[llvm] [RISCV][GISel] Add manual isel for s16 load/store for the GPR bank. (PR #116111)

[Craig Topper via llvm-commits]

================
@@ -786,6 +803,54 @@ bool RISCVInstructionSelector::select(MachineInstr &MI) {
     return selectMergeValues(MI, MIB);
   case TargetOpcode::G_UNMERGE_VALUES:
     return selectUnmergeValues(MI, MIB);
+  case TargetOpcode::G_LOAD:
+  case TargetOpcode::G_STORE: {
+    GLoadStore &LdSt = cast<GLoadStore>(MI);
+    LLT PtrTy = MRI->getType(LdSt.getPointerReg());
+
+    if (PtrTy != LLT::pointer(0, STI.getXLen())) {
+      LLVM_DEBUG(dbgs() << "Load/Store pointer has type: " << PtrTy
+                        << ", expected: " << LLT::pointer(0, STI.getXLen())
+                        << '\n');
+      return false;
+    }
+
+#ifndef NDEBUG
+    const RegisterBank &PtrRB =
+        *RBI.getRegBank(LdSt.getPointerReg(), *MRI, TRI);
+    // Check that the pointer register is valid.
+    assert(PtrRB.getID() == RISCV::GPRBRegBankID &&
+           "Load/Store pointer operand isn't a GPR");
+#endif
+
+    unsigned MemSizeInBits = LdSt.getMemSizeInBits().getValue();
+
+    const Register ValReg = LdSt.getReg(0);
+    const RegisterBank &RB = *RBI.getRegBank(ValReg, *MRI, TRI);
+
+    const unsigned NewOpc =
+        selectLoadStoreOp(MI.getOpcode(), RB.getID(), MemSizeInBits);
+    if (NewOpc == MI.getOpcode())
+      return false;
+
+    // Check if we can fold anything into the addressing mode.
+    auto AddrModeFns = selectAddrRegImm(MI.getOperand(1));
+    if (!AddrModeFns)
+      return false;
+
+    // Folded something. Create a new instruction and return it.
+    auto NewInst = MIB.buildInstr(NewOpc, {}, {}, MI.getFlags());
+    if (isa<GStore>(MI))
+      NewInst.addUse(ValReg);
+    else
+      NewInst.addDef(ValReg);
+    NewInst.cloneMemRefs(MI);
+    for (auto &Fn : *AddrModeFns)
+      Fn(NewInst);
+    MI.eraseFromParent();
+
+    return constrainSelectedInstRegOperands(*NewInst, TII, TRI, RBI);
----------------
topperc wrote:

I think I've traced it back to `ForceArbitraryInstResultType`. Which just picks the first type in the set. Which is probably ordered by the MVT enum.

```
/// Given a pattern result with an unresolved type, see if we can find one       
/// instruction with an unresolved result type.  Force this result type to an    
/// arbitrary element if it's possible types to converge results.                
static bool ForceArbitraryInstResultType(TreePatternNode &N, TreePattern &TP) {
```

https://github.com/llvm/llvm-project/pull/116111