[9] RFR (S): 8152179: C2: Folding of unsigned loads is broken w/ @Stable

[Vladimir Kozlov]

Yes, it is better. Reviewed.

Thanks,
Vladimir

On 3/24/16 4:14 AM, Vladimir Ivanov wrote:
> On 3/24/16 2:15 AM, Vladimir Kozlov wrote:
>> I don't understand changes in is_mismatched_access(). It does not
>> guarantee/check that loads have masking. If there are no masking then
>> short[] and char[] loads should mismatch. Right?
> There's no way to distinguish between a mismatched access and a load fused w/ a mask, except
> MemNode::is_mismatched_access() flag. And it is set for mismatched accesses in LibraryCallKit::inline_unsafe_access().
>
> fold_stable_ary_elem() call is guarded by MemNode::is_mismatched_access() and is_mismatched_access() is used only from
> fold_stable_ary_elem() which does the masking.
>
> But I decided to refactor the code:
>    http://cr.openjdk.java.net/~vlivanov/8152179/webrev.01
>
> Do you find it clearer?
>
> Best regards,
> Vladimir Ivanov
>
>> On 3/23/16 9:56 AM, Vladimir Ivanov wrote:
>>> http://cr.openjdk.java.net/~vlivanov/8152179/webrev.00/
>>> https://bugs.openjdk.java.net/browse/JDK-8152179
>>>
>>> Byte and short loads with small masks are converted to unsigned loads.
>>> Constant folding logic for array elements doesn't respect unsigned
>>> loads and replace them with signed values.
>>>
>>> It usually happens when a small mask is applied to the value produced
>>> by a signed load (see AndINode::Ideal [1]):
>>>
>>>    byteArr [i] & 0x000F;  // AndI (LoadB 0x000F) => AndI (LoadUB 0xF)
>>>    shortArr[i] & 0x000F;  // AndI (LoadS 0x000F) => AndI (LoadUS 0xF)
>>>
>>>    byteArr [i] & 0x00FF;  // AndI (LoadB 0x00FF) => LoadUB
>>>    shortArr[i] & 0xFFFF;  // AndI (LoadS 0xFFFF) => LoadUS
>>>
>>> Field loads aren't affected since they are constant folded during
>>> parsing.
>>>
>>> Testing: regression tests, hotspot/compiler/stable, JPRT
>>>
>>> Thanks!
>>>
>>> Best regards,
>>> Vladimir Ivanov
>>>
>>> PS: while experimenting with the fix, I found that constant folding of
>>> @Stable loads doesn't always happen. Filed 8152590 [2].
>>>
>>> [1]
>>> http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/file/d9d0a63499ce/src/share/vm/opto/mulnode.cpp#l483
>>>
>>>
>>> [2] https://bugs.openjdk.java.net/browse/JDK-8152590