[llvm] r204972 - [X86][Vector Cost Model] Add a comment to explain the workaround

[Quentin Colombet]

Author: qcolombet
Date: Thu Mar 27 17:27:41 2014
New Revision: 204972

URL: http://llvm.org/viewvc/llvm-project?rev=204972&view=rev
Log:
[X86][Vector Cost Model] Add a comment to explain the workaround
in my previous commit (r204884).

<rdar://problem/16381225>

Modified:
    llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp

Modified: llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp?rev=204972&r1=204971&r2=204972&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp Thu Mar 27 17:27:41 2014
@@ -512,6 +512,11 @@ unsigned X86TTI::getCastInstrCost(unsign
     { 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 },