Daniël Mantione wrote:
Op Tue, 26 Feb 2008, schreef Luiz Americo Pereira Camara:
Yury Sidorov wrote:
The patch removes packed record for some platforms.
IMO packed can be removed for all platforms. It will gain some speed.
I'd like to understand more this issue.
Why are non packed records faster?
Cache trashing. One of the most underestimated performance killers in
modern software.
The difference occurs at memory allocation or at memory access?
Memory access. What happens is that the non-packed version causes more
cache misses. A cache miss costs many cycles on a modern cpu, a
misaligned read just costs an extra memory access (which is fast if
cached) on x86, and extra load instruction on ARM. This much cheaper
than a chache miss.
Thanks for all explanation. I'm sure that the change is worth.
One more question:
The VirtualTreeView tries to make the fields of the (packed) record
aligned at dword boundary by grouping together smaller (one or two byte
fields) or adding dummy fields. Does this trick overrides the unaligned
memory access?
The real beast:
TVirtualNodePacked = packed record
Index, //Offset 0
ChildCount: Cardinal; //Offset 4
NodeHeight: Word; //Offset 8
States: TVirtualNodeStates; //Offset 10 *
Align: Byte; //Offset 14 **
CheckState: TCheckState; //Offset 15 **
CheckType: TCheckType; //Offset 16
Dummy: Byte; //Offset 17
TotalCount: Cardinal; //Offset 18 *
[...]
For what i understand, the fields marked with * makes an unaligned
access because they are not in dword boundary. Right?
Fields with ** also are not dword boundary aligned, but since are one
byte fields there's not unaligned access. Right?
And about 64bit systems. Should the fields be qword aligned or dword is
still sufficient?
Luiz
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