On 04/10/2013 11:26 PM, Martin Buchholz wrote:
I'm willing to accept John as an authority on hotspot optimization.
I'm surprised that null checks aren't more close to free in part because
recent jsr166 code has been introducing more explicit null checks,
sometimes in code where the reference being checked is "known" not to be
null.
Martin
null check that are never null are free when the interpreter has never
(or rarely) sees a null
at a specific callsite because in that case, the JIT doesn't generate a
null check and
let the CPU/MMU do a fault (the VM has a signal handler to be able to
come back from death :)
If you set a field to null, the profiler will see the null and will not
use this optimization,
that why in this case, it's better to have an empty array instead of null.
BTW, the nullcheck in assembler cost you almost nothing anyway but the jump
associated with it has a cost. Modern CPUs do not like to jump.
Rémi
On Wed, Apr 10, 2013 at 11:12 AM, Mike Duigou <mike.dui...@oracle.com>wrote:
On Apr 9 2013, at 19:56 , Martin Buchholz wrote:
The use of an empty array rather than null was suggested by John Rose who
said:
I recommend an empty array rather than null as a sentinel value for two
reasons:
1. The JVM prefers to merge null checks into load or store instructions
(so-called "implicit null checks") because it removes an explicit branch.
But it only does so if the probability of nulls is zero or very low. But
using null as a sentinel for common states (e.g., empty collection) defeats
this optimization.
2. For power-of-two sized structures (HashMap) we can optimize away an
array range check in the presence of a zero-length check.
Since most uses of a variable-sized collection load and test the array
length, the sentinel check can easily be overloaded onto this test. If null
is not used, then the (safety-mandated) null check is (usually) merged into
the load of the length. If the table is power-of-two-sized, then only the
zero check remains, and the array range check may be removed. This is
thought to be the best code for a frequent load from a possibly-empty
collection.
Mike asked, "what about empty collection?" This is a reasonable thing to
use, but it has a cost. The JVM uses inline caches and type profiles to
simplify its optimized code; these techniques "win" when at a given use
point (individual call to Map.get, for example) there is only one class
present, even if the interface is totally general. (This is the so-called
"monomorphic" case.) If the application uses (say) only HashMaps for both
empty and non-empty maps, then this optimization can win big. It can be
broken, on the other hand, if the application begins to use one other type
for some other case (such as empty maps). In these cases, it is better to
overload the "am I empty?" test on some other loaded value, such as a null
or (better) an array length.
Mike
