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

in my previous commit (r204884).

<rdar://problem/16381225>

llvm-svn: 204972
This commit is contained in:
Quentin Colombet 2014-03-27 22:27:41 +00:00
parent 82569b6366
commit 85b904d875
1 changed files with 5 additions and 0 deletions
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5
llvm/lib/Target/X86/X86TargetTransformInfo.cpp
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@ -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 },