[Bug target/46975] New: Replace 32 bit instructions with 16 bit instructions in thumb2

["carrot at google dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46975

           Summary: Replace 32 bit instructions with 16 bit instructions
                    in thumb2
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: carrot@google.com


Compile the following c code with options -march=armv7-a -mthumb -Os

int gzeof (int s)
{
    return s == 1;
}

GCC 4.6 generates:

00000000 <gzeof>:
   0:    f1a0 0301     sub.w    r3, r0, #1    // A
   4:    4258          negs    r0, r3
   6:    eb40 0003     adc.w    r0, r0, r3      // B
   a:    4770          bx    lr

Notice that instructions A and B are 32 bits, we can change them to subs and
adcs so both will be 16 bits.

The code sequence is generated by the following peephole2

 8731 ;; Attempt to improve the sequence generated by the compare_scc splitters
 8732 ;; not to use conditional execution.
 8733 (define_peephole2
 8734   [(set (reg:CC CC_REGNUM)
 8735         (compare:CC (match_operand:SI 1 "register_operand" "")
 8736                     (match_operand:SI 2 "arm_rhs_operand" "")))
 8737    (cond_exec (ne (reg:CC CC_REGNUM) (const_int 0))
 8738               (set (match_operand:SI 0 "register_operand" "") (const_int
0)))
 8739    (cond_exec (eq (reg:CC CC_REGNUM) (const_int 0))
 8740               (set (match_dup 0) (const_int 1)))
 8741    (match_scratch:SI 3 "r")]
 8742   "TARGET_32BIT"
 8743   [(set (match_dup 3) (minus:SI (match_dup 1) (match_dup 2)))
 8744    (parallel
 8745     [(set (reg:CC CC_REGNUM)
 8746           (compare:CC (const_int 0) (match_dup 3)))
 8747      (set (match_dup 0) (minus:SI (const_int 0) (match_dup 3)))])
 8748    (set (match_dup 0)
 8749         (plus:SI (plus:SI (match_dup 0) (match_dup 3))
 8750                  (geu:SI (reg:CC CC_REGNUM) (const_int 0))))])
 8751 

We should change the new instructions to use subs and adcs.