offset 0x2e4 contains 6 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
00000028  00000b02 R_ARM_ABS32       00000004   a
0000002c  00000c02 R_ARM_ABS32       00000000   __addsf3
00000034  00000602 R_ARM_ABS32       00000004   b
00000038  00000802 R_ARM_ABS32       00000000   __subsf3
0000003c  00000902 R_ARM_ABS32       00000004   c
00000040  00000a02 R_ARM_ABS32       00000000   __mulsf3

Independent of the flag -mcpu=cortex-m0plus or -mcpu=cortex-m4 used on the gcc command, the symbols generated are the same.

The problem is that these symbols appear does not exist on cortex-m0plus.

The libgcc of cortex-m0plus (armv6-m) located at C:\Program Files (x86)\GNU Tools ARM Embedded\4.9 2015q2\lib\gcc\arm-none-eabi\4.9.3\armv6-m not have these symbols. It was verified with the command arm-none-eabi-nm.

Does anyone know why these symbols are used if they do not exist for the cortex-m0plus?

I am using the version 4.9 2015q2 of the GCC ARM Embedded.

It's an old and legacy project. Do you think the use of ATPCS is what is causing this problem? Using AAPCS would be better? — djunho
I don't think so. I did not use soft-float on a CM, so far. Not sure, but, the functions might in one of the other libs, 'libm' or so. libgcc is used for all devices; you should have a look into the sub-directories for the arch. No idea, how atpcs differs from AAPCS, but you might run into trouble if the libs are compiled for AAPCS (as normal if you use pre-built packages. Note that your PCS has to be thumb-compliant. — too honest for this site
Note: Since d = 1.100000f / d; is division by 0 (and that is detectable by a compiler), may want to drop that line from the investigate. — chux - Reinstate Monica
I think that as it is a global variable, maybe the compiler dont make such optmization. But, to not diverge from the problem, I will comment/remove this line. — djunho

Richard at ImageCraft Richard at ImageCraft · Accepted Answer · 2019-04-29T04:45:07

These functions are defined in GCC glibc (either newlib or nanolib). This post is almost 4 years old, and I do not have experience with the 2015 GCC. However, the recent (e.g. 2018 etc.) GCC definitely has these FP software routines in the libraries.

floating point library on cortex-m0plus

1 Answers