Re: [Qemu-devel] [PATCH v3 19/29] tcg-aarch64: Introduce tcg_fmt_Rd_uimm_s

[Richard Henderson <rth@twiddle.net>]

On 09/05/2013 06:32 AM, Claudio Fontana wrote:
>>  {
>> -    uint32_t half, base, shift, movk = 0;
>> -    /* construct halfwords of the immediate with MOVZ/MOVK with LSL */
>> -    /* using MOVZ 0x52800000 | extended reg.. */
>> -    base = (value > 0xffffffff) ? 0xd2800000 : 0x52800000;
>> -    /* count trailing zeros in 16 bit steps, mapping 64 to 0. Emit the
>> -       first MOVZ with the half-word immediate skipping the zeros, with a shift
>> -       (LSL) equal to this number. Then morph all next instructions into MOVKs.
>> -       Zero the processed half-word in the value, continue until empty.
>> -       We build the final result 16bits at a time with up to 4 instructions,
>> -       but do not emit instructions for 16bit zero holes. */
> 
> Please do not remove these comments.
> In my judgement this part of the code profits from some verbose clarification.
> What is happening might be obvious to you, but not to others trying to step in.

Fair enough.

> In general I'd prefer to keep movi as it was (functionally-wise) for the
> time being, replacing it with a more efficient version once we can get some
> numbers (which will be soon) with which to justify (or not) the added code
> complexity.

The most important thing we're not doing at the moment is handling negative
numbers efficiently.  E.g. we're using 4 insns to load -1.



r~