[RegAlloc] Add scaffold for -regalloc=segmenttree (NFC)

Adds a minimal factory + regalloc registration for .
Currently delegates to Greedy allocator. No functional change.

Author: steven-studio

llvm/include/llvm/CodeGen/RegAllocSegmentTree.h

@@ -0,0 +1,17 @@
+//===- RegAllocSegmentTree.h - RA segtree scaffold --------------*- C++ -*-===//
+//
+// This file declares createRegAllocSegmentTree() factory (scaffold only).
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_REGALLOCSEGMENTTREE_H
+#define LLVM_CODEGEN_REGALLOCSEGMENTTREE_H
+
+namespace llvm {
+class FunctionPass;
+
+// Factory (for -regalloc=segtre). For now it delegates to Greedy (NFC).
+FunctionPass *createRegAllocSegmentTree();
+} // end namespace llvm
+
+#endif

llvm/lib/CodeGen/CodeGen.cpp

@@ -113,6 +113,7 @@ void llvm::initializeCodeGen(PassRegistry &Registry) {
   initializeRABasicPass(Registry);
   initializeRAGreedyLegacyPass(Registry);
   initializeRegAllocFastPass(Registry);
+  // initializeRegAllocSegmentTreePass(Registry);  // Add this line
   initializeRegUsageInfoCollectorLegacyPass(Registry);
   initializeRegUsageInfoPropagationLegacyPass(Registry);
   initializeRegisterCoalescerLegacyPass(Registry);

llvm/lib/CodeGen/RegAllocSegmentTree.cpp

@@ -0,0 +1,21 @@
+//===- RegAllocSegmentTree.cpp - RA segtree scaffold ----------------------===//
+//
+// Scaffold only: register -regalloc=segmenttree that delegates to Greedy (NFC).
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/RegAllocSegmentTree.h"
+#include "llvm/CodeGen/RegAllocRegistry.h"
+
+using namespace llvm;
+
+// 工廠：目前直接委派回 Greedy（零行為變更）
+FunctionPass *llvm::createRegAllocSegmentTree() {
+  extern FunctionPass *createGreedyRegisterAllocator();
+  return createGreedyRegisterAllocator();
+}
+
+// 把選項掛進 -regalloc= 名單（名稱、描述、工廠）
+static RegisterRegAlloc
+    RAReg("segmenttree", "Segment Tree Register Allocator (scaffold)",
+          createRegAllocSegmentTree);