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

Andrew Haley aph at redhat.com
Wed Mar 18 11:38:43 UTC 2015

On 03/18/2015 11:19 AM, Tobias Hartmann wrote:
> On 18.03.2015 11:27, Andrew Haley wrote:
>> On 03/18/2015 09:51 AM, Tobias Hartmann wrote:
>>> Byte array access:
>>>   0x0000007f7453d8b4: add	x10, x10, w1, sxtw
>>>   0x0000007f7453d8b8: ldrsb	w0, [x10,#16]
>>> Char array access:
>>>   0x0000007f7cb6f8b4: add	x8, x10, #0x10
>>>   0x0000007f7cb6f8b8: ldrh	w0, [x8,w1,sxtw #1]
>> Do you know why these are different?  The second form is definitely
>> preferable.
> Yes, it's because we have no operand for the indirect-with-offset
> (no scaling) case and therefore it is matched to 'addP_reg_reg_ext'.
> For this reason I added the two new operands indIndexOffsetI2L and
> indIndexOffsetI2LN. But even with those operands, it only works if
> we reduce their cost below INSN_COST. Otherwise, 'AddP (AddP reg
> (ConvI2L ireg)) off' is matched to 'addP_reg_reg_ext' (cost

Aha!  That's the bug.  'addP_reg_reg_ext' should be 2*INSN_COST,
along with all the ALU extend and/or shift instructions.  My
bad, probably.


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