8355635: Do not collect C strings in C2 scratch buffer

[vnkozlov]

JDK-8349479 added call to code_string() for Halt node mach node.
I am observing several more creation and clearing C strings collections during C2 compilation:
[17.405s][debug][codestrings] Clear 2 asm-remarks.
[17.405s][debug][codestrings] Clear 1 dbg-string.

Most are coming from temporary scratch buffer C2 uses for code size calculation. I suggest to not collect strings in this buffer.

Note, CodeSection::set_scratch_emit() is only called from PhaseOutput::scratch_emit_size() for scratch buffer.

I verified with -XX:CompileCommand=print,<classpath>::<method> that hsdis output is the same.

Running on linux-x64 with fastdebug VM:

before:
$  java -XX:-TieredCompilation -Xlog:codestrings=debug com.sun.tools.javac.Main HelloWorld.java | grep codestrings |wc
   1644    6576   80618

after again:
$  java -XX:-TieredCompilation -Xlog:codestrings=debug com.sun.tools.javac.Main HelloWorld.java | grep codestrings |wc
      0       0       0

It is more dramatic with -Xcomp we use for testing:

Before
$  java -XX:-TieredCompilation -Xcomp -Xlog:codestrings=debug com.sun.tools.javac.Main HelloWorld.java | grep codestrings |wc
  70924  283696 3533261

After fix
$  java -XX:-TieredCompilation -Xcomp -Xlog:codestrings=debug com.sun.tools.javac.Main HelloWorld.java | grep codestrings |wc
      0       0       0

I was curious why it is 0 - we do deoptimize nmethod. But with big default CodeCache GC does not collect them.
Reducing CodeCache to 8Mb shows deallocation:

$ java -XX:-TieredCompilation -Xcomp -Xlog:codestrings=debug -Xlog:codecache=debug -XX:+PrintCodeCache -XX:ReservedCodeCacheSize=8M com.sun.tools.javac.Main HelloWorld.java
...
[40.196s][debug][codestrings] Clear 42 asm-remarks.
[40.196s][debug][codestrings] Clear 1 dbg-string.
[40.196s][debug][codecache  ] *flushing  nmethod 5811/0x00007f71d74b7188. Live blobs:3370/Free CodeCache:2953Kb

Tested tier1-5, Xcomp,comp-stress.

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Issue

• JDK-8355635: Do not collect C strings in C2 scratch buffer (Enhancement - P4)

Reviewers

• John R Rose (@rose00 - Reviewer)
• Tobias Hartmann (@TobiHartmann - Reviewer)
• Aleksey Shipilev (@shipilev - Reviewer)

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8355635: Do not collect C strings in C2 scratch buffer

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At the time when this comment was updated there had been 13 new commits pushed to the master branch:

• 3140de4: 8345125: Aarch64: Add aarch64 backend for Float16 scalar operations
• a05ff55: 8355657: RISC-V: Improve PrintOptoAssembly output of vector-scalar instructions
• 7df1bfe: 8354811: clock_tics_per_sec code duplication between os_linux and os_posix
• ... and 10 more: https://git.openjdk.org/jdk/compare/e57fd710496b2ac8aa93eb3d4ff2234170fa2e37...master

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[rose00]

Yeah, the zero was puzzling to me too; looked like evidence that we killed all the strings, not just the ones in the scratch assembly. But your extra demonstration shows that we collect the strings we want for the non-scratch assemblies.

Good change.

[vnkozlov]

Thank you, @rose00

[TobiHartmann]

Good catch! The fix looks good to me.

[vnkozlov]

/integrate

[openjdk]

Going to push as commit 3eaec04.
Since your change was applied there have been 17 commits pushed to the master branch:

• a7359a5: 8354453: Don't strcpy in os::strdup, use memcpy instead
• b0c3485: 8348282: Add option for syntax highlighting in javadoc snippets
• 2447b98: 8355498: [AIX] Adapt code for C++ VLA rule
• ... and 14 more: https://git.openjdk.org/jdk/compare/e57fd710496b2ac8aa93eb3d4ff2234170fa2e37...master

Your commit was automatically rebased without conflicts.

[openjdk]

@vnkozlov Pushed as commit 3eaec04.

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