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

[Sam Tebbs via llvm-commits]

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@@ -4878,8 +4879,232 @@ 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) {
+  // 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.
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SamTebbs33 wrote:

register -> registers.

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