[Bug rtl-optimization/30354] New: -Os doesn't optimize a/CONST even if it saves size.

[vda dot linux at googlemail dot com]

gcc -O2 usually optimizes a/CONST. In many cases code is not only significantly
faster, but also smaller:

unsigned f(unsigned a) { return a/10; }

gcc 4.1.1 -O2:
        movl    $-858993459, %eax
        mull    4(%esp)
        shrl    $3, %edx
        movl    %edx, %eax
        ret

gcc 4.1.1 -Os:
        movl    4(%esp), %eax
        movl    $10, %edx
        movl    %edx, %ecx
        xorl    %edx, %edx
        divl    %ecx
        ret

Unfortunately, gcc -S never uses this optimization.

Note that with code proposed in bug 28417 a/CONST can be optimized even further
(we can use smaller mul constant and avoid shrl) when we know from VRP that
value of a is always small enough.


-- 
           Summary: -Os doesn't optimize a/CONST even if it saves size.
           Product: gcc
           Version: 4.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: vda dot linux at googlemail dot com
 GCC build triplet: i386-pc-linux-gnu
  GCC host triplet: i386-pc-linux-gnu
GCC target triplet: i386-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30354