[llvm] [llvm][transforms] Add a new algorithm to SplitModule (PR #95941)

[Mehdi Amini via llvm-commits]

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@@ -268,6 +268,38 @@ void llvm::SplitModule(
   ClusterIDMapType ClusterIDMap;
   findPartitions(M, ClusterIDMap, N);
 
+  // Find empty modules and functions not mapped to modules in ClusterIDMap.
+  // Map these functions to the empty modules so that they skip being
+  // distributed by isInPartition() based on function name hashes below.
+  // This provides better uniformity of distribution of functions to modules
+  // in some cases - for example when the number of functions equals to N.
+  if (TryToAvoidEmptyModules) {
+    DenseSet<unsigned> NonEmptyModules;
+    SmallVector<const GlobalValue *> UnmappedFunctions;
+    for (const auto &F : M.functions()) {
+      if (F.isDeclaration() ||
+          F.getLinkage() != GlobalValue::LinkageTypes::ExternalLinkage)
+        continue;
+      auto It = ClusterIDMap.find(&F);
+      if (It == ClusterIDMap.end())
+        UnmappedFunctions.push_back(&F);
+      else
+        NonEmptyModules.insert(It->second);
+    }
+    SmallVector<unsigned> EmptyModules;
+    for (unsigned I = 0; I < N; ++I) {
+      if (!NonEmptyModules.contains(I))
+        EmptyModules.push_back(I);
+    }
+    auto NextEmptyModuleIt = EmptyModules.begin();
+    for (const auto F : UnmappedFunctions) {
+      if (NextEmptyModuleIt == EmptyModules.end())
+        break;
+      ClusterIDMap.insert({F, *NextEmptyModuleIt});
+      ++NextEmptyModuleIt;
+    }
----------------
joker-eph wrote:

> Our main use case for this is to request the number of modules equal to the number of functions

I see: that is quite specific, it also makes the algorithm (and flags) only really suited for this case it seems. The name  "TryToAvoidEmptyModules" is explicit, but one may wonder "why would anyone use this algorithm as is?" without knowing the implicit assumption about "the number of modules equal to the number of functions".


https://github.com/llvm/llvm-project/pull/95941