[9] RFR(XS): 8135252: IdealLoopTree::dump_head() prints negative trip count

[Tobias Hartmann]

Hi,

please review the following patch.

https://bugs.openjdk.java.net/browse/JDK-8135252
http://cr.openjdk.java.net/~thartmann/8135252/webrev.00/

IdealLoopTree::dump_head() used by -XX:TraceLoopOpts prints a negative trip count if CountedLoopNode::profile_trip_cnt() is large: 

 compute_profile_trip_cnt lp: 1676 cnt: 2147483648.000000 
 Loop: N1676/N1546 limit_check predicated counted [0,100),+8 (-2147483648 iters)

The problem is that we cast a large float (2147483648.0) to int

 1904     tty->print(" (%d iters) ", (int)cl->profile_trip_cnt());

causing an overflow and leading to undefined behaviour [1]. The output is sometimes 2147483647 and sometimes -2147483648 depending on the generated assembly code. [2] demonstrates this undefined behaviour nicely.

Thanks,
Tobias


[1] C++ standard, 4.9 Floating-integral conversions
"A prvalue of a floating point type can be converted to a prvalue of an integer type. The conversion truncates;
that is, the fractional part is discarded. The behavior is undefined if the truncated value cannot be
represented in the destination type."
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4527.pdf

[2] https://ideone.com/4ZalQW