NFC: The Cost Model specialization, by Andrey Tischenko

The current Cost Model implementation is very inaccurate and has to be
updated, improved, re-implemented to be able to take into account the
concrete CPU models and the concrete targets where this Cost Model is
being used. For example, the Latency Cost Model should be differ from
Code Size Cost Model, etc.
This patch is the first step to launch the developing and implementation
of a new Cost Model generation.

Differential Revision: https://reviews.llvm.org/D25186

llvm-svn: 284012

lib/Target/X86/X86TargetTransformInfo.cpp

@@ -13,6 +13,31 @@
 /// independent and default TTI implementations handle the rest.
 ///
 //===----------------------------------------------------------------------===//
+/// About Cost Model numbers used below it's necessary to say the following:
+/// the numbers correspond to some "generic" X86 CPU instead of usage of
+/// concrete CPU model. Usually the numbers correspond to CPU where the feature
+/// apeared at the first time. For example, if we do Subtarget.hasSSE42() in
+/// the lookups below the cost is based on Nehalem as that was the first CPU
+/// to support that feature level and thus has most likely the worst case cost.
+/// Some examples of other technologies/CPUs:
+///   SSE 3   - Pentium4 / Athlon64
+///   SSE 4.1 - Penryn
+///   SSE 4.2 - Nehalem
+///   AVX     - Sandy Bridge
+///   AVX2    - Haswell
+///   AVX-512 - Xeon Phi / Skylake
+/// And some examples of instruction target dependent costs (latency)
+///                   divss     sqrtss          rsqrtss
+///   AMD K7            11-16     19              3
+///   Piledriver        9-24      13-15           5
+///   Jaguar            14        16              2
+///   Pentium II,III    18        30              2
+///   Nehalem           7-14      7-18            3
+///   Haswell           10-13     11              5
+/// TODO: Develop and implement  the target dependent cost model and
+/// specialize cost numbers for different Cost Model Targets such as throughput,
+/// code size, latency and uop count.
+//===----------------------------------------------------------------------===//
 
 #include "X86TargetTransformInfo.h"
 #include "llvm/Analysis/TargetTransformInfo.h"