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

[Roland Westrelin]

On Thu, 28 Mar 2024 11:53:02 GMT, Emanuel Peter <epeter 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 ...
>
> src/hotspot/share/opto/castnode.cpp line 222:
> 
>> 220:   if (!_range_check_dependency) {
>> 221:     res = widen_type(phase, res, T_INT);
>> 222:   }
> 
> Can you explain why you changed this, and why it is ok?

`ConvI2L` has a similar transformation. Let's say we have 2 `ConvI2L` nodes with identical inputs but different types:

(ConvI2L _ input [2..max_jint])
(ConvI2L _ input [1..max_jint])

They are transformed to:

(ConvI2L _ input [0..max_jint])
(ConvI2L _ input [0..max_jint])

so they can common. With range checks, the pattern is:

(ConvI2L _ (CastII control input [2..max_jint]) [2..max_jint])
(ConvI2L _ (CastII control input [1..max_jint]) [1..max_jint]

Without this patch, the range checks `CastII` are removed after loop opts so having the transformation be done only on `ConvI2L` nodes is sufficient. With this change the `CastII` are left in the IR so they need to be transformed the same way:


(ConvI2L _ (CastII control input [0..max_jint]) [0..max_jint])
(ConvI2L _ (CastII control input [0..max_jint]) [0..max_jint]


so they can common and the `ConvI2L` then common.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18377#discussion_r1542993546