Re: optimize __gp location

[Keith Owens <kaos@sgi.com>]

On Mon, 24 Jan 2005 14:44:22 +0100, 
Christian Hildner <christian.hildner@hob.de> wrote:
>Keith Owens schrieb:
>>When jiffies is within 22 bit range of __gp, the linker writes the
>>sequence as
>>
>>    addl r20=offset_of(jiffies,__gp),r1;;
>>    mov r16=r20;;
>>    ld8.acq r23=[r16]	// value of jiffies
>>
>Is there a restriction to not rewrite to
>
>    addl r16=offset_of(jiffies,__gp),r1;;
>    ld8.acq r23=[r16]	// value of jiffies
>    nop.i 0
>
>because that would save at least one cycle and would make bundling easier (dependend of additional instructions, of course).

The code snippet was a simplification of what gcc actually does.  If
you look at some object code, you will find that the 3 instructions are
already spread over multiple bundles.  Moving the final ld8 upwards
cannot save any cycles, you still have to execute the same number of
bundles.  A real example from kernel/sched.o

    4830:       09 50 20 42 00 21       [MMI]       adds r10=8,r33
                        4832: LTOFF22X  jiffies
    4836:       20 81 84 00 42 c0                   adds r18\x16,r33
    483c:       01 08 00 90                         addl r14=0,r1;;
    4840:       08 00 08 1e d8 19       [MMI]       stf.spill [r15]ò
                        4841: LDXMOV    jiffies
                        4842: LTOFF22X  __per_cpu_offset
    4846:       b0 00 38 30 20 40                   ld8 r11=[r14]
    484c:       03 08 00 90                         addl r26=0,r1
    4850:       08 a0 00 02 00 24       [MMI]       addl r20=0,r1
                        4850: LTOFF22X  .data.percpu+0x440
    4856:       90 00 01 20 40 e0                   shladd r9=r32,1,r0
    485c:       02 00 59 00                         sxt4 r23=r32
    4860:       08 40 00 14 18 10       [MMI]       ld8 r8=[r10]
    4866:       10 01 48 30 20 e0                   ld8 r17=[r18]
    486c:       04 00 c4 00                         mov r39°
    4870:       05 00 00 00 01 40       [MLX]       nop.m 0x0
    4876:       10 00 00 00 00 60                   movl r27=0x10624dd3;;
    487c:       33 55 6c 62 
    4880:       10 00 00 00 01 00       [MIB]       nop.m 0x0
    4886:       f0 40 e0 f0 29 00                   shl r15=r8,7
    488c:       00 00 00 20                         nop.b 0x0
    4890:       09 c0 00 34 18 10       [MMI]       ld8 r24=[r26]
                        4890: LDXMOV    __per_cpu_offset
    4896:       30 00 2c 70 21 40                   ld8.acq r3=[r11]

The LDXMOV relocation is designed to make it simple to convert the
instruction from ld8 r11=[r14] to mov r11=r14, it is easy to do in
place.  Moving an entire slot around is a lot messier, for no
performance gain.