[fpc-devel] Two floating point issues for the price of one on Cortex-M0 / Trunk

[Michael Ring]

Jeppe already told me that floating point could be a little shaky on 
Cortex-M, and he was right...

I am creating examples for the Raspberry Pi Pico and the ADC example 
requires some floating point math.

All testing was done with trunk version of fpc from today

RTL was compiled with empty CROSSOPTS options, for details please see 
end of text.


The first issue smells a lot like an optimization issue, when I compile 
rtl with -O- then it is gone.


First issue is about mutiplication/division.

The same code run with single datatype creates (wrong) negative result 
for this multiplication:

   rawvalue := 887;
   value := rawvalue*3.3/(1 shl 12);


The 2nd one is a bit more strange, here I am asking myself if I forgot 
something:

   str(value:6:3,valuestr);

When I compile the code with double my string version of the number is 
prefixed by 10 or so spaces.

when I compile with single datatype output of str is fine, no spaces as 
prefix.

When I comment out the three codelines behind the str command and 
compile with double I get correct transformation without the prefixed 
spaces.

When I compile RTL with -O- then double shows correctly and single is 
prefixed with spaces.


Commandline to compile code:

fpc -Tembedded -Parm -Cparmv6m -godwarfsets -godwarfcpp 
-Wp-WpNUCLEOG071RB floatmult.lpr

(I compile here for NUCLEOG071RB as Raspberry Pico is not yet in trunk)

Sample Code:

program floatmult;
{$MODE OBJFPC}
{$H-}
var
   //value : single; // gives -0.715 in line 11+12
   value : double; // gives 0.715 in line 11+12
   rawvalue : longWord;
   valuestr : string;
begin
   rawvalue := 887;
   value := rawvalue*3.3/(1 shl 12);
   str(value:6:3,valuestr);
   //value := value - 0.706;
   //value := value / 0.001721;
   //value := 27 - value;
end.


Building fpc RTL for armv6m
+ make --directory=/Users/ring/devel/fpc FPCDIR=/Users/ring/devel/fpc 
FPCMAKE=/Users/ring/devel/fpc/utils/fpcm/fpcmake 
PPUMOVE=/Users/ring/fpcupdeluxe-embedded/fpc/bin/x86_64-darwin/ppumove 
INSTALL_PREFIX=/Users/ring/fpcupdeluxe-embedded/fpc 
INSTALL_BINDIR=/Users/ring/fpcupdeluxe-embedded/fpc/bin/x86_64-darwin 
CROSSBINDIR=/Users/ring/fpcupdeluxe-embedded/fpc/bin/x86_64-darwin 
FPC=/Users/ring/fpcupdeluxe-embedded/fpc/bin/x86_64-darwin/ppcarm 
rtl_clean rtl_all CROSSINSTALL=1 CPU_SOURCE=x86_64 OS_SOURCE=darwin 
OS_TARGET=embedded CPU_TARGET=arm SUBARCH=armv6m NOGDBMI=1 
BINUTILSPREFIX=arm-none-eabi- 'OPT=-vw-n-h-l-d-u-t-p-c- -ap 
-Fl/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib' 
CROSSOPT= 
INSTALL_UNITDIR=/Users/ring/fpcupdeluxe-embedded/fpc/units/arm-embedded/armv6m/rtl
+ pv '--name=Build  ' --line-mode --size 50