Request for reviews (S): 7179138: Incorrect result with String concatenation optimization

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/7179138/webrev

7179138: Incorrect result with String concatenation optimization

The problem happens when one StringBuilder append/toString sequence uses the 
result of previous StringBuilder sequence and both sequences are merged by 
string concatenation optimization. Additional condition is the usage of 
String.valueOf(s) call as argument of StringBuilder constructor (which is 
implicitly generated by Eclipse java compiler).

In normal case, when toString() result is directly referenced by the 
constructor, string concatenation optimization will use input arguments of 
previous StringBuilder append/toString sequence as additional arguments of 
merged sequence instead of toString() result itself. It is done because string 
concatenation optimization replaces the original code with new one and it will 
throw away original calls, including intermediate toString() calls.

The problem with bug's case is toString() result is separated by Phi node from 
diamond shaped code from valueOf(s) method (s==null ? "null" : s) and it is kept 
as argument to new String concatenation code. But the input to valueOf(s) check 
become dead after the call to toString() is removed from graph. As result "null" 
string is used incorrectly instead.

The fix is to look for diamond shaped code which checks for NULL the result of 
toString() call and look through it as we do now for directly referenced 
toString() results.

I also fixed call nodes elimination code which did not remove Initialize nodes 
from merged StringBuilder sequences. Currently it removes only first one.

Added regression tests from bug report.

Tested with Hotspot regression tests, jdk java/lang tests, CTW.

Thanks,
Vladimir