This patch simplifies the calculation of certain costs in
getInstructionCost when isScalarAfterVectorization() returns a true value.
There are a few places where we multiply a cost by a number N, i.e.
unsigned N = isScalarAfterVectorization(I, VF) ? VF.getKnownMinValue() : 1; return N * TTI.getArithmeticInstrCost(...
After some investigation it seems that there are only these cases that occur
- VF is a scalar, in which case N = 1.
- VF is a vector. We can only get here if: a) the instruction is a GEP/bitcast with scalar uses, or b) this is an update to an induction variable that remains scalar.
I have changed the code so that N is assumed to always be 1. For GEPs
the cost is always 0, since this is calculated later on as part of the
load/store cost. For all other cases I have added an assert that none of the
users needs scalarising, which didn't fire in any unit tests.
Only one test required fixing and I believe the original cost for the scalar
add instruction to have been wrong, since only one copy remains after