RFR: 8324517: C2: crash in compiled code because of dependency on removed range check CastIIs [v4]

[Roland Westrelin]

On Fri, 3 May 2024 10:11:25 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> Range check `CastII` nodes are removed once loop opts are over. The
>> test case for this change includes 3 cases where elimination of a
>> range check `CastII` causes a crash in compiled code because either a
>> out of bounds array load or a division by zero happen.
>> 
>> In `test1`:
>> 
>> - the range checks for the `array[otherArray.length]` loads constant
>>   fold: `otherArray.length` is a `CastII` of i at the `otherArray`
>>   allocation. `i` is less than 9. The `CastII` at the allocation
>>   narrows the type down further to `[0-9]`.
>>   
>> - the `array[otherArray.length]` loads are control dependent on the
>> unrelated:
>> 
>> 
>> if (flag == 0) {
>> 
>> 
>> test. There's an identical dominating test which replaces that one. As
>> a consequence, the `array[otherArray.length]` loads become control
>> dependent on the dominating test.
>> 
>> - The `CastII` nodes at the `otherArray` allocations are replaced by a
>>   dominating range check `CastII` nodes for:
>>   
>> 
>> newArray[i] = 42;
>> 
>> 
>> - After loop opts, the range check `CastII` nodes are removed and the
>>   2 `array[otherArray.length]` loads common at the first:
>>   
>> 
>> if (flag == 0) {
>> 
>> 
>> test before the:
>> 
>> 
>> float[] otherArray = new float[i];
>> 
>> 
>> and
>> 
>> 
>> newArray[i] = 42;
>> 
>> 
>> that guarantee `i` is positive.
>> 
>> - `test1` is called with `i = -1`, the array load proceeds with an out
>>   of bounds index and the crash occurs.
>>   
>>   
>> `test2` and `test3` are mostly identical except for the check that's
>> eliminated (a null divisor check) and the instruction that causes a
>> fault (an integer division).
>>   
>> The fix I propose is to not eliminate range check `CastII` nodes after
>> loop opts. When range check`CastII` nodes were introduced, performance
>> was observed to regress. Removing them after loop opts was found to
>> preserve both correctness and performance. Today, the performance
>> regression still exists when `CastII` nodes are left in. So I propose
>> we keep them until the end of optimizations (so the 2 array loads
>> above don't lose a dependency and wrongly common) but remove them at
>> the end of all optimizations.
>> 
>> In the case of the array loads, they are dependent on a range check
>> for another array through a range check `CastII` and we must not lose
>> that dependency otherwise the array loads could float above the range
>> check at gcm time. I propose we deal with that problem the way it's
>> handled for `CastPP` nodes: add the dependency to the load (or
>> division)nodes ...
>
> Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   test fix

Thanks for reviewing this.

> Did you check if the other usages of `_range_check_dependency` via `CastIINode::has_range_check` are still needed? Seems to me as if at least the checks in `PhaseIdealLoop::match_fill_loop` can be removed.

I did but was fairly conservative. In the case of  `PhaseIdealLoop::match_fill_loop`, I don't think this change makes a difference: if we don't need the check for `CastIINode::has_range_check` there then it's true with or without that change.

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PR Comment: https://git.openjdk.org/jdk/pull/18377#issuecomment-2092708764