[Bug rtl-optimization/47454] New: registers are not allocated according to its preferred order

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47454

           Summary: registers are not allocated according to its preferred
                    order
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: carrot@google.com
            Target: arm-linux-androideabi


Created attachment 23115
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23115
testcase

The attached test case is extracted from zlib, when compiled by gcc4.6 with
options -march=armv7-a -mthumb -Os, I got the following code

    push    {r4, r5, r6, r7, r8, lr}
    mov    r6, r0
    mov    r4, r1
    cmp    r0, #0
    beq    .L2
    ldr    r5, [r0, #0]
    cmp    r5, #0
    beq    .L2
    cmp    r1, #0
    bge    .L3
    negs    r4, r1
    movs    r7, #0
    b    .L4
.L3:
    asrs    r7, r1, #4
    adds    r7, r7, #1
    cmp    r1, #47
    it    le
    andle    r4, r1, #15
.L4:
    adds    r3, r4, #0
    sub    r8, r4, #8
    it    ne
    movne    r3, #1
    cmp    r8, #7
    ite    ls
    movls    r8, #0
    andhi    r8, r3, #1
    cmp    r8, #0
    bne    .L2
    ldr    r1, [r5, #8]
    cbz    r1, .L5
    ldr    r3, [r5, #4]
    cmp    r3, r4
    beq    .L5
    ldr    r3, [r6, #4]
    ldr    r0, [r6, #8]
    blx    r3
    str    r8, [r5, #8]
.L5:
    str    r7, [r5, #0]
    mov    r0, r6
    str    r4, [r5, #4]
    pop    {r4, r5, r6, r7, r8, lr}
    b    inflateReset
.L2:
    mvn    r0, #1
    pop    {r4, r5, r6, r7, r8, pc}

Note that register r8 is used many times, but register r2 is never used. In
thumb2 r8 is high register, its usage will cause 32bit instructions. If we
replace r8 with r2, a lot of code size will be reduced in this case.

In arm.h REG_ALLOC_ORDER is defined as
3,  2,  1,  0, 12, 14,  4,  5, 6,  7,  8, 10,  9, 11, 13, 15 ...

We can see that r2 should be used before r8, but the result is not.