surprising optimization results

[Paul Burchard]

Inlines which call and return by value (even struct value) are showing
up consistently faster in my tests than the equivalent inlines which
call and return using non-const reference args (gcc configuration
below).  Assuming this is a real effect, it is surprising to me -- but
perhaps not to those more familiar with gcc's optimization strategies
(maybe by-value makes alias analysis easier?).  Any insight into tuning
my code for gcc-optimizability is appreciated...



Reading specs from /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95/specs
gcc version 2.95 19990728 (release)
 /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95/cpp -lang-c++ -v
-D__GNUC__=2 -D__GNUG__=2 -D__GNUC_MINOR__=95 -D__cplusplus -D__ELF__
-Dunix -D__i386__ -Dlinux -D__ELF__ -D__unix__ -D__i386__ -D__linux__
-D__unix -D__linux -Asystem(posix) -D__EXCEPTIONS -D__OPTIMIZE__ -Wall
-Winline -Acpu(i386) -Amachine(i386) -Di386 -D__i386 -D__i386__ -Di686
-Dpentiumpro -D__i686 -D__i686__ -D__pentiumpro -D__pentiumpro__
-DNO_EXPORT test2.cc /tmp/ccOTW7Ie.ii
GNU CPP version 2.95 19990728 (release) (i386 Linux/ELF)
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95/../../../../include/g++-3
 /usr/local/include

/usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95/../../../../i686-pc-linux-gnu/include
 /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95/include
 /usr/include
End of search list.
The following default directories have been omitted from the search
path:
End of omitted list.
 /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95/cc1plus /tmp/ccOTW7Ie.ii
-quiet -dumpbase test2.cc -O6 -Wall -Winline -version -o /tmp/ccn8UCEj.s
GNU C++ version 2.95 19990728 (release) (i686-pc-linux-gnu) compiled by
GNU C version 2.95 19990728 (release).
 as -V -Qy -o /tmp/ccJsJaNt.o /tmp/ccn8UCEj.s
GNU assembler version 2.9.1 (i386-redhat-linux), using BFD version
2.9.1.0.23
 /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95/collect2 -m elf_i386
-dynamic-linker /lib/ld-linux.so.2 -o test /usr/lib/crt1.o
/usr/lib/crti.o /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95/crtbegin.o
-L/usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95 -L/usr/local/lib
/tmp/ccJsJaNt.o -lstdc++ -lgcc -lc -lgcc
/usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95/crtend.o /usr/lib/crtn.o

-- 
----------------------------------------------------------------------
Paul Burchard   <burchard@pobox.com>   http://www.pobox.com/~burchard/
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