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

[Tobias Hartmann]

Thanks, Vladimir.

Best,
Tobias

On 11.09.2015 05:38, Vladimir Kozlov wrote:
> Looks good.
> 
> Thanks,
> Vladimir
> 
> On 9/9/15 6:21 AM, Tobias Hartmann wrote:
>> 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
>>