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

[Laurent Desnogues]

On Monday, May 27, 2013, Richard Henderson <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