[SPIRV] Implement translation for llvm.modf.* intrinsics #147556

maarquitos14 · 2025-07-08T15:45:14Z

Based on KhronosGroup/SPIRV-LLVM-Translator#3100, I'm adding translation for llvm.modf.* intrinsics.

maarquitos14 · 2025-07-08T15:45:28Z

@MrSidims please take a look :)

llvmbot · 2025-07-08T15:46:00Z

@llvm/pr-subscribers-backend-spir-v

Author: Marcos Maronas (maarquitos14)

Changes

Based on KhronosGroup/SPIRV-LLVM-Translator#3100, I'm adding translation for llvm.modf.* intrinsics.

Full diff: https://github.com/llvm/llvm-project/pull/147556.diff

2 Files Affected:

(modified) llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp (+76)
(modified) llvm/test/CodeGen/SPIRV/llvm-intrinsics/fp-intrinsics.ll (+22)

diff --git a/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp b/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
index 40a0bd97adaf9..ad6025804608d 100644
--- a/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
+++ b/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
@@ -296,6 +296,8 @@ class SPIRVInstructionSelector : public InstructionSelector {
   bool selectImageWriteIntrinsic(MachineInstr &I) const;
   bool selectResourceGetPointer(Register &ResVReg, const SPIRVType *ResType,
                                 MachineInstr &I) const;
+  bool selectModf(Register ResVReg, const SPIRVType *ResType,
+                  MachineInstr &I) const;
 
   // Utilities
   std::pair<Register, bool>
@@ -3207,6 +3209,9 @@ bool SPIRVInstructionSelector::selectIntrinsic(Register ResVReg,
   case Intrinsic::spv_discard: {
     return selectDiscard(ResVReg, ResType, I);
   }
+  case Intrinsic::modf: {
+    return selectModf(ResVReg, ResType, I);
+  }
   default: {
     std::string DiagMsg;
     raw_string_ostream OS(DiagMsg);
@@ -3990,6 +3995,77 @@ bool SPIRVInstructionSelector::selectLog10(Register ResVReg,
                        .constrainAllUses(TII, TRI, RBI);
 }
 
+bool SPIRVInstructionSelector::selectModf(Register ResVReg,
+                                          const SPIRVType *ResType,
+                                          MachineInstr &I) const {
+  // llvm.modf has a single arg --the number to be decomposed-- and returns a
+  // struct { restype, restype }, while OpenCLLIB::modf has two args --the
+  // number to be decomposed and a pointer--, returns the fractional part and
+  // the integral part is stored in the pointer argument. Therefore, we can't
+  // use directly the OpenCLLIB::modf intrinsic. However, we can do some
+  // scaffolding to make it work. The idea is to create an alloca instruction
+  // to get a ptr, pass this ptr to OpenCL::modf, and then load the value
+  // from this ptr to place it in the struct. llvm.modf returns the fractional
+  // part as the first element of the result, and the integral part as the
+  // second element of the result.
+
+  // At this point, the return type is not a struct anymore, but rather two
+  // independent elements of SPIRVResType. We can get each independent element
+  // from I.getDefs() or I.getOperands().
+  ExtInstList ExtInsts = {{SPIRV::InstructionSet::OpenCL_std, CL::modf},
+                          {SPIRV::InstructionSet::GLSL_std_450, GL::Modf}};
+  for (const auto &Ex : ExtInsts) {
+    SPIRV::InstructionSet::InstructionSet Set = Ex.first;
+    uint32_t Opcode = Ex.second;
+    if (STI.canUseExtInstSet(Set)) {
+      MachineIRBuilder MIRBuilder(I);
+      // Get pointer type for alloca variable.
+      const SPIRVType *PtrType = GR.getOrCreateSPIRVPointerType(
+          ResType, MIRBuilder, SPIRV::StorageClass::Input);
+      // Create new register for the pointer type of alloca variable.
+      Register NewRegister =
+          MIRBuilder.getMRI()->createVirtualRegister(&SPIRV::iIDRegClass);
+      MIRBuilder.getMRI()->setType(NewRegister, LLT::pointer(0, 64));
+      // Assign SPIR-V type of the pointer type of the alloca variable to the
+      // new register.
+      GR.assignSPIRVTypeToVReg(PtrType, NewRegister, MIRBuilder.getMF());
+      // Build the alloca variable.
+      Register Variable = GR.buildGlobalVariable(
+          NewRegister, PtrType, "placeholder", nullptr,
+          SPIRV::StorageClass::Function, nullptr, true, false,
+          SPIRV::LinkageType::Import, MIRBuilder, false);
+      // Modf must have 4 operands, the first two are the 2 parts of the result,
+      // the third is the operand, and the last one is the floating point value.
+      assert(I.getNumOperands() == 4 &&
+             "Expected 4 operands for modf instruction");
+      MachineBasicBlock &BB = *I.getParent();
+      // Create the OpenCLLIB::modf instruction.
+      auto MIB = BuildMI(BB, I, I.getDebugLoc(), TII.get(SPIRV::OpExtInst))
+                     .addDef(ResVReg)
+                     .addUse(GR.getSPIRVTypeID(ResType))
+                     .addImm(static_cast<uint32_t>(Set))
+                     .addImm(Opcode)
+                     .setMIFlags(I.getFlags())
+                     .add(I.getOperand(3)) // Floating point value.
+                     .addUse(Variable);    // Pointer to integral part.
+      // Assign the integral part stored in the ptr to the second element of the
+      // result.
+      Register IntegralPartReg = I.getOperand(1).getReg();
+      if (IntegralPartReg.isValid()) {
+        // Load the value from the pointer to integral part.
+        auto LoadMIB = BuildMI(BB, I, I.getDebugLoc(), TII.get(SPIRV::OpLoad))
+                           .addDef(IntegralPartReg)
+                           .addUse(GR.getSPIRVTypeID(ResType))
+                           .addUse(Variable);
+        return LoadMIB.constrainAllUses(TII, TRI, RBI);
+      }
+
+      return MIB.constrainAllUses(TII, TRI, RBI);
+    }
+  }
+  return false;
+}
+
 // Generate the instructions to load 3-element vector builtin input
 // IDs/Indices.
 // Like: GlobalInvocationId, LocalInvocationId, etc....
diff --git a/llvm/test/CodeGen/SPIRV/llvm-intrinsics/fp-intrinsics.ll b/llvm/test/CodeGen/SPIRV/llvm-intrinsics/fp-intrinsics.ll
index 3d46b527bf14f..91c6174e63d77 100644
--- a/llvm/test/CodeGen/SPIRV/llvm-intrinsics/fp-intrinsics.ll
+++ b/llvm/test/CodeGen/SPIRV/llvm-intrinsics/fp-intrinsics.ll
@@ -337,3 +337,25 @@ entry:
 }
 
 declare float @llvm.fma.f32(float, float, float)
+
+; CHECK: OpFunction
+; CHECK: %[[#d:]] = OpFunctionParameter %[[#]]
+; CHECK: %[[#fracPtr:]] = OpFunctionParameter %[[#]]
+; CHECK: %[[#integralPtr:]] = OpFunctionParameter %[[#]]
+; CHECK: %[[#varPtr:]] = OpVariable %[[#]] Function
+; CHECK: %[[#frac:]] = OpExtInst %[[#var2]] %[[#extinst_id]] modf %[[#d]] %[[#varPtr]]
+; CHECK: %[[#integral:]] = OpLoad %[[#var2]] %[[#varPtr]]
+; CHECK: OpStore %[[#fracPtr]] %[[#frac]]
+; CHECK: OpStore %[[#integralPtr]] %[[#integral]]
+; CHECK: OpFunctionEnd
+define void @foo(double %d, ptr addrspace(1) %frac, ptr addrspace(1) %integral) {
+entry:
+  %4 = tail call { double, double } @llvm.modf.f64(double %d)
+  %5 = extractvalue { double, double } %4, 0
+  %6 = extractvalue { double, double } %4, 1
+  store double %5, ptr addrspace(1) %frac, align 8
+  store double %6, ptr addrspace(1) %integral, align 8
+  ret void
+}
+
+declare { double, double } @llvm.modf.f64(double)

Naghasan · 2025-07-08T16:02:34Z

llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp

+  for (const auto &Ex : ExtInsts) {
+    SPIRV::InstructionSet::InstructionSet Set = Ex.first;
+    uint32_t Opcode = Ex.second;


why a loop and no a if / else to init Opcode and Set ? Looks a bit convoluted for no reason

Agree with Victor, that it's probably better to use if/else here, but for a different reason. From https://registry.khronos.org/SPIR-V/specs/unified1/GLSL.std.450.html : Modf is deprecated, use ModfStruct instead. and ModfStruct has a different semantic. I don't have strong opinion whether to generate ModfStruct in this patch right away (up to you to decide), but obviously handling for ModfStruct will be different.

BTW, please leave FIXME comment in case if you don't want to generate ModfStruct right away.

well spotted, ModfStruct was added in version 0.99, so IMO there is no reason to use GL::Modf at all

Naghasan · 2025-07-09T06:53:46Z

llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp

+      MachineIRBuilder MIRBuilder(I);
+      // Get pointer type for alloca variable.
+      const SPIRVType *PtrType = GR.getOrCreateSPIRVPointerType(
+          ResType, MIRBuilder, SPIRV::StorageClass::Input);


Suggested change

ResType, MIRBuilder, SPIRV::StorageClass::Input);

ResType, MIRBuilder, SPIRV::StorageClass::Function);

Naghasan · 2025-07-09T06:55:50Z

llvm/test/CodeGen/SPIRV/llvm-intrinsics/fp-intrinsics.ll

+; CHECK: %[[#d:]] = OpFunctionParameter %[[#]]
+; CHECK: %[[#fracPtr:]] = OpFunctionParameter %[[#]]
+; CHECK: %[[#integralPtr:]] = OpFunctionParameter %[[#]]
+; CHECK: %[[#varPtr:]] = OpVariable %[[#]] Function


can you add a spirv-val check for the test as well, the pointer was ill-formed and nothing caught it

Naghasan · 2025-07-09T07:00:13Z

llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp

+          SPIRV::StorageClass::Function, nullptr, true, false,
+          SPIRV::LinkageType::Import, MIRBuilder, false);


can you add comments to the bool params please (https://llvm.org/docs/CodingStandards.html#comment-formatting) ?

Naghasan · 2025-07-09T07:07:14Z

llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp

+      // new register.
+      GR.assignSPIRVTypeToVReg(PtrType, NewRegister, MIRBuilder.getMF());
+      // Build the alloca variable.
+      Register Variable = GR.buildGlobalVariable(


I think this works because you have 1 call to the intrinsics in your test, this is the first thing the function does

Module *M = MIRBuilder.getMF().getFunction().getParent(); GVar = M->getGlobalVariable(Name); if (GVar == nullptr) { const Type *Ty = getTypeForSPIRVType(BaseType); // TODO: check type. // Module takes ownership of the global var. GVar = new GlobalVariable(*M, const_cast<Type *>(Ty), false, GlobalValue::ExternalLinkage, nullptr, Twine(Name)); }

You probably going to end up with strange things if you have 2 calls in 2 distinct functions.

Naghasan · 2025-07-09T07:08:48Z

llvm/test/CodeGen/SPIRV/llvm-intrinsics/fp-intrinsics.ll

+; CHECK: OpFunctionEnd
+define void @foo(double %d, ptr addrspace(1) %frac, ptr addrspace(1) %integral) {
+entry:
+  %4 = tail call { double, double } @llvm.modf.f64(double %d)


I'd like to see a test where llvm.modf is called outside the entry block (SPIR-V mandates the OpVariable to be in the entry block)

Naghasan · 2025-07-09T07:10:28Z

llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp

+      // Create new register for the pointer type of alloca variable.
+      Register NewRegister =
+          MIRBuilder.getMRI()->createVirtualRegister(&SPIRV::iIDRegClass);
+      MIRBuilder.getMRI()->setType(NewRegister, LLT::pointer(0, 64));


Suggested change

MIRBuilder.getMRI()->setType(NewRegister, LLT::pointer(0, 64));

MIRBuilder.getMRI()->setType(NewRegister, LLT::pointer(storageClassToAddressSpace(SPIRV::StorageClass::Function), GR->getPointerSize()));

MrSidims · 2025-07-09T12:57:47Z

llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp

+      assert(I.getNumOperands() == 4 &&
+             "Expected 4 operands for modf instruction");
+      MachineBasicBlock &BB = *I.getParent();
+      // Create the OpenCLLIB::modf instruction.


nit: Not necessary OpenCLLIB:: as the code also handles GLSL_std_450

MrSidims · 2025-07-09T13:03:25Z

llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp

+  for (const auto &Ex : ExtInsts) {
+    SPIRV::InstructionSet::InstructionSet Set = Ex.first;
+    uint32_t Opcode = Ex.second;


Agree with Victor, that it's probably better to use if/else here, but for a different reason. From https://registry.khronos.org/SPIR-V/specs/unified1/GLSL.std.450.html : Modf is deprecated, use ModfStruct instead. and ModfStruct has a different semantic. I don't have strong opinion whether to generate ModfStruct in this patch right away (up to you to decide), but obviously handling for ModfStruct will be different.

BTW, please leave FIXME comment in case if you don't want to generate ModfStruct right away.

maarquitos14 added 2 commits July 8, 2025 17:08

Implement translation for llvm.modf.* intrinsics.

5bfcf33

Add testing for llvm.modf.* intrinsics.

7d87ed3

llvmbot added the backend:SPIR-V label Jul 8, 2025

Naghasan requested changes Jul 9, 2025

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MrSidims reviewed Jul 9, 2025

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[SPIRV] Implement translation for llvm.modf.* intrinsics #147556

[SPIRV] Implement translation for llvm.modf.* intrinsics #147556

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	ResType, MIRBuilder, SPIRV::StorageClass::Input);
	ResType, MIRBuilder, SPIRV::StorageClass::Function);

		SPIRV::StorageClass::Function, nullptr, true, false,
		SPIRV::LinkageType::Import, MIRBuilder, false);

	MIRBuilder.getMRI()->setType(NewRegister, LLT::pointer(0, 64));
	MIRBuilder.getMRI()->setType(NewRegister, LLT::pointer(storageClassToAddressSpace(SPIRV::StorageClass::Function), GR->getPointerSize()));

[SPIRV] Implement translation for llvm.modf.* intrinsics #147556

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[SPIRV] Implement translation for llvm.modf.* intrinsics #147556

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