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

[Thomas Stuefe]

On Wed, 10 Jan 2024 09:49:25 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> God you are fast. 
>> 
>> movptr looks suitable. Why did you name it "ptr" if its not for pointers?
>
> 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

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/17340#discussion_r1447140322