[fpc-devel] data alignment and int64 (or qword, maybe any singlepiece of size 8 if any)

[Пётр Косаревский]

Main warning at the end of the message.

> This is not always true. Sparc requires 64bit alignment for doubles. And
> i386 requires 128bit alignment for XMM.
1. I think, that when you optimize for size, default options should be "avoid alignment when possible".
2. SSE2 is a special case and, by the way, will FPC support integer SSE2 operations? (Current mmx unit is not widely used, so Florian K. said it is low priority.)

Compile this with "-OG" and notice, that while record has size 9, individual variables of that type are aligned by 16-byte. (FPC 2.0.3, i386, win32)
And it does not depend on processor model: -Cp386 and -CpPENTIUM4 yield the same results.

{$A-}
type
  a= record
     a: int64;
     b: byte;
     end;

var
  x: a;
  y: byte;        // these are
  yy: longword;   //   9
  zz: longword;   //     bytes
  z: a;

begin
writeln(sizeof(a));
writeln(longint(@x));
writeln(longint(@y));
writeln(longint(@z));
end.


This program makes me feel strange: it aligns records by 16 bytes for no reason (last two addresses are odd). (Don't forget {$A-})

{$A-}
type
  a= record
     b: byte;
     a: int64;
     end;

var
  x: a;
  y: byte;
  yy: longword;
  zz: longword;
  z: a;

begin
writeln(sizeof(a));
writeln;
writeln(longint(@x));
writeln(longint(@y));
writeln(longint(@z));
writeln;
writeln(longint(@(x.a)));
writeln(longint(@(z.a)));
end.

Get rid of {$A-}, use "-O3rp4", "-CpPENTIUM4", and one of int64 addresses is not 4 bytes aligned! And records are persistently 16 bytes aligned!