[llvm] [CodeGen] Add OffloadBlockUniformityAnalysis for offload PGO (PR #178417)

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,117 @@
+//===- OffloadBlockUniformity.cpp - Offload block uniformity info --------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/OffloadBlockUniformity.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/IR/Metadata.h"
+#include "llvm/Support/raw_ostream.h"
+#include <optional>
+
+using namespace llvm;
+
+static std::optional<bool> getIRBlockUniformity(const BasicBlock &BB) {
+  const Instruction *TI = BB.getTerminator();
+  if (!TI)
+    return std::nullopt;
+
+  MDNode *MD = TI->getMetadata(OffloadBlockUniformityInfo::MetadataName);
+  if (!MD)
+    return std::nullopt;
+
+  // Metadata format: !{i1 IsUniform} - structural validity assumed (verifier).
+  return mdconst::extract<ConstantInt>(MD->getOperand(0))->isOne();
+}
+
+void OffloadBlockUniformityInfo::compute(const MachineFunction &MF) {
+  HasAnyUniformity = false;
+  DivergentBlocks.clear();
+  DivergentBlocks.resize(MF.getNumBlockIDs());
+
+  // First determine whether any uniformity annotation exists for this function.
+  for (const MachineBasicBlock &MBB : MF) {
+    const BasicBlock *BB = MBB.getBasicBlock();
+    if (!BB)
+      continue;
+    if (getIRBlockUniformity(*BB).has_value()) {
+      HasAnyUniformity = true;
+      break;
+    }
+  }
+
+  if (!HasAnyUniformity)
+    return;
+
+  // Conservative behavior: if uniformity exists for the function but we cannot
+  // classify a particular (Machine)basic block, treat it as divergent.
+  for (const MachineBasicBlock &MBB : MF) {
+    const unsigned Num = MBB.getNumber();
+    bool IsDivergent = true;
+    if (const BasicBlock *BB = MBB.getBasicBlock()) {
+      if (auto U = getIRBlockUniformity(*BB))
+        IsDivergent = !*U;
+    }
+    if (Num < DivergentBlocks.size() && IsDivergent)
+      DivergentBlocks.set(Num);
+  }
+}
+
+void OffloadBlockUniformityInfo::print(raw_ostream &OS,
----------------
arsenm wrote:

This printing is going to be misleading for cases that are statically known uniform and don't have the metadata annotation 

https://github.com/llvm/llvm-project/pull/178417