Optimizing Errors Away

Escape Analysis

• -XX:+DoEscapeAnalysis (enabled by default)
• Detects NoEscape, ArgEscape or GlobalEscape state for locals
• -XX:+EliminateAllocations,
  -XX:+OptimizePtrCompare, -XX:+EliminateLocks, ...
• Heavily depends on inlining (especially EliminateAllocations)!
• Idea: J. Shoi, M. Gupta, M. Seffano, V. Sreedhar, S. Midkiff,
  "Escape Analysis for Java"
• Implementation: opt/escape.{cpp,hpp} and ci/bcEscapeAnaylzer.{cpp,hpp}

Scalar Replacement

Intrinsics

“ ..an intrinsic function (a.k.a. builtin function) is a function available for use in a given programming language whose implementation is handled specially by the compiler. Typically, it substitutes a sequence of automatically generated instructions for the original function call.. ”

“ ..the compiler has an intimate knowledge of the intrinsic function and can therefore better integrate it and optimize it for the situation.. ”

HotSpot Intrinsics

• Only for system classes!
• Must be annotated with jdk.internal.HotSpotIntrinsicCandidate (since Java 9)
  • Checked by -XX:+CheckIntrinsics
• See: src/share/vm/classfile/vmSymbols.hpp for a full list
• Library intrinsics (~260)
  • Library methods replaced by assembly, IR or both
• Bytecode intrinsics (~40)
  • Late/always inlining candidates (e.g. StringBuffer::*, boxing/unboxing methods)
  • See: Compile::should_delay_string_inlining() in doCall.cpp
  • See: InlineTree::should_inline() in bytecodeInfo.cpp

System.arraycopy()

              
klass()->copy_array(s, src_p, d, dst_p, length);
}
]]>
              
            

              
$ java -XX:DisableIntrinsic=_arraycopy \
  org.simonis.ScalarReplacement arrayCopy1

...                            // arg0 already contains src
00c   	movq    RCX, RDX       // arg2 = dst
00f   	movl    R9, #8         // arg4 = 8
015   	xorl    RDX, RDX       // arg1 = 0
017   	xorl    R8, R8         // arg3 = 0
01b   	call,static  java.lang.System::arraycopy
...




.
              
            

              
$ java \
  org.simonis.ScalarReplacement arrayCopy1
...
movl   R10, [RSI + #16 (8-bit)]  // R10 = src.length
movl   R8, [RDX + #16 (8-bit)]   // R8  = dst.length
cmpl   R10, #8                   // if src.length < 8 throw
jb,us  _throw_exception
cmpl   R8, #8                    // if dst.length < 8 throw
jb,us  _throw_exception
movl   R10, [RSI + #24 (8-bit)]  // R10 = src[0]
movl   [RDX + #24 (8-bit)], R10  // dst[0] = R10
...
movl   R11, [RSI + #52 (8-bit)]  // R11 = src[7]
movl   [RDX + #52 (8-bit)], R11  // dst[7] = R11
...
              
            

System.arraycopy() - Specification

see http://docs.oracle.com/javase/8/docs/api/java/lang/System.html#arraycopy

System.arraycopy() - Test (C2)

Violating the Specification (C2)

              
$ java \
  org.simonis.ScalarReplacement arrayCopy4
Error in iteration 5376
Expected IndexOutOfBoundsException

$ java \
  -XX:+PrintEscapeAnalysis -XX:+PrintEliminateAllocations \
  org.simonis.ScalarReplacement arrayCopy4
JavaObject NoEscape 43 AllocateArray arrayCopy4@bci:4
Eliminated: 43 AllocateArray
Error in iteration 5376
Expected IndexOutOfBoundsException

$ java -XX:DisableIntrinsic=_arraycopy \
  org.simonis.ScalarReplacement arrayCopy4
$ java -XX:-EliminateAllocations \
  org.simonis.ScalarReplacement arrayCopy4
$ 
              
            

src/share/vm/opto/library_call.cpp

src/share/vm/opto/macro.cpp

              
is_ArrayCopy()) {
    // Disconnect ArrayCopy node
    ArrayCopyNode* ac = use->as_ArrayCopy();
    ...
    // Disconnect src: it can help find new opportunities
    Node* src = ac->in(ArrayCopyNode::Src);
]]>
              
            

src/share/vm/opto/macroArrayCopy.cpp

System.arraycopy() - Specification

see http://docs.oracle.com/javase/8/docs/api/java/lang/System.html#arraycopy

System.arraycopy() - Test (C1)

Violating the Specification (C1)

              
$ java \
  org.simonis.ScalarReplacement arrayCopy5                 
Error in iteration 256
Expected ArrayStoreException

$ java -Xint \
  org.simonis.ScalarReplacement arrayCopy5                 

$ java -XX:-TieredCompilation \
  org.simonis.ScalarReplacement arrayCopy5

$ 
              
            

Violating the Specification (C1)

src/cpu/x86/vm/c1_LIRAssembler_x86.cpp

              
entry());
    __ jcc(Assembler::zero, *stub->continuation()); // <---
  }
  // check if array element types are compatible
  if (flags & LIR_OpArrayCopy::type_check) {
  ...
  address entry = StubRoutines::select_arraycopy_function()
  __ call_VM_leaf(entry, 0);
  __ bind(*stub->continuation());                   // <---
]]>