[llvm] [SPIRV] Support G_IS_FPCLASS (PR #148637)

[Dmitry Sidorov via llvm-commits]

================
@@ -378,6 +385,238 @@ bool SPIRVLegalizerInfo::legalizeCustom(
     }
     return true;
   }
-  // TODO: implement legalization for other opcodes.
+  }
+}
+
+// Note this code was copied from LegalizerHelper::lowerISFPCLASS and adjusted
+// to ensure that all instructions created during the lowering have SPIR-V types
+// assigned to them.
+bool SPIRVLegalizerInfo::legalizeIsFPClass(
+    LegalizerHelper &Helper, MachineInstr &MI,
+    LostDebugLocObserver &LocObserver) const {
+  auto [DstReg, DstTy, SrcReg, SrcTy] = MI.getFirst2RegLLTs();
+  FPClassTest Mask = static_cast<FPClassTest>(MI.getOperand(2).getImm());
+
+  auto &MIRBuilder = Helper.MIRBuilder;
+  auto &MF = MIRBuilder.getMF();
+  MachineRegisterInfo &MRI = MF.getRegInfo();
+
+  Type *LLVMDstTy =
+      IntegerType::get(MIRBuilder.getContext(), DstTy.getScalarSizeInBits());
+  if (DstTy.isVector())
+    LLVMDstTy = VectorType::get(LLVMDstTy, DstTy.getElementCount());
+  SPIRVType *SPIRVDstTy = GR->getOrCreateSPIRVType(
+      LLVMDstTy, MIRBuilder, SPIRV::AccessQualifier::ReadWrite,
+      /*EmitIR*/ true);
+
+  unsigned BitSize = SrcTy.getScalarSizeInBits();
+  const fltSemantics &Semantics = getFltSemanticForLLT(SrcTy.getScalarType());
+
+  LLT IntTy = LLT::scalar(BitSize);
+  Type *LLVMIntTy = IntegerType::get(MIRBuilder.getContext(), BitSize);
+  if (SrcTy.isVector()) {
+    IntTy = LLT::vector(SrcTy.getElementCount(), IntTy);
+    LLVMIntTy = VectorType::get(LLVMIntTy, SrcTy.getElementCount());
+  }
+  SPIRVType *SPIRVIntTy = GR->getOrCreateSPIRVType(
+      LLVMIntTy, MIRBuilder, SPIRV::AccessQualifier::ReadWrite,
+      /*EmitIR*/ true);
+
+  // Clang doesn't support capture of structured bindings:
+  LLT DstTyCopy = DstTy;
+  const auto assignSPIRVTy = [&](MachineInstrBuilder &&MI) {
+    // Assign this MI's (assumed only) destination to one of the two types we
+    // expect: either the G_IS_FPCLASS's destination type, or the integer type
+    // bitcast from the source type.
+    LLT MITy = MRI.getType(MI.getReg(0));
+    assert((MITy == IntTy || MITy == DstTyCopy) &&
+           "Unexpected LLT type while lowering G_IS_FPCLASS");
+    auto *SPVTy = MITy == IntTy ? SPIRVIntTy : SPIRVDstTy;
+    GR->assignSPIRVTypeToVReg(SPVTy, MI.getReg(0), MF);
+    return MI;
+  };
+
+  // Helper to build and assign a constant in one go
+  const auto buildSPIRVConstant = [&](LLT Ty, auto &&C) -> MachineInstrBuilder {
+    if (!Ty.isFixedVector())
+      return assignSPIRVTy(MIRBuilder.buildConstant(Ty, C));
+    auto ScalarC = MIRBuilder.buildConstant(Ty.getScalarType(), C);
+    assert((Ty == IntTy || Ty == DstTyCopy) &&
+           "Unexpected LLT type while lowering constant for G_IS_FPCLASS");
+    SPIRVType *VecEltTy = GR->getOrCreateSPIRVType(
+        (Ty == IntTy ? LLVMIntTy : LLVMDstTy)->getScalarType(), MIRBuilder,
+        SPIRV::AccessQualifier::ReadWrite,
+        /*EmitIR*/ true);
+    GR->assignSPIRVTypeToVReg(VecEltTy, ScalarC.getReg(0), MF);
+    return assignSPIRVTy(MIRBuilder.buildSplatBuildVector(Ty, ScalarC));
+  };
+
+  if (Mask == fcNone) {
+    MIRBuilder.buildCopy(DstReg, buildSPIRVConstant(DstTy, 0));
+    MI.eraseFromParent();
+    return true;
+  }
+  if (Mask == fcAllFlags) {
+    MIRBuilder.buildCopy(DstReg, buildSPIRVConstant(DstTy, 1));
+    MI.eraseFromParent();
+    return true;
+  }
+
+  // Note that rather than creating a COPY here (between a floating-point and
+  // integer type of the same size) we create a SPIR-V bitcast immediately. We
+  // can't create a G_BITCAST because the LLTs are the same, and we can't seem
+  // to correctly lower COPYs to SPIR-V bitcasts at this moment.
+  Register ResVReg = MRI.createGenericVirtualRegister(IntTy);
+  MRI.setRegClass(ResVReg, GR->getRegClass(SPIRVIntTy));
+  GR->assignSPIRVTypeToVReg(SPIRVIntTy, ResVReg, Helper.MIRBuilder.getMF());
+  auto AsInt = MIRBuilder.buildInstr(SPIRV::OpBitcast)
+                   .addDef(ResVReg)
+                   .addUse(GR->getSPIRVTypeID(SPIRVIntTy))
+                   .addUse(SrcReg);
+  AsInt = assignSPIRVTy(std::move(AsInt));
+
+  // Various masks.
+  APInt SignBit = APInt::getSignMask(BitSize);
+  APInt ValueMask = APInt::getSignedMaxValue(BitSize);     // All bits but sign.
+  APInt Inf = APFloat::getInf(Semantics).bitcastToAPInt(); // Exp and int bit.
+  APInt ExpMask = Inf;
+  APInt AllOneMantissa = APFloat::getLargest(Semantics).bitcastToAPInt() & ~Inf;
+  APInt QNaNBitMask =
+      APInt::getOneBitSet(BitSize, AllOneMantissa.getActiveBits() - 1);
+  APInt InversionMask = APInt::getAllOnes(DstTy.getScalarSizeInBits());
+
+  auto SignBitC = buildSPIRVConstant(IntTy, SignBit);
+  auto ValueMaskC = buildSPIRVConstant(IntTy, ValueMask);
+  auto InfC = buildSPIRVConstant(IntTy, Inf);
+  auto ExpMaskC = buildSPIRVConstant(IntTy, ExpMask);
+  auto ZeroC = buildSPIRVConstant(IntTy, 0);
+
+  auto Abs = assignSPIRVTy(MIRBuilder.buildAnd(IntTy, AsInt, ValueMaskC));
+  auto Sign = assignSPIRVTy(
+      MIRBuilder.buildICmp(CmpInst::Predicate::ICMP_NE, DstTy, AsInt, Abs));
+
+  auto Res = buildSPIRVConstant(DstTy, 0);
+
+  const auto appendToRes = [&](MachineInstrBuilder &&ToAppend) {
+    Res = assignSPIRVTy(
+        MIRBuilder.buildOr(DstTyCopy, Res, assignSPIRVTy(std::move(ToAppend))));
+  };
+
+  // Tests that involve more than one class should be processed first.
+  if ((Mask & fcFinite) == fcFinite) {
+    // finite(V) ==> abs(V) u< exp_mask
+    appendToRes(MIRBuilder.buildICmp(CmpInst::Predicate::ICMP_ULT, DstTy, Abs,
----------------
MrSidims wrote:

Probably it's indeed better to reuse existing instructions from SPIR-V, though practically probably after SPIR-V consumption the result would be similar to the current implementation.

In case of rewriting - here is a similar logic in the translator [here](https://github.com/KhronosGroup/SPIRV-LLVM-Translator/blob/main/lib/SPIRV/SPIRVWriter.cpp#L5053), it can be used as a reference, but watch out for bugs (as I've mentioned in a self-asked-self-resolved comment from above, it seems like the translator is not handling correctly sign for the subnormals).

https://github.com/llvm/llvm-project/pull/148637