[llvm] [RISCV][GlobalISel] Create generic instructions to handle legalization and selection of G_GLOBAL_VALUE for medlow code model (PR #69562)

[Matt Arsenault via llvm-commits]

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@@ -176,10 +181,39 @@ RISCVLegalizerInfo::RISCVLegalizerInfo(const RISCVSubtarget &ST) {
 
 bool RISCVLegalizerInfo::legalizeCustom(LegalizerHelper &Helper,
                                         MachineInstr &MI) const {
+  MachineIRBuilder &MIB = Helper.MIRBuilder;
+
   switch (MI.getOpcode()) {
   default:
     // No idea what to do.
     return false;
+  case TargetOpcode::G_GLOBAL_VALUE: {
+    auto &TM = ST->getTargetLowering()->getTargetMachine();
+
+    // FIXME: Handle position independent code.
+    if (TM.isPositionIndependent())
+      return false;
+
+    Register DstReg = MI.getOperand(0).getReg();
+    MachineOperand &GlobalOp = MI.getOperand(1);
+    const GlobalValue *GV = GlobalOp.getGlobal();
+    int64_t Offset = GlobalOp.getOffset();
+
+    switch (TM.getCodeModel()) {
+    case CodeModel::Small: {
+      auto AddrHi = MIB.buildInstr(RISCV::G_RISCV_HI, {LLT::pointer(0, 64)}, {})
+                        .addGlobalAddress(GV, Offset, RISCVII::MO_HI);
+
+      MIB.buildInstr(RISCV::G_RISCV_ADD_LO, {DstReg}, {AddrHi})
+                      .addGlobalAddress(GV, Offset, RISCVII::MO_LO);
+
+      MI.eraseFromParent();
+      return true;
+    }
+    default:
+      llvm_unreachable("Code model not handled!");
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arsenm wrote:

should prefer to fail legalization by returning false instead of asserting 

https://github.com/llvm/llvm-project/pull/69562