<i18n dev> RFR: 8304245: Speed up CharacterData.of by avoiding bit shifting in the latin1 fast-path test [v2]

[Eirik Bjorsnos]

On Sat, 18 Mar 2023 17:06:52 GMT, jmehrens <duke at openjdk.org> wrote:

>> We could, but benchmarks show no performance improvements over the current PR. I think the current code is perhaps slightly more readable.
>
> Does non-short-circuit logical AND operator perform similar to baseline?  In this very specific case there is no risk of side effects due to method calls, no risk of null pointer exception because of primitives, and there is not an excessive amount of conditions.  Therefore, I would think either `&` or `&&` are viable options.
> 
> `if (ch >= 0 & ch <= 0xFF) {`
> 
> I ask just for the sake of my own learning on this topic as it relates to performance as opposed to code readability preferences.

Both choices of operators en up with identical generated code on my Intel Mac.

For reference, I did a comparison between the generated code for the current code, and the one in this PR:

Current generated code start like this:


movl		%eax, -0x14000(%rsp)
pushq		%rbp
subq		$0x20, %rsp
cmpl		$1, 0x20(%r15)
jne		0x119cae072
movl		%esi, %r11d
shrl		$8, %r11d
testl		%r11d, %r11d
jne		0x119cae043
movabsq		$0x70fe03c90, %rax  ;   {oop(a 'java/lang/CharacterDataLatin1'{0x000000070fe03c90})}
``` 

While the PR generated code starts like this:


movl		%eax, -0x14000(%rsp)
pushq		%rbp
subq		$0x20, %rsp
cmpl		$1, 0x20(%r15)
jne		0x1171d41f2
nopw		(%rax, %rax)
cmpl		$0x100, %esi
jae		0x1171d41c5
movabsq		$0x70fe03c90, %rax  ;   {oop(a 'java/lang/CharacterDataLatin1'{0x000000070fe03c90})}


The rest of the code seems identical.

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

PR: https://git.openjdk.org/jdk/pull/13040