Updates:
Cc: [email protected]
Comment #2 on issue 3325 by [email protected]: Context allocated values
are not constant folded.
http://code.google.com/p/v8/issues/detail?id=3325
@yangguo: the issue is actually the other way around, if you look at the
results for Chrome >=34 then you will notice that situation is weird test
case of concatenation of literals inside the function is *slower* then
concatenating context variables.
The benchmark is obviously flawed, but it reveals a problem in V8:
outsideScope function is inlined into the benchmarking loop after which
concatenation is hoisted out of the loop with LICM leading to an empty loop
measurement
insideScope function is also inlined but then V8 goes highwire: it emits
expanded ConsString allocation instead of constant folding concatenation
due to wrong order of conditions in the graph builder. Expanded allocation
can't be hoisted out by LICM - hence the loop is actually doing the concat
which is slower compared to not doing a concat in the loop. The fix is
below:
--- src/hydrogen.cc (revision 21348)
+++ src/hydrogen.cc (working copy)
@@ -9639,6 +9639,14 @@
return left;
}
+ if (FLAG_fold_constants &&
+ left->IsConstant() && HConstant::cast(left)->HasStringValue() &&
+ right->IsConstant() && HConstant::cast(right)->HasStringValue()) {
+ return AddUncasted<HStringAdd>(
+ left, right, allocation_mode.GetPretenureMode(),
+ STRING_ADD_CHECK_NONE, allocation_mode.feedback_site());
+ }
+
// Register the dependent code with the allocation site.
if (!allocation_mode.feedback_site().is_null()) {
ASSERT(!graph()->info()->IsStub());
[I was under impression that I have filed a bug on this before, but somehow
it fell through]
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