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8230665: (bf spec) ByteBuffer::alignmentOffset spec misleading when address is misaligned

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Reviewed-by: alanb, darcy, psandoz
This commit is contained in:
Brian Burkhalter 2020-01-14 11:06:00 -08:00
parent 72a35c899e
commit 03cd98e15b
3 changed files with 112 additions and 12 deletions
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32
src/java.base/share/classes/java/nio/X-Buffer.java.template
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@ -1,5 +1,5 @@
/*
* Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2000, 2020, 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
@ -1869,14 +1869,26 @@ public abstract class $Type$Buffer
/**
* Returns the memory address, pointing to the byte at the given index,
* modulus the given unit size.
*
* <p> A return value greater than zero indicates the address of the byte at
* the index is misaligned for the unit size, and the value's quantity
* indicates how much the index should be rounded up or down to locate a
* byte at an aligned address. Otherwise, a value of {@code 0} indicates
* that the address of the byte at the index is aligned for the unit size.
* modulo the given unit size.
*
* <p> The return value is non-negative, with {@code 0} indicating that the
* address of the byte at the index is aligned for the unit size, and a
* positive value that the address is misaligned for the unit size. If the
* address of the byte at the index is misaligned, the return value
* represents how much the index should be adjusted to locate a byte at an
* aligned address. Specifically, the index should either be decremented by
* the return value, or incremented by the unit size minus the return value.
* Therefore given
* <blockquote><pre>
* int value = alignmentOffset(index, unitSize)</pre></blockquote>
* then the identities
* <blockquote><pre>
* alignmentOffset(index - value, unitSize) == 0</pre></blockquote>
* and
* <blockquote><pre>
* alignmentOffset(index + (unitSize - value), unitSize) == 0</pre></blockquote>
* must hold.
*
* @apiNote
* This method may be utilized to determine if unit size bytes from an
* index can be accessed atomically, if supported by the native platform.
@ -1892,7 +1904,7 @@ public abstract class $Type$Buffer
* @param unitSize
* The unit size in bytes, must be a power of {@code 2}
*
* @return The indexed byte's memory address modulus the unit size
* @return The indexed byte's memory address modulo the unit size
*
* @throws IllegalArgumentException
* If the index is negative or the unit size is not a power of
@ -1918,7 +1930,7 @@ public abstract class $Type$Buffer
if (unitSize > 8 && !isDirect())
throw new UnsupportedOperationException("Unit size unsupported for non-direct buffers: " + unitSize);
return (int) ((address + index) % unitSize);
return (int) ((address + index) & (unitSize - 1));
}
/**