[Bug c/20283] New: optimising muldiv() type operations

["ajrobb at bigfoot dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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
 /usr/include
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