[llvm] [ReachingDefAnalysis] Extend the analysis to stack objects. (PR #118097)

[Michael Maitland via llvm-commits]

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@@ -138,6 +141,13 @@ class ReachingDefAnalysis : public MachineFunctionPass {
   DenseMap<MachineInstr *, int> InstIds;
 
   MBBReachingDefsInfo MBBReachingDefs;
+  using MBBFrameObjsReachingDefsInfo =
+      std::vector<std::vector<std::vector<int>>>;
+  // MBBFrameObjsReachingDefs[i][j] is a list of instruction indicies (relative
+  // to begining of MBB) that define frame index (j +
+  // MF->getFrameInfo().getObjectIndexBegin()) in MBB i. This is used in
+  // answering reaching defenition queries.
+  MBBFrameObjsReachingDefsInfo MBBFrameObjsReachingDefs;
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michaelmaitland wrote:

We need to rethink this data structure based on @nikic's findings about memory usage.

Some points that may be helpful in reworking this:
* Use `empty()` instead of `-1`. See https://github.com/llvm/llvm-project/pull/118097#discussion_r1863677010
* NumStackEntries entries are allocated for every single MBB. We can probably optimize this to only the number of instructions that define a frame index in that MBB?
* Do we need to track this information for every single basic block, especially when it doesn't interact with frame objects?

https://github.com/llvm/llvm-project/pull/118097