cost of Java "assert" when disabled?
david.holmes at oracle.com
Fri Feb 17 01:27:36 UTC 2012
On 17/02/2012 9:40 AM, Vitaly Davidovich wrote:
> The asserts can be enabled/disabled at startup time, but I don't consider
> that an advantage over conditional compilation. In fact, it's less
> convenient in some cases, e.g. you can't conditionally add/remove class
> fields, can't surround blocks of code with condition, etc. There are
> workarounds, but it's not ideal.
I'm not going to get drawn into the whole "conditional compilation is
[not] evil" debate. :) If I recall correctly the suggested buld-time
idiom was to do:
static final boolean ASSERT = true; // or false
that way you could compile with ASSERT set true to get assertions in the
code; or false to have them elided by javac.
> C#/.Net have conditional compilation (conditional blocks + assert
> statements) and it's a handy tool and no need to worry about dead IL code
> causing opto issues - don't see a reason why java couldn't have done the
> same from the beginning.
Simply because the people defining the language didn't want it. I
suspect there's a blog or two out there somewhere discussing this.
> Sent from my phone
> On Feb 16, 2012 6:16 PM, "David Holmes"<david.holmes at oracle.com> wrote:
>> The corelibs side of things seems to have gotten dropped from the cc list
>> - added back.
>> On 17/02/2012 8:21 AM, Vitaly Davidovich wrote:
>>> Don't want to sidetrack this thread but I really wish javac had proper
>>> conditional compilation support, which would make this issue mostly moot.
>> But the whole point of Java assertions is to make them available at
>> runtime. I seem to recall a very similar question only recently on the
>> core-libs mailing list.
>> So summary is:
>> - Every assert requires checking if asserts are enabled
>> - JIT Compiler can elide the checks
>> - Presence of assert related bytecodes can impact JIT compiler inlining
>> Sent from my phone
>>> On Feb 16, 2012 5:14 PM, "John Rose"<john.r.rose at oracle.com
>>> <mailto:john.r.rose at oracle.com**>> wrote:
>>> On Feb 16, 2012, at 1:59 PM, Vitaly Davidovich wrote:
>>> I think one problem with them is that they count towards the
>>>> inlining budget since their bytecodes still take up space. Not
>>>> sure if newer C1/C2 compiler builds are "smarter" about this nowadays.
>>> Optimized object code has (probably) no trace of the assertions
>>> themselves, but as Vitaly said, they perturb the inlining budget.
>>> Larger methods have a tendency to "discourage" the inliner from
>>> inlining, causing more out-of-line calls and a rough net slowdown.
>>> Currently, the non-executed bytecodes for assertions (which can be
>>> arbitrarily complex) make methods look bigger than they really are.
>>> This is (IMO) a bug in the inlining heuristics, which should be
>>> fixed by examining inlining candidates with a little more care.
>>> Since the escape analysis does a similar method summarization,
>>> there isn't necessarily even a need for an extra pass over the methods.
>>> -- John
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