[X86][Vector Cost Model] Add a comment to explain the workaround

in my previous commit (r204884).

<rdar://problem/16381225>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204972 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Quentin Colombet 2014-03-27 22:27:41 +00:00
parent 6bdc4ebedd
commit d5f800aab3

View File

@ -512,6 +512,11 @@ unsigned X86TTI::getCastInstrCost(unsigned Opcode, Type *Dst, Type *Src) const {
{ ISD::UINT_TO_FP, MVT::v4f64, MVT::v4i8, 2 },
{ ISD::UINT_TO_FP, MVT::v4f64, MVT::v4i16, 2 },
{ ISD::UINT_TO_FP, MVT::v4f64, MVT::v4i32, 6 },
// The generic code to compute the scalar overhead is currently broken.
// Workaround this limitation by estimating the scalarization overhead
// here. We have roughly 10 instructions per scalar element.
// Multiply that by the vector width.
// FIXME: remove that when PR19268 is fixed.
{ ISD::UINT_TO_FP, MVT::v2f64, MVT::v2i64, 2*10 },
{ ISD::UINT_TO_FP, MVT::v4f64, MVT::v4i64, 4*10 },