Hal Finkel via llvm-dev
llvm-dev at lists.llvm.org
Fri Jan 20 06:53:26 PST 2017
On 01/20/2017 08:30 AM, Jonas Paulsson wrote:
> On 2017-01-20 14:31, Hal Finkel wrote:
>> On 01/20/2017 06:11 AM, Jonas Paulsson via llvm-dev wrote:
>>> I wonder why getScalarizationOverhead() does not take into account
>>> the number of operands of the instruction? This should influence the
>>> number of extracts needed, so instead of
>>> Scalarization cost = NumEls * (insert + extract)
>>> it would be better to do
>>> Scalarization cost = NumEls * (insert + (extract * numOperands))
>> I suspect this is an oversight (although we need to be a bit careful
>> here because if two operands are the same, which is not uncommon, we
>> don't want to double the cost).
> Do you in those cases of an identical operand want to count just a
> cost of "1" for a register move, instead of the "extraction cost"?
There should be no cost to reusing the operand. (mul a, a) should only
extract a once, the fact that it is used twice should not increase the cost.
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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