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

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)^;

After a little more fun in the debugger I came up with the following patch:

Index: generic.inc
=================================================================== --- generic.inc    (revision 40381)
+++ generic.inc    (working copy)
@@ -77,6 +77,8 @@
         begin
           { Align on native pointer size }            aligncount:=(PtrUInt(pdest) and (sizeof(PtrUInt)-1)); +          if aligncount > 0 then
+            aligncount := sizeof(PtrUInt)-aligncount;            dec(count,aligncount);
           pend:=psrc+aligncount;
           while psrc<pend do

from my point of view the routine is wrongly calculating the numbers of bytes that need to be read byte aligned before a pointer-sized copy can be done

Michael

Hi Michael,

there was a discussion in this issue
https://bugs.freepascal.org/view.php?id=33323

Cheers,
Benito

The purpose of the FPC_REQUIRES_PROPER_ALIGNMENT is mainly for compiler generated data structures like the RTTI which uses packed records on targets where the define is not set (e.g. x86).

Regards,
Sven

I see a fix in the bug description but I cannot see that it was implemented on trun:

current trunk:

{ Align on native pointer size }
79 	aligncount:=(PtrUInt(pdest) and (sizeof(PtrUInt)-1));
80 	dec(count,aligncount);
81

patch:

--- rtl/inc/generic.inc	(revision 38836)
+++ rtl/inc/generic.inc	(working copy)
@@ -59,7 +59,7 @@
          then
          begin
            { Align on native pointer size }
-          aligncount:=(PtrUInt(pdest) and (sizeof(PtrUInt)-1));
+          aligncount:=(sizeof(PtrUInt)-PtrInt(pdest)) and (sizeof(PtrUInt)-1);
            dec(count,aligncount);
            pend:=psrc+aligncount;
            while psrc<pend do