[CHERIoT] Mark CGetOffset as unavailable on CHERIoT, and provide an expansion for it.

llvm/lib/Target/RISCV/RISCVFeatures.td

@@ -1390,6 +1390,10 @@ def HasCheriot
     : Predicate<"Subtarget->hasVendorXCheriot()">,
       AssemblerPredicate<(all_of FeatureVendorXCheriot), "CHERIoT Extension">;
 
+def HasNoCheriot : Predicate<"!Subtarget->hasVendorXCheriot()">,
+                   AssemblerPredicate<(all_of(not FeatureVendorXCheriot)),
+                                      "CHERIoT Extension">;
+
 def FeatureRelax
     : SubtargetFeature<"relax", "EnableLinkerRelax", "true",
                        "Enable Linker relaxation.">;

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -10067,6 +10067,25 @@ SDValue RISCVTargetLowering::LowerINTRINSIC_WO_CHAIN(SDValue Op,
     // Expand CFromPtr since the dedicated instruction has been removed.
     return emitCFromPtrReplacement(DAG, DL, Op.getOperand(1), Op.getOperand(2),
                                    Op.getValueType(), XLenVT);
+
+  case Intrinsic::cheri_cap_offset_get: {
+    if (!Subtarget.hasVendorXCheriot())
+      return {};
+
+    // cheri_cap_offset_get is not directly implemented on Cheriot, so expand
+    // it to (sub (address_get), (base_get)). We lower this during operation
+    // legalization for that the subtraction has an opportunity to be
+    // optimized by DAGCombine.
+    auto Cap = Op->getOperand(1);
+    auto Addr = DAG.getNode(
+        ISD::INTRINSIC_WO_CHAIN, DL, XLenVT,
+        DAG.getConstant(llvm::Intrinsic::cheri_cap_address_get, DL, XLenVT),
+        Cap);
+    auto Base = DAG.getNode(
+        ISD::INTRINSIC_WO_CHAIN, DL, XLenVT,
+        DAG.getConstant(llvm::Intrinsic::cheri_cap_base_get, DL, XLenVT), Cap);
+    return DAG.getNode(ISD::SUB, DL, XLenVT, Addr, Base);
+  }
   case Intrinsic::cheri_cap_to_pointer:
     // Expand CToPtr since the dedicated instruction has been removed.
     // NB: DDC/PCC relocation has been removed, so we no longer subtract the

llvm/lib/Target/RISCV/RISCVInstrInfoXCheri.td

@@ -378,7 +378,6 @@ def CGetLen    : Cheri_r<0x3, "cgetlen">;
 let mayLoad = true in
   def CGetTag    : Cheri_r<0x4, "cgettag">;
 def CGetSealed : Cheri_r<0x5, "cgetsealed">;
-def CGetOffset : Cheri_r<0x6, "cgetoffset">;
 def CGetFlags  : Cheri_r<0x7, "cgetflags">;
 def CGetHigh   : Cheri_r<0x17, "cgethigh">;
 def CGetTop    : Cheri_r<0x18, "cgettop">;
@@ -389,6 +388,10 @@ def PseudoCGetAddr : Pseudo<(outs GPR:$rd), (ins GPCR:$cs1), [],
                             "cgetaddr", "$rd, $cs1">;
 }
 
+let Predicates = [HasCheri, HasNoCheriot] in {
+  def CGetOffset : Cheri_r<0x6, "cgetoffset">;
+}
+
 let Predicates = [HasCheriot] in {
   def : InstAlias<"ct.cgetperm $rd, $rs1", (CGetPerm GPR:$rd, GPCR:$rs1)>;
   def : InstAlias<"ct.cgettype $rd, $rs1", (CGetType GPR:$rd, GPCR:$rs1)>;
@@ -1408,7 +1411,8 @@ def : PatGpcr<int_cheri_cap_type_get, CGetType>;
 def : PatGpcr<int_cheri_cap_base_get, CGetBase>;
 def : PatGpcr<int_cheri_cap_length_get, CGetLen>;
 def : PatGpcr<riscv_cap_sealed_get, CGetSealed>;
-def : PatGpcr<int_cheri_cap_offset_get, CGetOffset>;
+let Predicates = [HasNoCheriot] in def : PatGpcr<int_cheri_cap_offset_get,
+                                                 CGetOffset>;
 def : PatGpcr<int_cheri_cap_flags_get, CGetFlags>;
 def : Pat<(XLenVT (int_cheri_cap_address_get GPCR:$cs1)), (PseudoCGetAddr GPCR:$cs1)>;
 def : PatGpcr<int_cheri_cap_high_get, CGetHigh>;

llvm/test/CodeGen/RISCV/cheri/cheriot-offset-get.ll

@@ -0,0 +1,14 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=riscv32cheriot --mcpu=cheriot -target-abi=cheriot -mattr=+xcheri,+cap-mode,+xcheriot -o - %s | FileCheck %s
+
+define i32 @test1(i8 addrspace(200)* %cap) nounwind {
+; CHECK-LABEL: test1:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    ct.cgetbase a1, ca0
+; CHECK-NEXT:    sub a0, a0, a1
+; CHECK-NEXT:    ct.cret
+  %newcap = call i32 @llvm.cheri.cap.offset.get.i32(i8 addrspace(200)* %cap)
+  ret i32 %newcap
+}
+
+declare i32 @llvm.cheri.cap.offset.get.i32(i8 addrspace(200)*)