[llvm] [FunctionAttrs] Add the "initializes" attribute inference (PR #97373)

[Haopeng Liu via llvm-commits]

================
@@ -580,6 +582,205 @@ struct ArgumentUsesTracker : public CaptureTracker {
   const SCCNodeSet &SCCNodes;
 };
 
+struct ArgumentUse {
+  Use *U;
+  std::optional<int64_t> Offset;
+};
+
+// A struct of argument access info. "Unknown" accesses are the cases like
+// unrecognized instructions, instructions that have more than one use of
+// the argument, or volatile memory accesses. "Unknown" implies "IsClobber"
+// and an empty access range.
+// Write or Read accesses can be clobbers as well for example, a Load with
+// scalable type.
+struct ArgumentAccessInfo {
+  enum AccessType { Write, Read, Unknown };
+  AccessType ArgAccessType;
+  ConstantRangeList AccessRanges;
+  bool IsClobber = false;
+};
+
+struct UsesPerBlockInfo {
+  DenseMap<Instruction *, ArgumentAccessInfo> Insts;
----------------
haopliu wrote:

Oh, good point! Changed the SmallDenseMap size here to 4 and the size of `UsesPerBlock` to 16. 
With 4\*3/4 (<=3) and 16\*3/4 (<=12), they covers 96% and 94% cases in an internal large benchmark. Thanks!

BTW, the size must be a power of 2 so choose 4 and 16. https://github.com/llvm/llvm-project/blob/8f159096e097dde5a95d14984623c51d58ed8e69/llvm/include/llvm/ADT/DenseMap.h#L933


https://github.com/llvm/llvm-project/pull/97373