[Bug tree-optimization/32593] New: Missed optimization of 'y = constant - x' operation

["astrange at ithinksw dot com" <gcc-bugzilla at gcc dot gnu dot org>]

> /usr/local/gcc43/bin/g++ -v
Using built-in specs.
Target: i386-apple-darwin8.10.1
Configured with: ../gcc/configure --prefix=/usr/local/gcc43 --disable-multilib
--with-arch=pentium-m --with-tune=nocona --enable-target-optspace
--disable-bootstrap --with-gmp=/sw --with-system-zlib
--enable-languages=c,c++,objc,obj-c++
Thread model: posix
gcc version 4.3.0 20070702 (experimental)

> /usr/local/gcc43/bin/gcc -Os -fno-pic -fomit-frame-pointer -S sub.c

"i= 7 - ff_h264_norm_shift[x>>(CABAC_BITS-1)];" generates:

        movl    $7, %ecx
        subl    %eax, %ecx
        sall    %cl, %edx

It would be better if it generated:

        negl %eax
        addl $7, %eax
        sall    %al, %edx

which would leave a register free (which helps if this function is inlined);
this is safe since eax isn't used again later.

You can do this by transforming 'y = constant - x' into 'y = -x + constant'.
It looks like gcc actually does the reverse; if I change the source to match
the best output, it generates the same thing.


-- 
           Summary: Missed optimization of 'y = constant - x' operation
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: astrange at ithinksw dot com
 GCC build triplet: i386-apple-darwin8.10.1
  GCC host triplet: i386-apple-darwin8.10.1
GCC target triplet: i386-apple-darwin8.10.1


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32593