[9] RFR(S): 8075136: Unnecessary sign extension for byte array access

[Tobias Hartmann]

Thanks, Vladimir.

Here's the final webrev:
http://cr.openjdk.java.net/~thartmann/8075136/webrev.05/

Best,
Tobias

On 17.03.2015 19:26, Vladimir Kozlov wrote:
> On 3/17/15 5:33 AM, Tobias Hartmann wrote:
>>
>> On 13.03.2015 19:03, Vladimir Kozlov wrote:
>>> Looks good. What about other platforms? I see the same optimization in aarch64.ad
>>
>> Thanks, Vladimir. I fixed the issue on ARM64 as well. Looks like other platforms are not affected.
>>
>> http://cr.openjdk.java.net/~thartmann/8075136/webrev.03/
> 
> Looks good for me. Please, finish discussion with Andrew Dinn to make sure he is okay with changes.
> 
> Thanks,
> Vladimir
> 
>>
>> While verifying, I noticed that the costs of memory operands in aarch64.ad are inconsistent:
>>
>> The following operands have cost 0:
>> - indirect
>> - indIndexScaledI2L
>> - indIndexScaled
>> - indIndex
>> - indOffL
>> - indirectN
>> - indIndexScaledOffsetIN
>> - indIndexScaledI2LN
>> - indIndexScaledN
>> - indIndexN
>> - indOffIN
>> - indOffLN
>>
>> Whereas the following operands have cost 'INSN_COST':
>> - indIndexScaledOffsetI
>> - indIndexScaledOffsetL
>> - indIndexScaledOffsetI2L
>> - indOffI
>> - indIndexScaledOffsetLN
>> - indIndexScaledOffsetI2LN
>>
>> This does not make sense to me. In this case 'AddP (AddP reg (ConvI2L ireg)) off' is matched to 'addP_reg_reg_ext' (cost INSN_COST) and 'indOffL' (cost 0) instead of the newly added 'indIndexOffsetI2L' (cost INSN_COST) because the sum of the costs is equal.
>>
>> I filed JDK-8075324 [1] to take care of this. I would suggest that we set the cost of a memory operand to either 0 (if only immediates) or 10 (if more complex) like we do on x86.
>>
>> I verified that the patch (together with the fix for JDK-8075324) solves the issue.
>>
>> Before:
>>    034   B3: #    N1 <- B2  Freq: 0.999998
>>    034 +     add R10, R10, R1, sxtw    # ptr
>>    038 +     ldrsbw  R0, [R10, #16]    # byte
>> After:
>>    034   B3: #    N1 <- B2  Freq: 0.999998
>>    034 +     ldrsbw  R0, R10, R1, #16 I2L    # byte
>>
>> Thanks,
>> Tobias
>>
>>
>> [1] https://bugs.openjdk.java.net/browse/JDK-8075324
>>
>>
>>> Thanks,
>>> Vladimir
>>>
>>> On 3/13/15 7:15 AM, Tobias Hartmann wrote:
>>>> Hi,
>>>>
>>>> please review the following patch.
>>>>
>>>> https://bugs.openjdk.java.net/browse/JDK-8075136
>>>> http://cr.openjdk.java.net/~thartmann/8075136/webrev.00/
>>>>
>>>> Problem:
>>>> C2 adds an unnecessary 'movslq' sign extension while compiling a byte array access:
>>>>
>>>>     public static byte accessByte(int index) {
>>>>       return byteArr[index];
>>>>     }
>>>>
>>>>     0x00007f76f4747208: movslq %esi,%r11
>>>>     0x00007f76f474720b: movsbl 0x10(%r10,%r11,1),%eax
>>>>
>>>> Where the 'movslq' is not necessary because we emit range checks guaranteeing that index %esi is not negative.
>>>> For a char array access no such sign extension is created:
>>>>
>>>>     public static char accessChar(int index) {
>>>>       return charArr[index];
>>>>     }
>>>>
>>>>     0x00007fab3916b188: movzwl 0x10(%r10,%rsi,2),%eax
>>>>
>>>> This is because we only have matching rules to fold the corresponding ConvI2LNode in the following case:
>>>>
>>>>     match(AddP (AddP (DecodeN reg) (LShiftL (ConvI2L idx) scale)) off);
>>>>
>>>> This applies to charAccess [1] but not to byteAccess [2] because the byte array access does not require a scaling factor.
>>>>
>>>> Solution:
>>>> I added the corresponding matching rule to fold the ConvI2LNode.
>>>>
>>>>     match(AddP (AddP (DecodeN reg) (ConvI2L idx)) off);
>>>>
>>>> Testing:
>>>> - Testcase
>>>> - JPRT
>>>>
>>>> Thanks,
>>>> Tobias
>>>>
>>>> [1] https://bugs.openjdk.java.net/secure/attachment/26108/accessChar.png
>>>> [2] https://bugs.openjdk.java.net/secure/attachment/26107/accessByte.png
>>>>