On 11 Oct 2012, at 13:59, dhkblas...@zeelandnet.nl wrote:
I did some performance tests on win32 and it appears that
both packed and unpacked objects and records all show exactly the same
performance. Writing the individual variables in a record or object to
file takes about 5.5 times longer than writing them at once. If
someone
wants my test app to run it on other platforms please let me know
then I
can post the code. I will do more testing later on mac and linux32.
I'm
interested how win64 and linux64 behave in this respect. So if someone
has these architectures please let me know.
As mentioned before, it not only depends on the platform, but also on
the contents of the object/record. E.g., a badly misaligned double
will generally give much worse performance even on Intel.
This makes me wonder if
choosing a proper value for $PACKRECORDS could make my file readable
safely on all platforms, only needing to convert the endianess if
applicable. This would not force me to do manual padding in my
structs.
Say I use a value of 16 would that cover all ABI's FPC currently
supports?
Yes.
Jonas: do you have an overview of the alignment on all
architectures that FPC supports?
The information is not just architecture-specific, but also OS-
specific (e.g. the alignment of int64 is 4 on Darwin/i386, but 8 on
all other i386 platforms). This is defined in the platform ABI
documents (application binary interface).
Perhaps you could pinpoint where in the
compiler this is handled? If appreciated I could make a patch to
include
this info in the documentation in the future.
It's a combination of tdef.alignment (and its overridden methods in
compiler/symdef.pas), tdef.structalignment (idem) and the varalign
information in compiler/systems/i_*.pas. And the latter information in
turn can be overridden by the programmer with -Oa switch and the
{$codealign ...} directive, or is sometimes also adjusted by us when
e.g. new data types are introduced, when bugs are found or when
support for a new ABI is added that has different requirements (some
OSes support multiple ABIs).
I don't think documenting it in our manual is a good idea. It's not
something people should depend on beyond what the official platform
ABIs say, and those documents are maintained separately from our
manual (and unfortunately seldom have stable URLs that can be referred
to).
Jonas
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