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

[Roland Westrelin]

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

Looks good.

Roland.

> 
> 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
>>>>>