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

Andrew Dinn adinn at redhat.com
Tue Mar 17 16:32:07 UTC 2015

On 17/03/15 16:11, Andrew Dinn wrote:

Apologies for jumping the gun wiht my previous comment. Having reviewed the earlier traffic in this thread I now understand why the two new rules are needed (i.e. to deal with the case where there is no LShift in the matched operand). Since they require use of an offset and an (unscaled) index register both these new operands will need to be encoded using two instructions. So that has two implications:

The cost of the operand will need to be INSNCOST to account for the extra add. The encoding function loadStore (the first one which operates on integer registers but not the second one which operates on float registers) will need to be updated to recognise these two operand types and pass in a zero scaling: . . . switch (opcode) { case INDINDEXSCALEDOFFSETI2L: case INDINDEXSCALEDI2L: case INDINDEXSCALEDOFFSETI2LN: case INDINDEXSCALEDI2LN: scale = Address::sxtw(size); break; + case INDINDEXOFFSETI2L: + case INDINDEXOFFSETI2LN: + scale = Address::lsl(0); + break; default: scale = Address::lsl(size); . . . [as Andrew Haley's comment points out the need for case handling here is horribly ugly].

Doh, apologies -- I'm afraid I am still one step behind!

The two new rules both specify an unscaled 32-bit integer register as index and, of course, the scale value is installed as 0 in the rule. So, the correct patch to function storeLoad is to include these two switch cases in the set which specify a sign-extend:

. . .
switch (opcode) {
case INDINDEXSCALEDOFFSETI2L:
case INDINDEXSCALEDI2L:
case INDINDEXSCALEDOFFSETI2LN:
case INDINDEXSCALEDI2LN:

regards,

Andrew Dinn

Senior Principal Software Engineer Red Hat UK Ltd Registered in UK and Wales under Company Registration No. 3798903 Directors: Michael Cunningham (USA), Matt Parson (USA), Charlie Peters (USA), Michael O'Neill (Ireland)

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