[llvm] [RISCV][GlobalISel] Select G_GLOBAL_VALUE (PR #70091)

[Craig Topper via llvm-commits]

================
@@ -483,6 +489,66 @@ bool RISCVInstructionSelector::selectConstant(MachineInstr &MI,
   return true;
 }
 
+bool RISCVInstructionSelector::selectGlobalValue(
+    MachineInstr &MI, MachineIRBuilder &MIB, MachineRegisterInfo &MRI) const {
+  assert(MI.getOpcode() == TargetOpcode::G_GLOBAL_VALUE &&
+         "Expected G_GLOBAL_VALUE");
+  auto GV = MI.getOperand(1).getGlobal();
+  if (GV->isThreadLocal()) {
+    // TODO: implement this case.
+    return false;
+  }
+
+  switch (TM.getCodeModel()) {
+  default:
+    report_fatal_error("Unsupported code model for lowering");
+  case CodeModel::Small: {
+    // Must lie within a single 2 GiB address range and must lie between
+    // absolute addresses -2 GiB and +2 GiB. This generates the pattern (addi
+    // (lui %hi(sym)) %lo(sym)).
+    Register AddrHiDest = MRI.createVirtualRegister(&RISCV::GPRRegClass);
+    MachineInstr *AddrHi = MIB.buildInstr(RISCV::LUI)
+                               .addDef(AddrHiDest)
+                               .addGlobalAddress(GV, RISCVII::MO_HI);
+    if (!constrainSelectedInstRegOperands(*AddrHi, TII, TRI, RBI))
+      return false;
+
+    Register DstReg = MI.getOperand(0).getReg();
+    MachineInstr *Result = MIB.buildInstr(RISCV::ADDI)
+                               .addDef(DstReg)
+                               .addReg(AddrHiDest)
+                               .addGlobalAddress(GV, 0, RISCVII::MO_LO);
+    if (!constrainSelectedInstRegOperands(*Result, TII, TRI, RBI))
+      return false;
+
+    MI.eraseFromParent();
+    return true;
+  }
+  case CodeModel::Medium: {
+    // Must lie within *any* 2GiB range. The generates (addi (auipc
+    // %pcrel_hi(sym)) %pcrel_lo(auipc)).
+    Register AddrHiDest = MRI.createVirtualRegister(&RISCV::GPRRegClass);
+    MachineInstr *AddrHi = MIB.buildInstr(RISCV::AUIPC)
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topperc wrote:

This needs to be emitted as PseudoLGA and get expanded later. The pcrel_lo relocation needs to reference a label that points to the AUIPC instruction itself, not the global. We can't do that from inside the instruction selector.

https://github.com/llvm/llvm-project/pull/70091