[fpc-devel] Where to set proper alignment default of const strings?

[Michael Ring]

The more I dig the more I ask myself if not the rtl routine is to blame 
for the issue in the first place because it only takes alignment of the 
destination into account when doing it's job.

Aligncount is calculated base on destination (which is RAM), for this 
reason the last line always crashes because psrc can be unaligned.

I most likely completely misunderstand the meaning of 
FPC_REQUIRES_PROPER_ALIGNMENT but usually ram can be written byte 
aligned but flash has often limitations on read access so alignment 
should look for psrc as this can come from flash.

Unless of course FPC_REQUIRES_PROPER_ALIGNMENT is  primarily meant for 
speed improovement only, then it would make sense to look at pdest to 
try to optimize write performance by having as much aligned access as 
possible.I only found an old discussion in lazarus list from 2011 on 
this topic, where it looks like structs were manually aligned in 
lazarus, but I did not look too deep into the patches.

procedure Move(const source;var dest;count:SizeInt);[public, alias: 
'FPC_MOVE']; from generic.inc line77:

         begin
           { Align on native pointer size }
--> *aligncount:=(PtrUInt(pdest) and (sizeof(PtrUInt)-1));*
           dec(count,aligncount);
           pend:=psrc+aligncount;
           while psrc<pend do
             begin
               pdest^:=psrc^;
               inc(pdest);
               inc(psrc);
             end;
           { use sizeuint typecast to force shr optimization }
           pptruint(pend):=pptruint(psrc)+(sizeuint(count) div 
sizeof(ptruint));
           while psrc<pend do
             begin
               pptruint(pdest)^:=pptruint(psrc)^;

Am 28.11.18 um 17:49 schrieb Michael Ring:
> Hi,
>
> I am debugging an issue that I see on mipsel embedded target (and 
> perhaps also on arm-embedded for corteX-m0, I had crashes in the same 
> rtl routine a while ago)
>
> because RAM is small on embedded devices I use:
>
> {$WRITEABLECONST OFF}
>
> which means that const are read from flash and not from ram.
>
> problem is now that const strings are sometimes not 32-bit aligned and 
> this causes an unaligned access exception on pic32mx because date from 
> flash must be read 32-bit aligned.
>
> I thought that I had seen the correct place to configure this 
> alignment in the past but my google/grep foo is weak at the moment, I 
> cannot find something in the fpc sourcecode that rings a bell.
>
> {$CODEALIGN CONSTMIN=4}
>
> also does not work, I recompiled and in my case the const is still 
> improperly alligned.
>
>
> Thanks in advance,
>
>
> Michael
>
>
>
> _______________________________________________
> fpc-devel maillist  -  fpc-devel at lists.freepascal.org
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