[fpc-pascal] Maybe one stupid question

JoshyFun joshyfun at gmail.com
Fri Jan 23 16:46:54 CET 2009

Hello Eduardo,

Friday, January 23, 2009, 1:46:59 PM, you wrote:

>>I'm using FPC 2.2.2 running over WinXP SP2. Are there any technical
>>reasons that expains that a program runs faster using all checks ( I/O,
>>Range, Overflow, Stack and Verify calls ) with optimization level 1
>>and using heaptrace, than the same one compiled without any check,
>>optimization level 2 or 3 and not using heaptrace ?

EM> Perhaps your level 1 code fits in level 1-2 cache, and your level 2-3
EM> don't, causing more cache misses. Try to compile with -Os  instead
EM> level 1-2-3 and see if it's faster.

Test done also, the -Os does not provide a significative change. Just
to show some numbers:

with -Os, all checks (overflow...) iteration time: 3.650 secs.
with -Os, all checks + heaptrace iteration time: 3.517 secs.

So, heap trace makes programs faster :-?

The cache theory is fine, but the -O2,-O3 produces smaller code than
-O1 (less calls and memory accesses) but program runs slower.

I had also tried to use cmem instead the default memory manager and as
expected the results are worst, iteration time 3.870 seconds.

Also gprof is not helping too much because there are no clear
differences about calls in both optimized and non optimized modes,
except the calls to range check, object check and others as expected
in the "with checks" version which runs faster :-?

With gprof I saw in my code a lot of FillChar calls (which are not in
my code at all), so I think that they comes from the internal
initializations of variables and records, unfortunatly the calls
counter does not appear (spontaneous), and the fact that heaptrace
produces faster code makes me think in the memory manager, but without
a conclusion to this "trivial".

Best regards,

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