[llvm] [LoongArch] Pass 'half' in the lower 16 bits of an f32 value with F/D ABI (PR #109368)

[via llvm-commits]

================
@@ -1354,6 +1358,40 @@ SDValue LoongArchTargetLowering::lowerVECTOR_SHUFFLE(SDValue Op,
   return SDValue();
 }
 
+SDValue LoongArchTargetLowering::lowerFP_TO_FP16(SDValue Op,
+                                                 SelectionDAG &DAG) const {
+  // Custom lower to ensure the libcall return is passed in an FPR on hard
+  // float ABIs.
+  SDLoc DL(Op);
+  MakeLibCallOptions CallOptions;
+  SDValue Op0 = Op.getOperand(0);
+  SDValue Chain = SDValue();
+  RTLIB::Libcall LC = RTLIB::getFPROUND(Op0.getValueType(), MVT::f16);
+  SDValue Res;
+  std::tie(Res, Chain) =
+      makeLibCall(DAG, LC, MVT::f32, Op0, CallOptions, DL, Chain);
+  if (Subtarget.is64Bit())
+    return DAG.getNode(LoongArchISD::MOVFR2GR_S_LA64, DL, MVT::i64, Res);
+  return DAG.getBitcast(MVT::i32, Res);
+}
+
+SDValue LoongArchTargetLowering::lowerFP16_TO_FP(SDValue Op,
+                                                 SelectionDAG &DAG) const {
+  // Custom lower to ensure the libcall argument is passed in an FPR on hard
+  // float ABIs.
----------------
heiher wrote:

In my view, `__gnu_h2f_ieee` is primarily designed for converting `f16` values to `f32`, particularly in scenarios where hardware lacks native `f16` support, requiring software emulation instead. This function does not inherently define any specific argument-passing conventions; rather, those are determined by the architecture's ABI. For instance, on RISC-V, the lower 16 bits of a floating point argument register are used (given that hardware support for `f16` is not enabled), while some other architectures use integer registers. In terms of implementation, `__extendhfsf2` serves as an alias for `__gnu_h2f_ieee` and are used by arm.

https://github.com/rust-lang/compiler-builtins/blob/compiler_builtins-v0.1.126/src/float/extend.rs#L87-L89

```rust
pub extern "C" fn __extendhfsf2(a: f16) -> f32 {
    extend(a)
}
```

https://github.com/rust-lang/compiler-builtins/blob/compiler_builtins-v0.1.126/src/float/extend.rs#L95-L97

```rust
pub extern "C" fn __gnu_h2f_ieee(a: f16) -> f32 {
    extend(a)
}
```

https://github.com/llvm/llvm-project/pull/109368