CODEWITHSUNDEEP
javajvmcode-generation
Cherry Vanc
Cherry Vanc

Reputation: 831

Does JVM have machine specific optimization flags?

I understand that a JVM interprets / JITs the bytecodes; and that JVM provides the platform specific machine code generation functionality (and runtime) for running the program, in the current context.

I would like to understand what platform specific code generation tuning options are available in JVM. I ran into https://docs.oracle.com/cd/E21764_01/web.1111/e13814/jvm_tuning.htm#PERFM167, which are actually only flags to tune the JVM runtime.

I am interested in knowing if JVM allows for tuning options for code generation for a specific platform, much like gcc on x86 https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html#x86-Options.

Upvotes: 0

Views: 914

Answers (1)

Stephen C
Stephen C

Reputation: 718708

First of all, JVM flags are fundamentally different to GCC's optimization flags. The GCC flags affect the code generated at compile time. The JVM options (to the extent that they affect JIT compilation at all) do NOT affect the bytecode.

There are various flags that affect the native code emitted by the JIT compiler; for example:

  • The -ea options determine whether assertions are checked.
  • The -server option (in part) controls when the code is JIT compiler which affects its optimization. The -Xcomp option does the same.
  • The -Xint and -Xmixed affect whether code is JIT compiled at all.

Then there are options that specifically change specific optimization-related things. These include -XX:ObjectAlignmentInBytes, -XX:CompressedOops, -XX:AggressiveOpts, the -XX:AllocatePrefetch options, -XX:DoEscapeAnalysis, the -XX:Inline options, -XX:OptimizeStringConcat, the -XX:UseAES options and the -XX:UseSHA options.

For a more complete listing, refer to the Oracle documentation for your platform.

Note that:

  • All JIT code optimization is platform specific in the sense that the JIT compiler optimizes for the hardware that you are actually running on.
  • Conversely, the JIT compiler doesn't need to be told which platform to optimize for.
  • Some of the above options are platform specific / have platform specific effects (see documentation)
  • The -X and -XX options may change between Java versions.

Java programmers typically let the platform select the optimization settings. Java programs are less likely to break due to choice of the optimization settings than (say) C and C++ programs 1.

1 - For a single-threaded Java code, optimization settings should not alter the behavior of a program. For a multi-threaded code, the behavior of an incorrect application could change due to inadequate synchronization. While this might be sensitive to optimizer settings, it could also be influenced by hardware characteristics, Java version and so on.

Upvotes: 3

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