[llvm] [RISCV][GISel] Add FP calling convention support (PR #69138)

[Brandon Wu via llvm-commits]

================
@@ -93,11 +93,59 @@ struct RISCVOutgoingValueHandler : public CallLowering::OutgoingValueHandler {
 
   void assignValueToReg(Register ValVReg, Register PhysReg,
                         CCValAssign VA) override {
+    // If we're passing an f32 value into an i64, anyextend before copying.
+    if (VA.getLocVT() == MVT::i64 && VA.getValVT() == MVT::f32)
+      ValVReg = MIRBuilder.buildAnyExt(LLT::scalar(64), ValVReg).getReg(0);
+
     Register ExtReg = extendRegister(ValVReg, VA);
     MIRBuilder.buildCopy(PhysReg, ExtReg);
     MIB.addUse(PhysReg, RegState::Implicit);
   }
 
+  unsigned assignCustomValue(CallLowering::ArgInfo &Arg,
+                             ArrayRef<CCValAssign> VAs,
+                             std::function<void()> *Thunk) override {
+    assert(VAs.size() >= 2 && "Expected at least 2 VAs.");
+    const CCValAssign &VALo = VAs[0];
+    const CCValAssign &VAHi = VAs[1];
+
+    assert(VAHi.needsCustom() && "Value doesn't need custom handling");
+    assert(VALo.getValNo() == VAHi.getValNo() &&
+           "Values belong to different arguments");
+
+    assert(VALo.getLocVT() == MVT::i32 && VAHi.getLocVT() == MVT::i32 &&
+           VALo.getValVT() == MVT::f64 && VAHi.getValVT() == MVT::f64 &&
+           "unexpected custom value");
+
+    Register NewRegs[] = {MRI.createGenericVirtualRegister(LLT::scalar(32)),
+                          MRI.createGenericVirtualRegister(LLT::scalar(32))};
+    MIRBuilder.buildUnmerge(NewRegs, Arg.Regs[0]);
+
+    if (VAHi.isMemLoc()) {
+      LLT MemTy(VAHi.getLocVT());
+
+      MachinePointerInfo MPO;
+      Register StackAddr = getStackAddress(
+          MemTy.getSizeInBytes(), VAHi.getLocMemOffset(), MPO, Arg.Flags[0]);
+
+      assignValueToAddress(NewRegs[1], StackAddr, MemTy, MPO,
+                           const_cast<CCValAssign &>(VAHi));
+    }
+
+    if (Thunk) {
+      *Thunk = [=]() {
----------------
4vtomat wrote:

One minor comment is that since Thunk is the same as the logic after the if condition, can we fold them and replace with the same lambda call?
for example,
```
auto AssignFunc = [=]() {
    assignValueToReg(NewRegs[0], VALo.getLocReg(), VALo);
     if (VAHi.isRegLoc())
       assignValueToReg(NewRegs[1], VAHi.getLocReg(), VAHi);
}

if (Thunk) {
    *Thunk = AssignFunc;
    return 1;
}

AssignFunc();
return 1;
```
something like that~

https://github.com/llvm/llvm-project/pull/69138