[Bug rtl-optimization/47373] New: avoid goto table to reduce code size when optimized for size

[carrot at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47373

           Summary: avoid goto table to reduce code size when optimized
                    for size
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: carrot@google.com
              Host: linux
            Target: arm-linux-androideabi


Created attachment 23040
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23040
modified testcase

When I compiled the infback.c from zlib 1.2.5 with options -march=armv7-a
-mthumb -Os, gcc 4.6 generates following code for a large switch statement:

    subs    r3, r3, #11
    cmp    r3, #18
    bhi    .L16
    tbh    [pc, r3, lsl #1]
.L23:
    .2byte    (.L17-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L18-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L154-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L20-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L16-.L23)/2
    .2byte    (.L21-.L23)/2
    .2byte    (.L121-.L23)/2
.L17:

GCC generates a goto table for 19 cases. The table and the instructions which
manipulate it occupies 19*2 + 10 = 48 bytes.

Actually most of the targets in the table are same. There are only 6 targets
other than .L16. So if we generate a sequence of cmp & br instructions, we need
only 6 cmp&br and one br to default, that's only 4*6+2=26 bytes.

When I randomly modified the source code, gcc sometimes generate the absolute
address in the goto table, double the table size, make result worse. The
modified source code is attached.