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Date: Fri, 08 Nov 2013 11:08:00 -0000
Message-ID: <CAO9OKOO6NbmPwuSPWwoGFRWbxx0DqrZ8neN_wWNhUyiaZWiZmQ@mail.gmail.com>
Subject: m68k optimisations?
From: Fredrik Olsson <peylow@gmail.com>
To: gcc@gcc.gnu.org
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X-SW-Source: 2013-11/txt/msg00137.txt.bz2

I have this simple functions:
int sum_vec(int c, ...) {
    va_list argptr;
    va_start(argptr, c);
    int sum = 0;
    while (c--) {
        int x = va_arg(argptr, int);
        sum += x;
    }
    va_end(argptr);
    return sum;
}


When compiling with "-fomit-frame-pointer -Os -march=68000 -c -S
-mshort" I get this assembly (I have manually added comments with
clock cycles per instruction and a total for a count of 0, 8 and n>0):
    .even
    .globl _sum_vec
_sum_vec:
    lea (6,%sp),%a0         | 8
    move.w 4(%sp),%d1       | 12
    clr.w %d0               | 4
    jra .L1                 | 12
.L2:
    add.w (%a0)+,%d0        | 8
.L1:
    dbra %d1,.L2            | 16,12
    rts                     | 16
| c==0: 8+12+4+12+12+16=64
| c==8: 8+12+4+12+(16+8)*8+12+16=256
| c==n: =64+24n

When instead compiling with "-fomit-frame-pointer -O3 -march=68000 -c
-S -mshort" I expect to get more aggressive optimisation than -Os, or
at least just as performant, but instead I get this:
    .even
    .globl _sum_vec
_sum_vec:
    move.w 4(%sp),%d0       | 12
    jeq .L2                 | 12,8
    lea (6,%sp),%a0         | 8
    subq.w #1,%d0           | 4
    and.l #65535,%d0        | 16
    add.l %d0,%d0           | 8
    lea 8(%sp,%d0.l),%a1    | 16
    clr.w %d0               | 4
.L1:
    add.w (%a0)+,%d0        | 8
    cmp.l %a0,%a1           | 8
    jne .L1                 | 12|8
    rts                     | 16
.L2:
    clr.w %d0               | 4
    rts                     | 16
| c==0: 12+12+4+16=44
| c==8: 12+8+8+4+16+8+16+4+(8+8+12)*4-4+16=316
| c==n: =88+28n

The count==0 case is better. I can see what optimisation has been
tried for the loop, but it just not working since both the ini for the
loop and the loop itself becomes more costly.

Being a GCC beginner I would like a few pointers as to how I should go
about to fix this?

// Fredrik