RFR: 8341415: Optimize RawBytecodeHelper::next

[Shaojin Wen]

On Wed, 2 Oct 2024 13:28:24 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> src/java.base/share/classes/jdk/internal/classfile/impl/RawBytecodeHelper.java line 350:
>> 
>>> 348:      */
>>> 349:     public boolean isWide() {
>>> 350:         return (opcode & (WIDE << 8)) != 0;
>> 
>> Suggestion:
>> 
>>         return (opcode >>> 8) == WIDE;
>
> WIDE is a constant, WIDE << 8 will do constant folding, your suggestion will be longer in bytecode.

public isWide()Z
   L0
    LINENUMBER 8 L0
    ALOAD 0
    GETFIELD internal/classfile/impl/RawBytecodeHelper.opcode : I
    LDC 50176
    IAND
    IFEQ L1
    ICONST_1
    GOTO L2
   L1
   FRAME SAME
    ICONST_0
   L2
   FRAME SAME1 I
    IRETURN
   L3
    LOCALVARIABLE this Linternal/classfile/impl/RawBytecodeHelper; L0 L3 0
    MAXSTACK = 2
    MAXLOCALS = 1
}

public isWide1()Z
   L0
    LINENUMBER 12 L0
    ALOAD 0
    GETFIELD internal/classfile/impl/RawBytecodeHelper.opcode : I
    BIPUSH 8
    IUSHR
    SIPUSH 196
    IF_ICMPNE L1
    ICONST_1
    GOTO L2
   L1
   FRAME SAME
    ICONST_0
   L2
   FRAME SAME1 I
    IRETURN
   L3
    LOCALVARIABLE this Linternal/classfile/impl/RawBytecodeHelper; L0 L3 0
    MAXSTACK = 2
    MAXLOCALS = 1
}



The above are the bytecodes of the two implementations. It can be seen that the original implementation is more concise.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21300#discussion_r1784575918