Div operation for AVR target includes exception related code
lazarus / 64-FPC development

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ccrause · 2017-07-02 06:20

When including a div operation in a calculation the generated code
size increases dramatically (in terms of memory generally available on
an embedded target).  It appears that the compiler uses generic
software math routines from the rtl/inc folder (generic.inc and
int64.inc).  These routines includes an error check
(HandleErrorAddrFrameInd) which pulls in a bunch of code
(get_caller_frame, pushexceptionobject, dounhandledexception,
raiseexception etc.).

I've adapted (formerly) Atmel's application note AVR200 div routines
to fpc to get a comparison between fpc's build-in code and an
alternative approach without exception handling (see attached demo
program).  I've compiled the example withCode: [Select]ppcrossavr
-Tembedded -Wpatmega328p -Cpavr5 -Pavr -O3 -g -a -al -XPavr-
Compiling attached example with build-in div :
67 lines compiled, 0.1 sec, 2630 bytes code, 310 bytes data

Compiling attached example with custom div function:
67 lines compiled, 0.1 sec, 290 bytes code, 0 bytes data

1st question - is exception handling code required (by language
convention) on embedded targets?
2nd question - is there a need for more compact math routines for the
AVR embedded target?

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Thaddy · 2017-07-02 06:33

1. They are not part of the language, but part of the implementation.
Based on the algorithmic descriptions in e.g.
http://www.atmel.com/Images/doc0936.pdf they are not necessary at all.
2. In my opinion: yes, of course.

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ccrause · 2017-07-03 09:24

I've got 8, 16 and 32 bit unsigned div functions so far, how do I
include them in the embedded avr target RTL?  I am not familiar with
writing code for the compiler or RTL.

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Thaddy · 2017-07-03 10:12

I would create a patch and submit it on the bug tracker.
If you are unable to do that, I would still create a suggestion on the
bugtracker and provide the code as an attachment.
Note it would be useful for mul too as that suffers the same...
The devs will pick it up.

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ccrause · 2017-07-03 19:47

Issue reported...