RFR: JDK-8323497: On x64, use 32-bit immediate moves for narrow klass base if possible

[Quan Anh Mai]

On Wed, 10 Jan 2024 09:56:07 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> I haven't changed the name of the method. `movptr` means "move something of the width of the machine pointer", like everywhere else in assembler code. That fits your case, I think?
>
> Hmm, movptr only works its magic if the input is smaller than signed int max (2gb), and it needs one byte more than my variant.
> 
> base < 2g:
> 
> 
>    35  ;; decode_klass_not_null
>    36   0x00007efdcc9e51c4:   mov    $0x27000000,%r11                                                                                                                                                                                                                               
>    37   0x00007efdcc9e51cb:   add    %r11,%r10
> 
> 
> base > 2g:
> 
> 
>    35  ;; decode_klass_not_null
>    36   0x00007ff7a89e51c4:   movabs $0x82000000,%r11
>    37   0x00007ff7a89e51ce:   add    %r11,%r10

FYI the logic for immediate matching is:

    if (is_uimm32(imm)) {
        movl(dst, imm);
    } else if (is_simm32(imm)) {
        movq(dst, imm);
    } else {
        mov64(dst, imm);
    }

The reason is that `movl` is smaller than `movq` in code size.

Maybe we can change `movptr` to this. I hope that I do not miss anything here.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17340#discussion_r1447161579