TTI: Add comment clarifying the meaning of MemIntrinsicInfo::PtrVal

[• tstellarAMD]

Summary

EarlyCSE assumes that intrinsics with a defined PtrVal are functionally
equivalent to a load/store and it will CSE two intrinsics that have the
same PtrVal even if other arguments are different.

[mcrosier]

Adding Sanjin as a reviewer as he wrote the original patch.

[mcrosier]

LGTM, but please wait a day or two to see if Sanjin has any additional comments.

[ssijaric]

LGTM. The CSE code for commoning of intrinsic loads and stores should probably be refactored. I should revisit https://reviews.llvm.org/D8313 or https://reviews.llvm.org/D15616.

include/llvm/Analysis/TargetTransformInfo.h

struct MemIntrinsicInfo {
   bool ReadMem;
   bool WriteMem;
   /// True only if this memory operation is non-volatile, non-atomic, and
   /// unordered.  (See LoadInst/StoreInst for details on each)
   bool IsSimple;
   // Same Id is set by the target for corresponding load/store intrinsics.
   unsigned short MatchingId;
   int NumMemRefs;
+
+  /// This is the pointer that the intrinsic is loading from or storing to.
+  /// If this is non-null, then analysis/optimization passes can assume that
+  /// this intrinsic is functionally equivalent to a load/store from this
+  /// pointer.
   Value *PtrVal;
 };
 
 /// \brief This pass provides access to the codegen interfaces that are needed
 /// for IR-level transformations.
 class TargetTransformInfo {
 public:
   /// \brief Construct a TTI object using a type implementing the \c Concept