[llvm] [LV] Compute register usage for interleaving on VPlan. (PR #126437)

[Sander de Smalen via llvm-commits]

================
@@ -4871,8 +4872,242 @@ void LoopVectorizationCostModel::collectElementTypesForWidening() {
   }
 }
 
+/// Estimate the register usage for \p Plan and vectorization factors in \p VFs.
+/// Returns the register usage for each VF in \p VFs.
+static SmallVector<LoopVectorizationCostModel::RegisterUsage, 8>
+calculateRegisterUsage(VPlan &Plan, ArrayRef<ElementCount> VFs,
+                       const TargetTransformInfo &TTI,
+                       const SmallPtrSetImpl<const Value *> &ValuesToIgnore) {
+  // This function calculates the register usage by measuring the highest number
+  // of values that are alive at a single location. Obviously, this is a very
+  // rough estimation. We scan the loop in a topological order in order and
+  // assign a number to each recipe. We use RPO to ensure that defs are
+  // met before their users. We assume that each recipe that has in-loop
+  // users starts an interval. We record every time that an in-loop value is
+  // used, so we have a list of the first and last occurrences of each
+  // recipe. Next, we transpose this data structure into a multi map that
+  // holds the list of intervals that *end* at a specific location. This multi
+  // map allows us to perform a linear search. We scan the instructions linearly
+  // and record each time that a new interval starts, by placing it in a set.
+  // If we find this value in the multi-map then we remove it from the set.
+  // The max register usage is the maximum size of the set.
+  // We also search for instructions that are defined outside the loop, but are
+  // used inside the loop. We need this number separately from the max-interval
+  // usage number because when we unroll, loop-invariant values do not take
+  // more register.
----------------
sdesmalen-arm wrote:

Thanks, that helps! Part of my concern was that the detailed description at the top of the function might deviate over time from the implementation. Having the comments split up and closer to the code, makes them more likely to get updated.

https://github.com/llvm/llvm-project/pull/126437