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

[Tobias Hartmann]

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