Re: Optimizing empty functions in C

"Peter L. Montgomery" <Peter-Lawrence.Montgomery@cwi.nl> writes:


>"Mark Lacey \[MSFT\]" <mlacey@microsoft.com> writes:
>>
>>The Microsoft Visual C++ toolset will eliminate duplicated functions
>>through a process it calls "identical COMDAT folding" (ICF). By
>>specifying -O1 or -O2 for the compilation and -opt:icf as a link
>>option, you get this effect. For elimination of (statically)
>>unreferenced functions, you again need either -O1 or -O2 on the
>>compilation and -opt:ref as a link option. -opt:ref implies -opt:icf
>>by default.
>
>/* Does standard C require twice_1 to compare unequal to twice_2? */


Yes, it does. So a conforming compiler can't treat them as identical.


It can however replace the body of one function with a jump to the
other one. This achieves most of the space savings from merging
duplicate functions (e.g. those that arise from C++ templates), albeit
at the cost of a small increase in execution time for the extra jumps,
which may be outweighed by the reduction in icache misses.


--
Fergus Henderson <fjh@cs.mu.oz.au> | "I have always known that the pursuit
The University of Melbourne | of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh> | -- the last words of T. S. Garp.

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