[fpc-devel] ARM vs Thumb2 - can't have both

John Clymer john at johnclymer.net
Tue Aug 23 16:01:10 CEST 2011

Digging some more around it today, came up with the following idea...

In the rtl/embedded folder - there is the "system" file for "ARM" - it is ALL 
pascal - and compiles to either of Thumb2 or ARMV4 - but not both.

In that folder's Makefile.fpc, the units to be built are listed - the could be 
switched to listing directories to get built.

One folder for ARMV4, one for Thumb2.  A "system" file and rtl.cfg file sits in 
each folder.  The "system" file just bounces back down and includes the current 
system files from the rtl/embedded folder, but the library gets built in the 
core specific folder.

That's the easy part, the more difficult part will to be to get the compiler to 
choose the correct system file.  That is, the "usual" ARM folder where the 
libraries sit would need to have the same 2 seperate subdirectorie, the compiler 
would have to choose which one based on the core it's currently compiling for.


From: David Welch <dwelch at dwelch.com>
To: FPC developers' list <fpc-devel at lists.freepascal.org>
Sent: Tue, August 23, 2011 10:39:50 AM
Subject: Re: [fpc-devel] ARM vs Thumb2 - can't have both

Most if not all of my references to thumb meant the original ARMv4T thumb 
instruction set, definitely not the thumb2 extensions, nor ARMv5 or ARMv6 

If for example you had a thumb backend to fpc, you could easily solve this 
problem, all of these libraries would run on both platforms, one compiler, one 
set of libraries, compiled one time.

There is no thumb backend at the moment, this is the first problem to that 

I figure most folks would not want to sink to the lowest common denominator.

I would then recommend splitting the arm/arm7/ARMv4 architecture from the 
cortex-m3/ARMv7m, as implemented now they are two incompatible instruction 
sets.  One instruction set happens to share the name of the company, move beyond 
that sticking point and create two architectures.

The third alternative is do what others do and build two sets of libraries, one 
for each cpu type if that is the preferred term to distinguish arm and thumb2.  
Even if they are in the same library file but by name the linker extracts the 
arm cpu whatsit function from the thumb2 cpu whatsit function it is still two 
compilations of the whatsit function.

You really have to pick one of those solutions, same instruction set or compile 
the libraries twice either as two arches build one or the other but not both, or 
two cpus within an arch and both/all cpus for an arch get built when the arch 
compiler is built.


On 08/22/2011 01:15 AM, John Clymer wrote:
> Yes, all my references of Thumb meant Thumb2.

fpc-devel maillist  -  fpc-devel at lists.freepascal.org
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