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8288425: Footprint regression due MH creation when initializing StringConcatFactory #9154

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8288425: Footprint regression due MH creation when initializing StringConcatFactory #9154

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15 changes: 11 additions & 4 deletions src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java
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Expand Up @@ -722,8 +722,15 @@ private static int classIndex(Class<?> cl) {
private static final int[] PREPEND_FILTER_SECOND_PAIR_ARGS = new int[] { 0, 1, 4, 5 };

// Base MH for complex prepender combinators.
private static final MethodHandle PREPEND_BASE = MethodHandles.dropArguments(
MethodHandles.identity(long.class), 1, byte[].class);
private static @Stable MethodHandle PREPEND_BASE;
private static MethodHandle prependBase() {
MethodHandle base = PREPEND_BASE;
if (base == null) {
base = PREPEND_BASE = MethodHandles.dropArguments(
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I'm curious about this idiom - couldn't this result in the PREPEND_BASE field being assigned twice, with objects that are morally identical, but could have different identities? (thus violating @Stable contract)

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It's not a violation, it's relying on behavior not discussed by the @Stable doc, which refers to such behavior as "undefined".

It's a carefully correlated dance between the code, interpreter, and C2 relying on:

  1. safe publication of equivalent immutable structures with no dependence on their identity when operating on them;
  2. the field being set in the interpreter; and
  3. by the time C2 inlines prependBase the field is no longer be updated, thus C2 can replace the field access with its value.

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Thanks, I think I was missing (2)

MethodHandles.identity(long.class), 1, byte[].class);
}
return base;
}

private static final @Stable MethodHandle[][] DOUBLE_PREPENDERS = new MethodHandle[TYPE_COUNT][TYPE_COUNT];

Expand All @@ -733,7 +740,7 @@ private static MethodHandle prepender(String prefix, Class<?> cl, String prefix2
MethodHandle prepend = DOUBLE_PREPENDERS[idx1][idx2];
if (prepend == null) {
prepend = DOUBLE_PREPENDERS[idx1][idx2] =
MethodHandles.dropArguments(PREPEND_BASE, 2, cl, cl2);
MethodHandles.dropArguments(prependBase(), 2, cl, cl2);
}
prepend = MethodHandles.filterArgumentsWithCombiner(prepend, 0, prepender(prefix, cl),
PREPEND_FILTER_FIRST_ARGS);
Expand All @@ -750,7 +757,7 @@ private static MethodHandle prepender(int pos, String[] constants, Class<?>[] pt
return prepender(constants[pos], ptypes[pos], constants[pos + 1], ptypes[pos + 1]);
}
// build a tree from an unbound prepender, allowing us to bind the constants in a batch as a final step
MethodHandle prepend = PREPEND_BASE;
MethodHandle prepend = prependBase();
if (count == 3) {
prepend = MethodHandles.dropArguments(prepend, 2,
ptypes[pos], ptypes[pos + 1], ptypes[pos + 2]);
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