[llvm] r326956 - [TTI] add explanatory comments for getArithmeticInstrCost; NFC

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Wed Mar  7 14:43:08 2018
New Revision: 326956

URL: http://llvm.org/viewvc/llvm-project?rev=326956&view=rev
Log:
[TTI] add explanatory comments for getArithmeticInstrCost; NFC

Modified:
    llvm/trunk/include/llvm/Analysis/TargetTransformInfo.h

Modified: llvm/trunk/include/llvm/Analysis/TargetTransformInfo.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/TargetTransformInfo.h?rev=326956&r1=326955&r2=326956&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/TargetTransformInfo.h (original)
+++ llvm/trunk/include/llvm/Analysis/TargetTransformInfo.h Wed Mar  7 14:43:08 2018
@@ -710,10 +710,20 @@ public:
   /// and the number of execution units in the CPU.
   unsigned getMaxInterleaveFactor(unsigned VF) const;
 
-  /// \return The expected cost of arithmetic ops, such as mul, xor, fsub, etc.
-  /// \p Args is an optional argument which holds the instruction operands  
-  /// values so the TTI can analyize those values searching for special 
-  /// cases\optimizations based on those values.
+  /// This is an approximation of reciprocal throughput of a math/logic op.
+  /// A higher cost indicates less expected throughput.
+  /// From Agner Fog's guides, reciprocal throughput is "the average number of
+  /// clock cycles per instruction when the instructions are not part of a
+  /// limiting dependency chain."
+  /// Therefore, costs should be scaled to account for multiple execution units
+  /// on the target that can process this type of instruction. For example, if
+  /// there are 5 scalar integer units and 2 vector integer units that can
+  /// calculate an 'add' in a single cycle, this model should indicate that the
+  /// cost of the vector add instruction is 2.5 times the cost of the scalar
+  /// add instruction.
+  /// \p Args is an optional argument which holds the instruction operands
+  /// values so the TTI can analyze those values searching for special
+  /// cases or optimizations based on those values.
   int getArithmeticInstrCost(
       unsigned Opcode, Type *Ty, OperandValueKind Opd1Info = OK_AnyValue,
       OperandValueKind Opd2Info = OK_AnyValue,