Hello,
Am 22.08.2008 um 10:31 schrieb leledumbo:
Thomas Schatzl wrote:
Use fpc -i to query the available optimization options.
I know that, but what I need is an explanation about what each
optimization
does. Plus, what are the defaults?
From the programmer's manual about the $OPTIMIZATION switch (you
need very recent docs for this) and the "Optimization" chapter:
Default is no optimization at all, i.e. all of the below disabled.
REGVAR - Try to keep local values in registers, avoiding memory traffic.
UNCERTAIN - Use uncertain optimizations, i.e. make some assumptions
about pointer aliasing which give more opportunities in the other
optimizations but may result in incorrect code otherwise; see the
documentation for the rules.
SIZE - Try to generate smaller code; note that this counters some
other optimizations.
STACKFRAME - Do not generate stackframes if possible (e.g. for leaf
procedures).
PEEPHOLE - Peephole optimizations. Try to find code patterns that
either do nothing (and remove them) or replace them with more
efficient ones. See e.g. http://en.wikipedia.org/wiki/
Peephole_optimization for more information.
ASMCSE - Use common subexpression elimination at the assembler level.
LOOPUNROLL - Unroll loops: small loops are unrolled, i.e. loop bodies
inlined multiple times into the code (plus some setup code) - note
that specifying the target processor has great influence on what is
considered "small" (e.g. -Op<x>)
TAILREC - Automatically try to convert tail recursions into iterations.
CSE - Use common subexpression elimination, for more information see
e.g. http://en.wikipedia.org/wiki/Common_subexpression_elimination
DFA - Use data flow analysis, i.e. gathering more information about
the data flow in the program in order to make other optimizations
more effective. See e.g. http://en.wikipedia.org/wiki/
Data_flow_analysis for more information.
There are also LEVEL<1-3> "optimizations" that are shortcuts for some
of the above.
Regards,
Thomas
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