[Bug target/53447] New: missed optimization of 64bit ALU operation with small constant

[carrot at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53447

             Bug #: 53447
           Summary: missed optimization of 64bit ALU operation with small
                    constant
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: car...@google.com
            Target: arm-unknown-linux-gnueabi


Compile the following code with options -march=armv7-a -O2 -mthumb

void t0p(long long *p)
{
  *p += 1;
}

GCC 4.8  generates:


t0p:
    ldrd    r2, [r0]
    push    {r4, r5}
    movs    r4, #1       //A
    adds    r2, r2, r4   //B
    mov    r5, #0       //C
    adc    r3, r3, r5   //D
    strd    r2, [r0]
    pop    {r4, r5}
    bx    lr

Instructions ABCD can be simplified as

        adds   r2, r2, 1
        adc    r3, r3, 0

This sequence is smaller and faster than original code, it uses two less
registers, so the push/pop instructions can also be removed.

Both arm/thumb mode and Os/O2 generates similar code.

This optimization can also be applied to other alu operations, such as
sub/and/or/xor/cmp.