Got it. I think the only cases we have to worry about, then, are
buffer size overflows resulting in NegativeArraySizeException, or
possibly an explicitly thrown OutOfMemoryError (which is
StringBuilder's response when the buffer size tries to exceed
Integer.MAX_VALUE). I think we might conceivably deal with this
by rewriting the bytecode to -- I think we can improve on this
with jump hackery to avoid repetition, but essentially --
int length = 3; // sum of the constant strings; we can check that
this won't overflow at compile time but I think it couldn't no
matter what because of method length constraints
String mStr = m().toString();
length += mStr.length();
if (length < 0) {
throw new OutOfMemoryError();
}
String nStr = n().toString();
length += nStr.length();
if (length < 0) {
throw new OutOfMemoryError();
}
This continues to expand the bytecode, but probably manageably --
we don't actually need a local for length; if (int < 0) is easy
in bytecode, and we can have only one OOME site that all the ifs
jump to?
On Fri, Mar 13, 2015 at 12:59 PM Alex Buckley
<alex.buck...@oracle.com <mailto:alex.buck...@oracle.com>> wrote:
I do recognize that the proposed implementation doesn't
reorder the
evaluation of subexpressions.
When discussing the proposed implementation of '+' -- whose
key element
is calling append(String) with a pre-computed value -- I was
careful to
set aside asynchronous OOMEs, but I see that even synchronous
OOMEs are
sidetracking us. My concern is not heap pressure; my concern is
arbitrary unchecked exceptions arising from the <init>(int) and
append(String) calls.
For sake of argument, I'll simplify "unchecked exceptions" to
just
RuntimeExceptions, not Errors. If you can guarantee that no
RuntimeExceptions are thrown synchronously during the
execution of those
method bodies on the JVM, then '+' cannot fail and the timing of
subexpression evaluation is unobservable (ordering is still
observable,
as required). I think this guarantee is just a matter of
reviewing the
method bodies.
Alex
On 3/12/2015 6:01 PM, Louis Wasserman wrote:
> I confess I'm not sure how that "quality" goal would be
achievable in
> bytecode without deliberately allocating arrays we then
discard.
>
> For what it's worth, delaying or avoiding OOMEs seems a
desirable goal
> in general, and up to a constant multiplicative factor, this
> implementation seems to allocate the same amount in the
same order.
> That is, we're still computing m().toString() before
n().toString(), and
> up to a constant multiplicative factor, m().toString()
allocates the
> same number of bytes as the StringBuilder the status quo
generates. So
> if m() does something like allocate a
char[Integer.MAX_VALUE], we still
> OOM at the appropriate time.
>
> Other notes: this implementation would tend to decrease maximum
> allocation, so it'd reduce OOMEs. Also, since the
StringBuilder will
> never need further expansion and we're only using the
String and
> primitive overloads of append, the only way for append to
OOME would be
> in append(float) and append(double), which allocate a
FloatingDecimal
> (which may, in turn, allocate a new thread-local char[26]
if one isn't
> already there).
>
> On Thu, Mar 12, 2015 at 4:28 PM Alex Buckley
<alex.buck...@oracle.com <mailto:alex.buck...@oracle.com>
> <mailto:alex.buck...@oracle.com
<mailto:alex.buck...@oracle.com>>> wrote:
>
> More abstract presentation. Given the expression:
>
> "foo" + m() + n()
>
> you must not evaluate n() if evaluation of "foo" + m()
completes
> abruptly. The proposed implementation evaluates n()
regardless.
>
> All is not lost. In the proposed implementation, the
abrupt completion
> of "foo" + m() could occur because an append call fails
or (thanks to
> Jon for pointing this out) the StringBuilder ctor
fails. The
> quality-of-implementation issue is thus: if the
proposed implementation
> is of sufficiently high quality to guarantee that the
ctor and the first
> append both succeed, then the evaluation of "foo" + m()
will always
> complete normally, and it would be an unobservable
(thus acceptable)
> implementation detail to evaluate n() early.
>
> Alex
>
> On 3/11/2015 10:26 PM, Jeremy Manson wrote:
> > Isn't Louis's proposed behavior equivalent to saying
"the rightmost
> > concatenation threw an OOME" instead of "some
concatenation in the
> > middle threw an OOME"?
> >
> > It's true that the intermediate String
concatenations haven't
> occurred
> > at that point, but in the JDK's current
implementation, that's true,
> > too: the concatenations that have occurred at that
point are
> > StringBuilder ones, not String ones. If any of the
append operations
> > throws an OOME, no Strings have been created at all,
either in
> Louis's
> > implementation or in the JDK's.
> >
> > Ultimately, isn't this a quality of implementation
issue? And if so,
> > isn't it a quality of implementation issue that
doesn't provide any
> > additional quality? I can't imagine code whose
semantics relies on
> > this, and if they do, they are relying on something
> > implementation-dependent.
> >
> > Jeremy
> >
> > On Wed, Mar 11, 2015 at 6:13 PM, Alex Buckley
> <alex.buck...@oracle.com
<mailto:alex.buck...@oracle.com>
<mailto:alex.buck...@oracle.com <mailto:alex.buck...@oracle.com>>
> > <mailto:alex.buckley@oracle.
<mailto:alex.buckley@oracle.>__com
> <mailto:alex.buck...@oracle.com
<mailto:alex.buck...@oracle.com>>>> wrote:
> >
> > On 3/11/2015 2:01 PM, Louis Wasserman wrote:
> >
> > So for example, "foo" + myInt + myString +
"bar" + myObj
> would be
> > compiled to the equivalent of
> >
> > int myIntTmp = myInt;
> > String myStringTmp =
String.valueOf(myString); // defend
> against
> > null
> > String myObjTmp =
> String.valueOf(String.valueOf(____myObj)); // defend
> > against evil toString implementations
returning null
> >
> > return new StringBuilder(
> > 17 // length of "foo" (3) + max
length of myInt (11) +
> > length of
> > "bar" (3)
> > + myStringTmp.length()
> > + myObjTmp.length())
> > .append("foo")
> > .append(myIntTmp)
> > .append(myStringTmp)
> > .append("bar")
> > .append(myObjTmp)
> > .toString();
> >
> > As far as language constraints go, the JLS
is (apparently
> > deliberately)
> > vague about how string concatenation is
implemented. "An
> > implementation
> > may choose to perform conversion and
concatenation in one
> step
> > to avoid
> > creating and then discarding an intermediate
String
> object. To
> > increase
> > the performance of repeated string
concatenation, a Java
> > compiler may
> > use the StringBuffer class or a similar
technique to
> reduce the
> > number
> > of intermediate String objects that are
created by
> evaluation of an
> > expression." We see no reason this approach
would not
> qualify as a
> > "similar technique."
> >
> >
> > The really key property of the string
concatenation operator is
> > left-associativity. Later subexpressions must not be
> evaluated until
> > earlier subexpressions have been successfully
evaluated AND
> > concatenated. Consider this expression:
> >
> > "foo" + m() + n()
> >
> > which JLS8 15.8 specifies to mean:
> >
> > ("foo" + m()) + n()
> >
> > We know from JLS8 15.6 that if m() throws, then
foo+m()
> throws, and
> > n() will never be evaluated.
> >
> > Happily, your translation doesn't appear to
catch and swallow
> > exceptions when eagerly evaluating each
subexpression in
> turn, so I
> > believe you won't evaluate n() if m() already threw.
> >
> > Unhappily, a call to append(..) can in general
fail with
> > OutOfMemoryError. (I'm not talking about
asynchronous
> exceptions in
> > general, but rather the sense that append(..)
manipulates the
> heap
> > so an OOME is at least plausible.) In the OpenJDK
> implementation, if
> > blah.append(m()) fails with OOME, then n()
hasn't been
> evaluated yet
> > -- that's "real" left-associativity. In the proposed
> implementation,
> > it's possible that more memory is available when
evaluating
> m() and
> > n() upfront than at the time of an append call,
so n() is
> evaluated
> > even if append(<<tmp result of m()>>) fails --
that's not
> > left-associative.
> >
> > Perhaps you can set my mind at ease that
append(..) can't
> fail with
> > OOME?
> >
> > Alex
> >
> >
>
