[llvm] [AArch64] Legalize fp128 scalars as libcalls for G_FCMP (PR #98452)

[David Green via llvm-commits]

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@@ -980,6 +979,148 @@ LegalizerHelper::createSetStateLibcall(MachineIRBuilder &MIRBuilder,
                        LocObserver, nullptr);
 }
 
+/// Returns the corresponding libcall for the given Pred and
+/// the ICMP predicate that should be generated to compare with #0
+/// after the libcall.
+static std::pair<RTLIB::Libcall, CmpInst::Predicate>
+getFCMPLibcallDesc(const CmpInst::Predicate Pred) {
+
+  switch (Pred) {
+  case CmpInst::FCMP_OEQ:
+    return {RTLIB::OEQ_F128, CmpInst::ICMP_EQ};
+  case CmpInst::FCMP_UNE:
+    return {RTLIB::UNE_F128, CmpInst::ICMP_NE};
+  case CmpInst::FCMP_OGE:
+    return {RTLIB::OGE_F128, CmpInst::ICMP_SGE};
+  case CmpInst::FCMP_OLT:
+    return {RTLIB::OLT_F128, CmpInst::ICMP_SLT};
+  case CmpInst::FCMP_OLE:
+    return {RTLIB::OLE_F128, CmpInst::ICMP_SLE};
+  case CmpInst::FCMP_OGT:
+    return {RTLIB::OGT_F128, CmpInst::ICMP_SGT};
+  case CmpInst::FCMP_UNO:
+    return {RTLIB::UO_F128, CmpInst::ICMP_NE};
+  default:
+    return {RTLIB::UNKNOWN_LIBCALL, CmpInst::BAD_ICMP_PREDICATE};
+  }
+}
+
+LegalizerHelper::LegalizeResult
+LegalizerHelper::createFCMPLibcall(MachineIRBuilder &MIRBuilder,
+                                   MachineInstr &MI,
+                                   LostDebugLocObserver &LocObserver) {
+  auto &MF = MIRBuilder.getMF();
+  auto &Ctx = MF.getFunction().getContext();
+  const GFCmp *Cmp = cast<GFCmp>(&MI);
+
+  LLT OpLLT = MRI.getType(Cmp->getLHSReg());
+  if (OpLLT != LLT::scalar(128) || OpLLT != MRI.getType(Cmp->getRHSReg()))
+    return UnableToLegalize;
+
+  Type *OpType = getFloatTypeForLLT(Ctx, OpLLT);
+
+  // Libcall always return i32
+  constexpr LLT I32LLT = LLT::scalar(32);
+  constexpr LLT PredTy = LLT::scalar(1);
+
+  const Register DstReg = Cmp->getReg(0);
+  const Register Op1 = Cmp->getLHSReg();
+  const Register Op2 = Cmp->getRHSReg();
+  const auto Cond = Cmp->getCond();
+
+  // Generates a libcall followed by ICMP
+  const auto BuildLibcall = [&](const RTLIB::Libcall Libcall,
+                                const CmpInst::Predicate ICmpPred) -> Register {
+    Register Temp = MRI.createGenericVirtualRegister(I32LLT);
+    // Generate libcall, storing result into Temp
+    const auto Status =
+        createLibcall(MIRBuilder, Libcall, {Temp, Type::getInt32Ty(Ctx), 0},
+                      {{Op1, OpType, 0}, {Op2, OpType, 1}}, LocObserver, &MI);
+    if (!Status)
+      return {};
+
+    // FCMP libcall always returns an i32, we need to compare it with #0 to get
+    // the final result.
+    return MIRBuilder
+        .buildICmp(ICmpPred, PredTy, Temp, MIRBuilder.buildConstant(I32LLT, 0))
+        .getReg(0);
+  };
+
+  // Simple case if we have a direct mapping from predicate to libcall
+  if (const auto [Libcall, ICmpPred] = getFCMPLibcallDesc(Cond);
+      Libcall != RTLIB::UNKNOWN_LIBCALL &&
+      ICmpPred != CmpInst::BAD_ICMP_PREDICATE) {
+    if (const auto Res = BuildLibcall(Libcall, ICmpPred)) {
+      MIRBuilder.buildCopy(DstReg, Res);
----------------
davemgreen wrote:

If the type of the function is always an i32, might that need to be zext/sext/trunc to the predicate type?

https://github.com/llvm/llvm-project/pull/98452