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

Tobias Hartmann tobias.hartmann at oracle.com
Fri Mar 13 14:15:18 UTC 2015

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:

Thanks, Tobias

[1] https://bugs.openjdk.java.net/secure/attachment/26108/accessChar.png [2] https://bugs.openjdk.java.net/secure/attachment/26107/accessByte.png

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