[fpc-devel] about freepascal for mips

[Jonas Maebe]

Fuxin Zhang wrote on Fri, 25 May 2012:

> And my progress and problems:
>   I've got the source from subversion and tried to build it in ubuntu
> 11.10 x86-64 machine. With the following patch and commands, I can get a
> partly working ppcrossmipsel(it can at least build a helloworld and
> runnable):
> patch:
>   Index: Makefile.fpc
> ===================================================================
> --- Makefile.fpc	(版本 21354)
> +++ Makefile.fpc	(工作副本)
[snip]
> This is to fix various file not found errors. I am not sure whether it is
> 'right' fix.

No, definitely it's definitely not right. It's strange that you need  
those changes, since the Linux RTL Makefile is used daily. Did you  
need those changes even to compile it for i386?

> commands:
>   cd rtl/linux;
>   patch Makefile.fpc; fpcmake -Ti386-linux,mipsel-linux

It's better to use -Tall (since there are more Linux targets than just  
i386 and mipsel).

> /home/foxsen/fpc/rtl/units/mipsel-linux/sysutils.o: In function
> `SYSUTILS_$$_finalize':
> /home/foxsen/fpc/rtl/unix/sysutils.pp:1414: relocation truncated to fit:
> R_MIPS_PC16 against
> `SYSTEM$_$TINTERFACEDOBJECT_$__$$_QUERYINTERFACE$TGUID$formal$$LONGINT'
> /home/foxsen/fpc/rtl/unix/sysutils.pp:1414: relocation truncated to fit:
> R_MIPS_PC16 against `SYSTEM$_$TINTERFACEDOBJECT_$__$$__ADDREF$$LONGINT'
> /home/foxsen/fpc/rtl/unix/sysutils.pp:1414: relocation truncated to fit:
> R_MIPS_PC16 against `SYSTEM$_$TINTERFACEDOBJECT_$__$$__RELEASE$$LONGINT'
> pp.pas(224,36) Error: Error while linking
> pp.pas(224,36) Fatal: There were 1 errors compiling module, stopping
> Fatal: Compilation aborted> make[1]: *** [ppcmipsel] 错误 1
> make[1]:正在离开目录 `/home/foxsen/fpc/compiler'
> make: *** [cycle] 错误 2
>
> I know it is related to mips branch offset overflow.

A similar issue exists on PowerPC: conditional branches are limited to  
16 bit offsets, while unconditional branches can reach up to 26 bits  
or so. As far as I can see, it's similar on MIPS (the B* opcodes  
versus the J opcode). If so, you may be able to create a MIPS version  
of the fixup_jmps procedure in compiler/ppcgen/aasmcpu.pas and add it  
to compiler/mips/aasmcpu.pas (it's called from psub.pas, you'll have  
to modify the ifdef there to also activate it for MIPS and MIPSEL).

The actual errors you get are strange though. We never insert  
conditional branches from one routine to another (in the above case:  
from SYSUTILS_$$_finalize, or more likely from interface wrappers  
following that symbol, to various other routines). Looking at the code  
for tcgcpu.g_intf_wrapper in compiler/mips/cgcpu.pas, the code for  
interface wrappers uses the "B" as opposed to the "J" instruction to  
branch to the interface wrappers. Maybe that's a copy/paste error from  
another platform. I'm not sure what "B" means though, I don't see it  
listed at http://www.mrc.uidaho.edu/mrc/people/jff/digital/MIPSir.html

Anyway, I guess this patch will solve the above error:

Index: compiler/mips/cgcpu.pas
===================================================================
--- compiler/mips/cgcpu.pas	(revision 21359)
+++ compiler/mips/cgcpu.pas	(working copy)
@@ -1722,7 +1722,7 @@
      op_onr24methodaddr;
    end
    else
-    
list.concat(taicpu.op_sym(A_B,current_asmdata.RefAsmSymbol(procdef.mangledname)));
+    
list.concat(taicpu.op_sym(A_J,current_asmdata.RefAsmSymbol(procdef.mangledname)));
    { Delay slot }
    list.Concat(TAiCpu.Op_none(A_NOP));

> After some
> research,I've found that ppc code support -fPIC.

PIC normally won't help with branch offset overflows.

> But when I try to enable
> it,the other issue appears:

Yes, PIC requires cpu-specific support. It shouldn't be necessary to  
get an initial port up and running though.


Jonas