Re: [Qemu-devel] [PATCH 3/3] tcg/aarch64: implement new TCG target for aarch64

[Claudio Fontana]

On 27.05.2013 23:14, Laurent Desnogues wrote:
> 
> 
> On Monday, May 27, 2013, Richard Henderson <address@hidden 
> <mailto:address@hidden>> wrote:
>> On 2013-05-27 04:43, Claudio Fontana wrote:
>>>>
>>>> Hmm, true.  Although I'd been thinking more along the lines of
>>>> arranging the code such that we'd use movz to set the zero.
>>>
>>> I think we need to keep treating zero specially if we want to keep the 
>>> optimization where we don't emit needless MOVK instructions for half-words 
>>> of value 0000h.
>>>
>>> I can however make one single function out of movi32 and movi64, it could 
>>> look like this:
>>>
>>> if (!value) {
>>>      tcg_out_movr(s, 0, rd, TCG_REG_ZXR);
>>>      return;
>>> }
>>>
>>> base = (value > 0xffffffff) ? 0xd2800000 : 0x52800000;
>>>
>>> while (value) {
>>>      /* etc etc */
>>> }
>>
>>
>>     if (type == TCG_TYPE_I32) {
>>         value = (uint32_t)value;
>>         ext = 0;
>>     } else if (value <= 0xffffffff) {
>>         ext = 0;
>>     } else {
>>         ext = 0x80000000;
>>     }
>>
>>     base = 0x52800000;  /* MOVZ */
>>     do {
>>         int shift = ctz64(value) & (63 & -16);
>>         int half = (value >> shift) & 0xffff;
>>         tcg_out32(s, base | ext | half << 5 | rd);
>>         value &= ~(0xffffUL << shift);
>>         base = 0x72800000;  /* MOVK */
>>     } while (value != 0);
>>
>>
>> Since we go through the loop at least once, we emit the movz for zero input. 
>> No need for any extra tests.  And using ctz we can iterate fewer times.
> 
> You could probably go one step further and use the logical
> immediate encoding.  Look at build_immediate_table in binutils
> opcodes/aarch64-opc.c.  The problem would be the use of
> binary search; perhaps one could come up with some perfect
> hash function.
> 
> 
> Laurent

if it's ok I would go with a variation of Richard's approach.
A more sofisticated approach, like the one you suggest, could be added by a 
successive patch.

Thanks,

Claudio