[PPC64LE] Remove unnecessary swaps from lane-insensitive vector computations

[oota-llvm.git] / lib / Target / PowerPC / PPCInstrVSX.td

diff --git a/lib/Target/PowerPC/PPCInstrVSX.td b/lib/Target/PowerPC/PPCInstrVSX.td
index a98e58f18ca6ecfda118e88c5915f0b763dbf6f2..44f4e8ae46740e5baafa4f29be6de366b82e4390 100644 (file)
--- a/lib/Target/PowerPC/PPCInstrVSX.td
+++ b/lib/Target/PowerPC/PPCInstrVSX.td
@@ -11,6 +11,21 @@
 //
 //===----------------------------------------------------------------------===//
 
+// *********************************** NOTE ***********************************
+// ** For POWER8 Little Endian, the VSX swap optimization relies on knowing  **
+// ** which VMX and VSX instructions are lane-sensitive and which are not.   **
+// ** A lane-sensitive instruction relies, implicitly or explicitly, on      **
+// ** whether lanes are numbered from left to right.  An instruction like    **
+// ** VADDFP is not lane-sensitive, because each lane of the result vector   **
+// ** relies only on the corresponding lane of the source vectors.  However, **
+// ** an instruction like VMULESB is lane-sensitive, because "even" and      **
+// ** "odd" lanes are different for big-endian and little-endian numbering.  **
+// **                                                                        **
+// ** When adding new VMX and VSX instructions, please consider whether they **
+// ** are lane-sensitive.  If so, they must be added to a switch statement   **
+// ** in PPCVSXSwapRemoval::gatherVectorInstructions().                      **
+// ****************************************************************************
+
 def PPCRegVSRCAsmOperand : AsmOperandClass {
   let Name = "RegVSRC"; let PredicateMethod = "isVSRegNumber";
 }