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8326692: JVMCI Local.endBci is off-by-one #18087

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8326692: JVMCI Local.endBci is off-by-one #18087

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94c15b8
Add javadoc and minimal test for Local.getStart/EndBCI
gilles-duboscq Mar 1, 2024
90e96b4
Fix JVMCI Local endBCI off-by-one error
gilles-duboscq Mar 1, 2024
fe1ee47
Add note about zero-length locals
gilles-duboscq Mar 2, 2024
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4 changes: 2 additions & 2 deletions src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/HotSpotResolvedJavaMethodImpl.java
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/*
* Copyright (c) 2011, 2023, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2011, 2024, 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
Expand Down Expand Up @@ -632,7 +632,7 @@ public LocalVariableTable getLocalVariableTable() {

for (int i = 0; i < localVariableTableLength; i++) {
final int startBci = UNSAFE.getChar(localVariableTableElement + config.localVariableTableElementStartBciOffset);
final int endBci = startBci + UNSAFE.getChar(localVariableTableElement + config.localVariableTableElementLengthOffset);
final int endBci = startBci + UNSAFE.getChar(localVariableTableElement + config.localVariableTableElementLengthOffset) - 1;
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Just a question: Can the length of a local variable be 0?

If the code length is 0, the endBci here may be less than startBci.

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@dougxc dougxc Mar 2, 2024 •
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I don't see anything in JVMS 4.7.13 that says it cannot be 0. It basically means the LVT entry is useless (denotes a local that is never alive) but is otherwise harmless.
Maybe add this to the javadoc for getEndBci() to make the API user aware of this corner case:

If the value returned is less than {@link #getStartBCI}, this object denotes a local that is never live.

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The reason, which causes this problem, is that the Local::endBci includes itself instead of excluding it. But now, we can only fix the javadoc just as you suggested.

If the value returned is less than {@link #getStartBCI}, this object denotes a local that is never live.

a local variable may be better to a local above.

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I had checked the specs on that and came to the same conclusion.
I also think the current state is fine in that regards in terms of code since it just means that there is no bci where this local would be valid when checking both start and end bci.
Adding a note about that to the javadoc is a good idea. I'll do that.

final int nameCpIndex = UNSAFE.getChar(localVariableTableElement + config.localVariableTableElementNameCpIndexOffset);
final int typeCpIndex = UNSAFE.getChar(localVariableTableElement + config.localVariableTableElementDescriptorCpIndexOffset);
final int slot = UNSAFE.getChar(localVariableTableElement + config.localVariableTableElementSlotOffset);
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11 changes: 10 additions & 1 deletion src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/meta/Local.java
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/*
* Copyright (c) 2011, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2011, 2024, 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
Expand Down Expand Up @@ -41,10 +41,19 @@ public Local(String name, JavaType type, int startBci, int endBci, int slot) {
this.type = type;
}

/**
* Returns the first BCI at which this local has a value (inclusive).
*/
public int getStartBCI() {
return startBci;
}


/**
* Returns the last BCI at which this local has a value (inclusive).
* If the value returned is less than {@link #getStartBCI}, this object denotes a local
* variable that is never live.
*/
public int getEndBCI() {
return endBci;
}
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21 changes: 20 additions & 1 deletion ...piler/jvmci/jdk.vm.ci.runtime.test/src/jdk/vm/ci/runtime/test/TestResolvedJavaMethod.java
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Expand Up @@ -95,6 +95,8 @@

import jdk.vm.ci.meta.ConstantPool;
import jdk.vm.ci.meta.ExceptionHandler;
import jdk.vm.ci.meta.Local;
import jdk.vm.ci.meta.LocalVariableTable;
import jdk.vm.ci.meta.ResolvedJavaMethod;
import jdk.vm.ci.meta.ResolvedJavaMethod.Parameter;
import jdk.vm.ci.meta.ResolvedJavaType;
Expand Down Expand Up @@ -734,6 +736,24 @@ public void getOopMapAtTest() throws Exception {
Assert.assertTrue(processedMethodWithManyArgs[0]);
}

@Test
public void getLocalVariableTableTest() {
for (ResolvedJavaMethod m : methods.values()) {
LocalVariableTable table = m.getLocalVariableTable();
if (table == null) {
continue;
}
for (Local l : table.getLocals()) {
if (l.getStartBCI() < 0) {
throw new AssertionError(m.format("%H.%n(%p)") + " local " + l.getName() + " starts at " + l.getStartBCI());
}
if (l.getEndBCI() >= m.getCodeSize()) {
throw new AssertionError(m.format("%H.%n(%p)") + " (" + m.getCodeSize() + "bytes) local " + l.getName() + " ends at " + l.getEndBCI());
}
}
}
}

private Method findTestMethod(Method apiMethod) {
String testName = apiMethod.getName() + "Test";
for (Method m : getClass().getDeclaredMethods()) {
Expand All @@ -756,7 +776,6 @@ private Method findTestMethod(Method apiMethod) {
"canBeInlined",
"shouldBeInlined",
"getLineNumberTable",
"getLocalVariableTable",
"isInVirtualMethodTable",
"toParameterTypes",
"getParameterAnnotation",
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