changeset 55839:d7304cf430f1

8227680: FastJNIAccessors: Check for JVMTI field access event requests at runtime Summary: Check JvmtiExport::_field_access_count != 0 at runtime Reviewed-by: dholmes, eosterlund, bulasevich
author mdoerr
date Mon, 29 Jul 2019 18:22:55 +0200
parents be47f3ccdf12
children 9d82a35b6ff7
files src/hotspot/cpu/aarch64/jniFastGetField_aarch64.cpp src/hotspot/cpu/arm/jniFastGetField_arm.cpp src/hotspot/cpu/sparc/jniFastGetField_sparc.cpp src/hotspot/cpu/x86/jniFastGetField_x86_32.cpp src/hotspot/cpu/x86/jniFastGetField_x86_64.cpp src/hotspot/share/prims/jni.cpp test/hotspot/jtreg/runtime/jni/FastGetField/FastGetField.java test/hotspot/jtreg/runtime/jni/FastGetField/libFastGetField.c
diffstat 8 files changed, 401 insertions(+), 41 deletions(-) [+]
line wrap: on
line diff
--- a/src/hotspot/cpu/aarch64/jniFastGetField_aarch64.cpp	Mon Jul 29 09:09:23 2019 -0700
+++ b/src/hotspot/cpu/aarch64/jniFastGetField_aarch64.cpp	Mon Jul 29 18:22:55 2019 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2004, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2004, 2019, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2014, Red Hat Inc. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
@@ -79,33 +79,57 @@
   Address safepoint_counter_addr(rcounter_addr, offset);
   __ ldrw(rcounter, safepoint_counter_addr);
   __ tbnz(rcounter, 0, slow);
-  __ eor(robj, c_rarg1, rcounter);
-  __ eor(robj, robj, rcounter);               // obj, since
-                                              // robj ^ rcounter ^ rcounter == robj
-                                              // robj is address dependent on rcounter.
 
+  if (!UseBarriersForVolatile) {
+    // Field may be volatile. See other usages of this flag.
+    __ membar(MacroAssembler::AnyAny);
+    __ mov(robj, c_rarg1);
+  } else if (JvmtiExport::can_post_field_access()) {
+    // Using barrier to order wrt. JVMTI check and load of result.
+    __ membar(Assembler::LoadLoad);
+    __ mov(robj, c_rarg1);
+  } else {
+    // Using address dependency to order wrt. load of result.
+    __ eor(robj, c_rarg1, rcounter);
+    __ eor(robj, robj, rcounter);         // obj, since
+                                          // robj ^ rcounter ^ rcounter == robj
+                                          // robj is address dependent on rcounter.
+  }
+
+  if (JvmtiExport::can_post_field_access()) {
+    // Check to see if a field access watch has been set before we
+    // take the fast path.
+    unsigned long offset2;
+    __ adrp(result,
+            ExternalAddress((address) JvmtiExport::get_field_access_count_addr()),
+            offset2);
+    __ ldrw(result, Address(result, offset2));
+    __ cbnzw(result, slow);
+  }
+
+  // Both robj and rscratch1 are clobbered by try_resolve_jobject_in_native.
   BarrierSetAssembler* bs = BarrierSet::barrier_set()->barrier_set_assembler();
   bs->try_resolve_jobject_in_native(masm, c_rarg0, robj, rscratch1, slow);
 
   __ lsr(roffset, c_rarg2, 2);                // offset
+  __ add(result, robj, roffset);
 
   assert(count < LIST_CAPACITY, "LIST_CAPACITY too small");
   speculative_load_pclist[count] = __ pc();   // Used by the segfault handler
+  // Using acquire: Order JVMTI check and load of result wrt. succeeding check
+  // (LoadStore for volatile field).
   switch (type) {
-    case T_BOOLEAN: __ ldrb    (result, Address(robj, roffset)); break;
-    case T_BYTE:    __ ldrsb   (result, Address(robj, roffset)); break;
-    case T_CHAR:    __ ldrh    (result, Address(robj, roffset)); break;
-    case T_SHORT:   __ ldrsh   (result, Address(robj, roffset)); break;
-    case T_FLOAT:   __ ldrw    (result, Address(robj, roffset)); break;
-    case T_INT:     __ ldrsw   (result, Address(robj, roffset)); break;
+    case T_BOOLEAN: __ ldarb(result, result); break;
+    case T_BYTE:    __ ldarb(result, result); __ sxtb(result, result); break;
+    case T_CHAR:    __ ldarh(result, result); break;
+    case T_SHORT:   __ ldarh(result, result); __ sxth(result, result); break;
+    case T_FLOAT:   __ ldarw(result, result); break;
+    case T_INT:     __ ldarw(result, result); __ sxtw(result, result); break;
     case T_DOUBLE:
-    case T_LONG:    __ ldr     (result, Address(robj, roffset)); break;
+    case T_LONG:    __ ldar (result, result); break;
     default:        ShouldNotReachHere();
   }
 
-  // counter_addr is address dependent on result.
-  __ eor(rcounter_addr, rcounter_addr, result);
-  __ eor(rcounter_addr, rcounter_addr, result);
   __ ldrw(rscratch1, safepoint_counter_addr);
   __ cmpw(rcounter, rscratch1);
   __ br (Assembler::NE, slow);
--- a/src/hotspot/cpu/arm/jniFastGetField_arm.cpp	Mon Jul 29 09:09:23 2019 -0700
+++ b/src/hotspot/cpu/arm/jniFastGetField_arm.cpp	Mon Jul 29 18:22:55 2019 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2008, 2019, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -32,7 +32,7 @@
 
 #define __ masm->
 
-#define BUFFER_SIZE  96
+#define BUFFER_SIZE 120
 
 address JNI_FastGetField::generate_fast_get_int_field0(BasicType type) {
   const char* name = NULL;
@@ -99,10 +99,10 @@
   CodeBuffer cbuf(blob);
   MacroAssembler* masm = new MacroAssembler(&cbuf);
   fast_entry = __ pc();
+  Label slow_case;
 
   // Safepoint check
   InlinedAddress safepoint_counter_addr(SafepointSynchronize::safepoint_counter_addr());
-  Label slow_case;
   __ ldr_literal(Rsafepoint_counter_addr, safepoint_counter_addr);
 
   __ push(RegisterSet(R0, R3));  // save incoming arguments for slow case
@@ -112,9 +112,21 @@
 
   __ bic(R1, R1, JNIHandles::weak_tag_mask);
 
-  // Address dependency restricts memory access ordering. It's cheaper than explicit LoadLoad barrier
-  __ andr(Rtmp1, Rsafept_cnt, (unsigned)1);
-  __ ldr(Robj, Address(R1, Rtmp1));
+  if (JvmtiExport::can_post_field_access()) {
+    // Using barrier to order wrt. JVMTI check and load of result.
+    __ membar(MacroAssembler::Membar_mask_bits(MacroAssembler::LoadLoad), Rtmp1);
+
+    // Check to see if a field access watch has been set before we
+    // take the fast path.
+    __ ldr_global_s32(Rtmp1, (address)JvmtiExport::get_field_access_count_addr());
+    __ cbnz(Rtmp1, slow_case);
+
+    __ ldr(Robj, Address(R1));
+  } else {
+    // Address dependency restricts memory access ordering. It's cheaper than explicit LoadLoad barrier
+    __ andr(Rtmp1, Rsafept_cnt, (unsigned)1);
+    __ ldr(Robj, Address(R1, Rtmp1));
+  }
 
   Address field_addr;
   if (type != T_BOOLEAN
@@ -170,20 +182,18 @@
       ShouldNotReachHere();
   }
 
-  // Address dependency restricts memory access ordering. It's cheaper than explicit LoadLoad barrier
+  __ ldr_literal(Rsafepoint_counter_addr, safepoint_counter_addr);
 #ifdef __ABI_HARD__
   if (type == T_FLOAT || type == T_DOUBLE) {
-    __ ldr_literal(Rsafepoint_counter_addr, safepoint_counter_addr);
     __ fmrrd(Rres, Rres_hi, D0);
-    __ eor(Rtmp2, Rres, Rres);
-    __ ldr_s32(Rsafept_cnt2, Address(Rsafepoint_counter_addr, Rtmp2));
-  } else
+  }
 #endif // __ABI_HARD__
-  {
-    __ ldr_literal(Rsafepoint_counter_addr, safepoint_counter_addr);
-    __ eor(Rtmp2, Rres, Rres);
-    __ ldr_s32(Rsafept_cnt2, Address(Rsafepoint_counter_addr, Rtmp2));
-  }
+
+  // Order JVMTI check and load of result wrt. succeeding check
+  // (LoadStore for volatile field).
+  __ membar(MacroAssembler::Membar_mask_bits(MacroAssembler::LoadLoad | MacroAssembler::LoadStore), Rtmp2);
+
+  __ ldr_s32(Rsafept_cnt2, Address(Rsafepoint_counter_addr));
   __ cmp(Rsafept_cnt2, Rsafept_cnt);
   // discards saved R0 R1 R2 R3
   __ add(SP, SP, 4 * wordSize, eq);
--- a/src/hotspot/cpu/sparc/jniFastGetField_sparc.cpp	Mon Jul 29 09:09:23 2019 -0700
+++ b/src/hotspot/cpu/sparc/jniFastGetField_sparc.cpp	Mon Jul 29 18:22:55 2019 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2004, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2004, 2019, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -70,6 +70,15 @@
   __ andcc (G4, 1, G0);
   __ br (Assembler::notZero, false, Assembler::pn, label1);
   __ delayed()->srl (O2, 2, O4);
+
+  if (JvmtiExport::can_post_field_access()) {
+    // Check to see if a field access watch has been set before we
+    // take the fast path.
+    AddressLiteral get_field_access_count_addr(JvmtiExport::get_field_access_count_addr());
+    __ load_contents(get_field_access_count_addr, O5);
+    __ cmp_and_br_short(O5, 0, Assembler::notEqual, Assembler::pn, label1);
+  }
+
   __ mov(O1, O5);
 
   // Both O5 and G3 are clobbered by try_resolve_jobject_in_native.
@@ -153,6 +162,15 @@
   __ andcc (G4, 1, G0);
   __ br (Assembler::notZero, false, Assembler::pn, label1);
   __ delayed()->srl (O2, 2, O4);
+
+  if (JvmtiExport::can_post_field_access()) {
+    // Check to see if a field access watch has been set before we
+    // take the fast path.
+    AddressLiteral get_field_access_count_addr(JvmtiExport::get_field_access_count_addr());
+    __ load_contents(get_field_access_count_addr, O5);
+    __ cmp_and_br_short(O5, 0, Assembler::notEqual, Assembler::pn, label1);
+  }
+
   __ mov(O1, O5);
 
   // Both O5 and G1 are clobbered by try_resolve_jobject_in_native.
@@ -211,6 +229,15 @@
   __ andcc (G4, 1, G0);
   __ br (Assembler::notZero, false, Assembler::pn, label1);
   __ delayed()->srl (O2, 2, O4);
+
+  if (JvmtiExport::can_post_field_access()) {
+    // Check to see if a field access watch has been set before we
+    // take the fast path.
+    AddressLiteral get_field_access_count_addr(JvmtiExport::get_field_access_count_addr());
+    __ load_contents(get_field_access_count_addr, O5);
+    __ cmp_and_br_short(O5, 0, Assembler::notEqual, Assembler::pn, label1);
+  }
+
   __ mov(O1, O5);
 
   // Both O5 and G3 are clobbered by try_resolve_jobject_in_native.
--- a/src/hotspot/cpu/x86/jniFastGetField_x86_32.cpp	Mon Jul 29 09:09:23 2019 -0700
+++ b/src/hotspot/cpu/x86/jniFastGetField_x86_32.cpp	Mon Jul 29 18:22:55 2019 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2004, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2004, 2019, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -75,6 +75,14 @@
   __ mov32 (rcx, counter);
   __ testb (rcx, 1);
   __ jcc (Assembler::notZero, slow);
+
+  if (JvmtiExport::can_post_field_access()) {
+    // Check to see if a field access watch has been set before we
+    // take the fast path.
+    __ cmp32(ExternalAddress((address) JvmtiExport::get_field_access_count_addr()), 0);
+    __ jcc(Assembler::notZero, slow);
+  }
+
   __ mov(rax, rcx);
   __ andptr(rax, 1);                         // rax, must end up 0
   __ movptr(rdx, Address(rsp, rax, Address::times_1, 2*wordSize));
@@ -188,6 +196,14 @@
   __ mov32 (rcx, counter);
   __ testb (rcx, 1);
   __ jcc (Assembler::notZero, slow);
+
+  if (JvmtiExport::can_post_field_access()) {
+    // Check to see if a field access watch has been set before we
+    // take the fast path.
+    __ cmp32(ExternalAddress((address) JvmtiExport::get_field_access_count_addr()), 0);
+    __ jcc(Assembler::notZero, slow);
+  }
+
   __ mov(rax, rcx);
   __ andptr(rax, 1);                         // rax, must end up 0
   __ movptr(rdx, Address(rsp, rax, Address::times_1, 3*wordSize));
@@ -272,6 +288,14 @@
   __ mov32 (rcx, counter);
   __ testb (rcx, 1);
   __ jcc (Assembler::notZero, slow);
+
+  if (JvmtiExport::can_post_field_access()) {
+    // Check to see if a field access watch has been set before we
+    // take the fast path.
+    __ cmp32(ExternalAddress((address) JvmtiExport::get_field_access_count_addr()), 0);
+    __ jcc(Assembler::notZero, slow);
+  }
+
   __ mov(rax, rcx);
   __ andptr(rax, 1);                         // rax, must end up 0
   __ movptr(rdx, Address(rsp, rax, Address::times_1, 2*wordSize));
--- a/src/hotspot/cpu/x86/jniFastGetField_x86_64.cpp	Mon Jul 29 09:09:23 2019 -0700
+++ b/src/hotspot/cpu/x86/jniFastGetField_x86_64.cpp	Mon Jul 29 18:22:55 2019 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2004, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2004, 2019, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -41,11 +41,10 @@
 // c_rarg1:    obj
 // c_rarg2:    jfield id
 
-static const Register rtmp          = r8;
-static const Register robj          = r9;
-static const Register rcounter      = r10;
-static const Register roffset       = r11;
-static const Register rcounter_addr = r11;
+static const Register rtmp     = rax; // r8 == c_rarg2 on Windows
+static const Register robj     = r9;
+static const Register roffset  = r10;
+static const Register rcounter = r11;
 
 // Warning: do not use rip relative addressing after the first counter load
 // since that may scratch r10!
@@ -74,6 +73,15 @@
   __ mov   (robj, c_rarg1);
   __ testb (rcounter, 1);
   __ jcc (Assembler::notZero, slow);
+
+  if (JvmtiExport::can_post_field_access()) {
+    // Check to see if a field access watch has been set before we
+    // take the fast path.
+    assert_different_registers(rscratch1, robj, rcounter); // cmp32 clobbers rscratch1!
+    __ cmp32(ExternalAddress((address) JvmtiExport::get_field_access_count_addr()), 0);
+    __ jcc(Assembler::notZero, slow);
+  }
+
   __ mov   (roffset, c_rarg2);
   __ shrptr(roffset, 2);                         // offset
 
@@ -164,6 +172,13 @@
   __ testb (rcounter, 1);
   __ jcc (Assembler::notZero, slow);
 
+  if (JvmtiExport::can_post_field_access()) {
+    // Check to see if a field access watch has been set before we
+    // take the fast path.
+    __ cmp32(ExternalAddress((address) JvmtiExport::get_field_access_count_addr()), 0);
+    __ jcc(Assembler::notZero, slow);
+  }
+
   // Both robj and rtmp are clobbered by try_resolve_jobject_in_native.
   BarrierSetAssembler* bs = BarrierSet::barrier_set()->barrier_set_assembler();
   bs->try_resolve_jobject_in_native(masm, /* jni_env */ c_rarg0, robj, rtmp, slow);
--- a/src/hotspot/share/prims/jni.cpp	Mon Jul 29 09:09:23 2019 -0700
+++ b/src/hotspot/share/prims/jni.cpp	Mon Jul 29 18:22:55 2019 +0200
@@ -3776,8 +3776,7 @@
 
 void quicken_jni_functions() {
   // Replace Get<Primitive>Field with fast versions
-  if (UseFastJNIAccessors && !JvmtiExport::can_post_field_access()
-      && !VerifyJNIFields && !CountJNICalls && !CheckJNICalls) {
+  if (UseFastJNIAccessors && !VerifyJNIFields && !CountJNICalls && !CheckJNICalls) {
     address func;
     func = JNI_FastGetField::generate_fast_get_boolean_field();
     if (func != (address)-1) {
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/jtreg/runtime/jni/FastGetField/FastGetField.java	Mon Jul 29 18:22:55 2019 +0200
@@ -0,0 +1,124 @@
+/*
+ * Copyright (c) 2019 SAP SE and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/**
+ * @test
+ * @bug 8227680
+ * @summary Tests that all FieldAccess notifications for Get*Field
+            with primitive type are generated.
+ * @compile FastGetField.java
+ * @run main/othervm/native -agentlib:FastGetField -XX:+IgnoreUnrecognizedVMOptions -XX:-VerifyJNIFields FastGetField
+ * @run main/othervm/native -agentlib:FastGetField -XX:+IgnoreUnrecognizedVMOptions -XX:-VerifyJNIFields -XX:+UnlockDiagnosticVMOptions -XX:+ForceUnreachable -XX:+SafepointALot -XX:GuaranteedSafepointInterval=1 FastGetField
+ */
+
+import java.lang.reflect.Field;
+
+
+public class FastGetField {
+
+    private static final String agentLib = "FastGetField";
+
+    private native boolean initFieldIDs(Class c);
+    private native boolean initWatchers(Class c);
+    public native long accessFields(MyItem i);
+    public static native long getFieldAccessCount();
+
+    static final int loop_cnt = 10000;
+
+
+    class MyItem {
+        // Names should match JNI types.
+        boolean Z;
+        byte B;
+        short S;
+        char C;
+        int I;
+        long J;
+        float F;
+        double D;
+
+        public void change_values() {
+            Z = true;
+            B = 1;
+            C = 1;
+            S = 1;
+            I = 1;
+            J = 1l;
+            F = 1.0f;
+            D = 1.0;
+        }
+
+        public void reset_values() {
+            Z = false;
+            B = 0;
+            C = 0;
+            S = 0;
+            I = 0;
+            J = 0l;
+            F = 0.0f;
+            D = 0.0;
+        }
+    }
+
+    // Static initialization.
+    static {
+        try {
+            System.loadLibrary(agentLib);
+        } catch (UnsatisfiedLinkError ex) {
+            System.err.println("Failed to load " + agentLib + " lib");
+            System.err.println("java.library.path: " + System.getProperty("java.library.path"));
+            throw ex;
+        }
+    }
+
+    public void TestFieldAccess() throws Exception {
+        MyItem i = new MyItem();
+        if (!initFieldIDs(MyItem.class)) throw new RuntimeException("FieldID initialization failed!");
+
+        long duration = System.nanoTime();
+        for (int c = 0; c < loop_cnt; ++c) {
+            if (accessFields(i) != 0l) throw new RuntimeException("Wrong initial result!");
+            i.change_values();
+            if (accessFields(i) != 8l) throw new RuntimeException("Wrong result after changing!");
+            i.reset_values();
+        }
+        duration = System.nanoTime() - duration;
+        System.out.println(loop_cnt + " iterations took " + duration + "ns.");
+
+        if (getFieldAccessCount() != 0) throw new RuntimeException("Watch not yet active!");
+
+        // Install watchers.
+        if (!initWatchers(MyItem.class)) throw new RuntimeException("JVMTI missing!");
+
+        // Try again with watchers.
+        if (accessFields(i) != 0l) throw new RuntimeException("Wrong initial result!");
+        i.change_values();
+        if (accessFields(i) != 8l) throw new RuntimeException("Wrong result after changing!");
+        if (getFieldAccessCount() != 16) throw new RuntimeException("Unexpected event count!");
+    }
+
+    public static void main(String[] args) throws Exception {
+        FastGetField inst = new FastGetField();
+        inst.TestFieldAccess();
+    }
+}
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/jtreg/runtime/jni/FastGetField/libFastGetField.c	Mon Jul 29 18:22:55 2019 +0200
@@ -0,0 +1,137 @@
+/*
+ * Copyright (c) 2019 SAP SE and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+#include <stdio.h>
+#include <string.h>
+
+#include "jvmti.h"
+
+static jvmtiEnv *jvmti = NULL;
+
+static const char* fields[] = { "Z", "B", "C", "S", "I", "J", "F", "D" };
+#define NUM_FIELDS (sizeof fields / sizeof fields[0])
+static jfieldID fieldIDs[NUM_FIELDS];
+static jlong fieldAccessCount = 0;
+
+
+JNIEXPORT jboolean JNICALL Java_FastGetField_initFieldIDs(JNIEnv *env, jobject this, jclass c) {
+  for (int i = 0; i < (int)NUM_FIELDS; ++i) {
+    fieldIDs[i] = (*env)->GetFieldID(env, c, fields[i], fields[i]);
+    if (fieldIDs[i] == NULL) {
+      printf("field %d not found\n", i);
+      return JNI_FALSE;
+    }
+  }
+  return JNI_TRUE;
+}
+
+
+JNIEXPORT jboolean JNICALL Java_FastGetField_initWatchers(JNIEnv *env, jobject this, jclass c) {
+  if (jvmti == NULL) {
+    printf("jvmti is NULL\n");
+    return JNI_FALSE;
+  }
+
+  for (int i = 0; i < (int)NUM_FIELDS; ++i) {
+    jvmtiError err = (*jvmti)->SetFieldAccessWatch(jvmti, c, fieldIDs[i]);
+    if (err != JVMTI_ERROR_NONE) {
+      printf("SetFieldAccessWatch failed with error %d\n", err);
+      return JNI_FALSE;
+    }
+  }
+
+  return JNI_TRUE;
+}
+
+
+JNIEXPORT jlong JNICALL Java_FastGetField_accessFields(JNIEnv *env, jobject this, jobject obj) {
+  return
+      (*env)->GetBooleanField(env, obj, fieldIDs[0]) +
+      (*env)->GetByteField(env, obj, fieldIDs[1]) +
+      (*env)->GetCharField(env, obj, fieldIDs[2]) +
+      (*env)->GetShortField(env, obj, fieldIDs[3]) +
+      (*env)->GetIntField(env, obj, fieldIDs[4]) +
+      (*env)->GetLongField(env, obj, fieldIDs[5]) +
+      (jlong)((*env)->GetFloatField(env, obj, fieldIDs[6])) +
+      (jlong)((*env)->GetDoubleField(env, obj, fieldIDs[7]));
+}
+
+
+JNIEXPORT jlong JNICALL Java_FastGetField_getFieldAccessCount(JNIEnv *env, jclass c) {
+  return fieldAccessCount;
+}
+
+
+static void JNICALL onFieldAccess(jvmtiEnv *jvmti_env, JNIEnv* jni_env, jthread thread,
+                                  jmethodID method, jlocation location, jclass field_klass,
+                                  jobject object, jfieldID field) {
+  char *fname = NULL, *mname = NULL;
+
+  jvmtiError err = (*jvmti)->GetFieldName(jvmti, field_klass, field, &fname, NULL, NULL);
+  if (err != JVMTI_ERROR_NONE) {
+    printf("GetFieldName failed with error %d\n", err);
+    return;
+  }
+
+  err = (*jvmti)->GetMethodName(jvmti, method, &mname, NULL, NULL);
+  if (err != JVMTI_ERROR_NONE) {
+    printf("GetMethodName failed with error %d\n", err);
+    return;
+  }
+
+  printf("%s accessed field %s\n", mname, fname);
+
+  err = (*jvmti)->Deallocate(jvmti, (unsigned char*)fname);
+  if (err != JVMTI_ERROR_NONE) {
+    printf("Deallocate failed with error %d\n", err);
+    return;
+  }
+
+  err = (*jvmti)->Deallocate(jvmti, (unsigned char*)mname);
+  if (err != JVMTI_ERROR_NONE) {
+    printf("Deallocate failed with error %d\n", err);
+    return;
+  }
+
+  fieldAccessCount++;
+}
+
+
+JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM* vm, char* options, void* reserved) {
+  jvmtiCapabilities capa;
+  jvmtiEventCallbacks cbs = {0};
+
+  (*vm)->GetEnv(vm, (void**)&jvmti, JVMTI_VERSION_1_0);
+
+  memset(&capa, 0, sizeof(capa));
+  capa.can_generate_field_access_events = 1;
+  (*jvmti)->AddCapabilities(jvmti, &capa);
+
+  cbs.FieldAccess = &onFieldAccess;
+  (*jvmti)->SetEventCallbacks(jvmti, &cbs, sizeof(cbs));
+  (*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE, JVMTI_EVENT_FIELD_ACCESS, NULL);
+  printf("Loaded agent\n");
+  fflush(stdout);
+
+  return 0;
+}