8276926: Use String.valueOf() when initializing File.separator and File.pathSeparator

[stsypanov]

Looking into File.pathSeparator I've found out that currently we initialize it as

public static final String separator = "" + separatorChar;

Which after compilation turns into

NEW java/lang/StringBuilder
DUP
INVOKESPECIAL java/lang/StringBuilder.<init> ()V
LDC ""
INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
GETSTATIC java/io/File.pathSeparatorChar : C
INVOKEVIRTUAL java/lang/StringBuilder.append (C)Ljava/lang/StringBuilder;
INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
PUTSTATIC java/io/File.pathSeparator : Ljava/lang/String;

This can be simplified to

public static final String separator = String.valueOf(separatorChar);

Which is in turn complied into more compact

GETSTATIC java/io/File.pathSeparatorChar : C
INVOKESTATIC java/lang/String.valueOf (C)Ljava/lang/String;
PUTSTATIC java/io/File.pathSeparator : Ljava/lang/String;

The changed code is likely to slightly improve start-up time as it allocates less and does no bound checks.

---

Progress

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Issue

• JDK-8276926: Use String.valueOf() when initializing File.separator and File.pathSeparator

Reviewers

• Claes Redestad (@cl4es - Reviewer)
• Jim Laskey (@JimLaskey - Reviewer)

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[mlbridge]

Webrevs

• 00: Full (7b5bab38)

[mbien]

it should compile into invokedynamic makeConcatWithConstants on later JDKs, but valueOf(char) is most likely going to have smaller first-invocation overhead still.

[stsypanov]

it should compile into invokedynamic makeConcatWithConstants on later JDKs

As far as I understand StringConcatFactory is not available on bootstrap stage and StringBuilder is used instead. See #3464 (comment) and #4507 (comment) (pay attention to class loading order - j.i.File is loaded before StringConcatFactory

[cl4es]

FWIW the specific pattern of "" + foo could probably profitably translate into String.valueOf(foo) regardless of StringConcatFactory availability. (And translating String concat to use SCF in class initializers is likely a pessimization most of the time.) I'm no expert on javac but I suspect implementing (and worse: specifying) minor improvements to the translation might be more effort than it's worth, though, so point fixes like these seem fine to me.

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8276926: Use String.valueOf() when initializing File.separator and File.pathSeparator

Reviewed-by: redestad, jlaskey

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At the time when this comment was updated there had been 3 new commits pushed to the master branch:

• a0b8445: 8276846: JDK-8273416 is incomplete for UseSSE=1
• a3f710e: 8276215: Intrinsics matchers should handle native method flags better
• 0f463a7: 8276845: (fs) java/nio/file/spi/SetDefaultProvider.java fails on x86_32

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[stsypanov]

/integrate

[openjdk]

@stsypanov
Your change (at version 7b5bab3) is now ready to be sponsored by a Committer.

[cl4es]

/sponsor

[openjdk]

Going to push as commit 0f23c6a.
Since your change was applied there have been 3 commits pushed to the master branch:

• a0b8445: 8276846: JDK-8273416 is incomplete for UseSSE=1
• a3f710e: 8276215: Intrinsics matchers should handle native method flags better
• 0f463a7: 8276845: (fs) java/nio/file/spi/SetDefaultProvider.java fails on x86_32

Your commit was automatically rebased without conflicts.

[openjdk]

@cl4es @stsypanov Pushed as commit 0f23c6a.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

[JimLaskey]

It's unlikely that javac would do this kind of optimization (javac tends to produce code verbatim), but it might be worth filing a RFE.

[cl4es]

I've filed https://bugs.openjdk.java.net/browse/JDK-8276951

This pattern is quite common - both in the OpenJDK and elsewhere.