Reading specs from /usr/lib/gcc-lib/i586-suse-linux/3.3.4/specs
Configured with: ../configure --enable-threads=posix --prefix=/usr
--with-local-prefix=/usr/local --infodir=/usr/share/info --mandir=/usr/share/man
--enable-languages=c,c++,f77,objc,java,ada --disable-checking --libdir=/usr/lib
--enable-libgcj --with-gxx-include-dir=/usr/include/g++ --with-slibdir=/lib
--with-system-zlib --enable-shared --enable-__cxa_atexit i586-suse-linux
Thread model: posix
gcc version 3.3.4 (pre 3.3.5 20040809)
/usr/lib/gcc-lib/i586-suse-linux/3.3.4/cc1 -E -quiet -v -D__GNUC__=3
-D__GNUC_MINOR__=3 -D__GNUC_PATCHLEVEL__=4 muldiv.c -march=pentium -O3 muldiv.i
#include "..." search starts here:
#include <...> search starts here:
/usr/local/include
/usr/lib/gcc-lib/i586-suse-linux/3.3.4/include
/usr/i586-suse-linux/include
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End of search list.
/usr/lib/gcc-lib/i586-suse-linux/3.3.4/cc1 -fpreprocessed muldiv.i -quiet
-dumpbase muldiv.c -march=pentium -auxbase muldiv -O3 -version -o muldiv.s
GNU C version 3.3.4 (pre 3.3.5 20040809) (i586-suse-linux)
compiled by GNU C version 3.3.4 (pre 3.3.5 20040809).
GGC heuristics: --param ggc-min-expand=63 --param ggc-min-heapsize=63486
When compiling the following function:
gcc -O3 -S -march=pentium
int vat(int a)
{
return a * 47 / 40;
}
optimising misses the trick of combining the multiply and divide into a single
multiply and shift arithmetic right.
# assume value is already in EAX
movl 1261646643,%ecx
imul %ecx
sarl $30 # I mean shift %edx,%eax pair right by 30 bits
I realise I haven't chosen the value properly to give the same results as an
overflow when multiplying by 47, but is that defined in the standard?
--
Summary: optimising muldiv() type operations
Product: gcc
Version: 3.3.4
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: ajrobb at bigfoot dot com
CC: gcc-bugs at gcc dot gnu dot org
GCC host triplet: SuSE Linux 9.2
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20283
