[SPIR-V] Emit OpUndef for undefined values (#6686)

Before this change, OpConstantNull was emitted when an undef value was
required.
This causes an issue for some types which cannot have the OpConstantNull
value.

In addition, it mixed well-defined values with undefined values, which
prevents any kind of optimization/analysis later on.

Fixes #6653

---------

Signed-off-by: Nathan Gauër <brioche@google.com>

tools/clang/include/clang/SPIRV/SpirvBuilder.h

@@ -748,6 +748,7 @@ public:
                        llvm::ArrayRef<SpirvConstant *> constituents,
                        bool specConst = false);
   SpirvConstant *getConstantNull(QualType);
+  SpirvUndef *getUndef(QualType);
 
   SpirvString *createString(llvm::StringRef str);
   SpirvString *getString(llvm::StringRef str);

tools/clang/include/clang/SPIRV/SpirvInstruction.h

@@ -67,6 +67,9 @@ public:
     IK_ConstantComposite,
     IK_ConstantNull,
 
+    // OpUndef
+    IK_Undef,
+
     // Function structure kinds
 
     IK_FunctionParameter, // OpFunctionParameter
@@ -1302,6 +1305,22 @@ public:
   bool operator==(const SpirvConstantNull &that) const;
 };
 
+class SpirvUndef : public SpirvInstruction {
+public:
+  SpirvUndef(QualType type);
+
+  DEFINE_RELEASE_MEMORY_FOR_CLASS(SpirvUndef)
+
+  // For LLVM-style RTTI
+  static bool classof(const SpirvInstruction *inst) {
+    return inst->getKind() == IK_Undef;
+  }
+
+  bool operator==(const SpirvUndef &that) const;
+
+  bool invokeVisitor(Visitor *v) override;
+};
+
 /// \brief OpCompositeConstruct instruction
 class SpirvCompositeConstruct : public SpirvInstruction {
 public:

tools/clang/include/clang/SPIRV/SpirvModule.h

@@ -142,6 +142,9 @@ public:
   // Adds a constant to the module.
   void addConstant(SpirvConstant *);
 
+  // Adds an Undef to the module.
+  void addUndef(SpirvUndef *);
+
   // Adds given string to the module which will be emitted via OpString.
   void addString(SpirvString *);
 
@@ -202,6 +205,7 @@ private:
       decorations;
 
   std::vector<SpirvConstant *> constants;
+  std::vector<SpirvUndef *> undefs;
   std::vector<SpirvVariable *> variables;
   // A vector of functions in the module in the order that they should be
   // emitted. The order starts with the entry-point function followed by a

tools/clang/include/clang/SPIRV/SpirvVisitor.h

@@ -89,6 +89,7 @@ public:
   DEFINE_VISIT_METHOD(SpirvConstantFloat)
   DEFINE_VISIT_METHOD(SpirvConstantComposite)
   DEFINE_VISIT_METHOD(SpirvConstantNull)
+  DEFINE_VISIT_METHOD(SpirvUndef)
   DEFINE_VISIT_METHOD(SpirvCompositeConstruct)
   DEFINE_VISIT_METHOD(SpirvCompositeExtract)
   DEFINE_VISIT_METHOD(SpirvCompositeInsert)

tools/clang/lib/SPIRV/EmitVisitor.cpp

@@ -1006,6 +1006,13 @@ bool EmitVisitor::visit(SpirvConstantNull *inst) {
   return true;
 }
 
+bool EmitVisitor::visit(SpirvUndef *inst) {
+  typeHandler.getOrCreateUndef(inst);
+  emitDebugNameForInstruction(getOrAssignResultId<SpirvInstruction>(inst),
+                              inst->getDebugName());
+  return true;
+}
+
 bool EmitVisitor::visit(SpirvCompositeConstruct *inst) {
   initInstruction(inst);
   curInst.push_back(inst->getResultTypeId());
@@ -2010,6 +2017,8 @@ uint32_t EmitTypeHandler::getOrCreateConstant(SpirvConstant *inst) {
     return getOrCreateConstantNull(constNull);
   } else if (auto *constBool = dyn_cast<SpirvConstantBoolean>(inst)) {
     return getOrCreateConstantBool(constBool);
+  } else if (auto *constUndef = dyn_cast<SpirvUndef>(inst)) {
+    return getOrCreateUndef(constUndef);
   }
 
   llvm_unreachable("cannot emit unknown constant type");
@@ -2070,6 +2079,31 @@ uint32_t EmitTypeHandler::getOrCreateConstantNull(SpirvConstantNull *inst) {
   return inst->getResultId();
 }
 
+uint32_t EmitTypeHandler::getOrCreateUndef(SpirvUndef *inst) {
+  auto canonicalType = inst->getAstResultType().getCanonicalType();
+  auto found = std::find_if(
+      emittedUndef.begin(), emittedUndef.end(),
+      [canonicalType](SpirvUndef *cached) {
+        return cached->getAstResultType().getCanonicalType() == canonicalType;
+      });
+
+  if (found != emittedUndef.end()) {
+    // We have already emitted this constant. Reuse.
+    inst->setResultId((*found)->getResultId());
+    return inst->getResultId();
+  }
+
+  // Constant wasn't emitted in the past.
+  const uint32_t typeId = emitType(inst->getResultType());
+  initTypeInstruction(inst->getopcode());
+  curTypeInst.push_back(typeId);
+  curTypeInst.push_back(getOrAssignResultId<SpirvInstruction>(inst));
+  finalizeTypeInstruction();
+  // Remember this constant for the future
+  emittedUndef.push_back(inst);
+  return inst->getResultId();
+}
+
 uint32_t EmitTypeHandler::getOrCreateConstantFloat(SpirvConstantFloat *inst) {
   llvm::APFloat value = inst->getValue();
   const SpirvType *type = inst->getResultType();

tools/clang/lib/SPIRV/EmitVisitor.h

@@ -57,7 +57,7 @@ public:
         typeConstantBinary(typesVec), takeNextIdFunction(takeNextIdFn),
         emittedConstantInts({}), emittedConstantFloats({}),
         emittedConstantComposites({}), emittedConstantNulls({}),
-        emittedConstantBools() {
+        emittedUndef({}), emittedConstantBools() {
     assert(decVec);
     assert(typesVec);
   }
@@ -107,6 +107,7 @@ public:
   uint32_t getOrCreateConstantFloat(SpirvConstantFloat *);
   uint32_t getOrCreateConstantComposite(SpirvConstantComposite *);
   uint32_t getOrCreateConstantNull(SpirvConstantNull *);
+  uint32_t getOrCreateUndef(SpirvUndef *);
   uint32_t getOrCreateConstantBool(SpirvConstantBoolean *);
   template <typename vecType>
   void emitLiteral(const SpirvConstant *, vecType &outInst);
@@ -172,6 +173,7 @@ private:
       emittedConstantFloats;
   llvm::SmallVector<SpirvConstantComposite *, 8> emittedConstantComposites;
   llvm::SmallVector<SpirvConstantNull *, 8> emittedConstantNulls;
+  llvm::SmallVector<SpirvUndef *, 8> emittedUndef;
   SpirvConstantBoolean *emittedConstantBools[2];
   llvm::DenseSet<const SpirvInstruction *> emittedSpecConstantInstructions;
 
@@ -252,6 +254,7 @@ public:
   bool visit(SpirvConstantFloat *) override;
   bool visit(SpirvConstantComposite *) override;
   bool visit(SpirvConstantNull *) override;
+  bool visit(SpirvUndef *) override;
   bool visit(SpirvCompositeConstruct *) override;
   bool visit(SpirvCompositeExtract *) override;
   bool visit(SpirvCompositeInsert *) override;

tools/clang/lib/SPIRV/SpirvBuilder.cpp

@@ -1826,6 +1826,13 @@ SpirvConstant *SpirvBuilder::getConstantNull(QualType type) {
   return nullConst;
 }
 
+SpirvUndef *SpirvBuilder::getUndef(QualType type) {
+  // We do not care about making unique constants at this point.
+  auto *undef = new (context) SpirvUndef(type);
+  mod->addUndef(undef);
+  return undef;
+}
+
 SpirvString *SpirvBuilder::createString(llvm::StringRef str) {
   // Create a SpirvString instruction
   auto *instr = new (context) SpirvString(/* SourceLocation */ {}, str);

tools/clang/lib/SPIRV/SpirvEmitter.cpp

@@ -1517,10 +1517,9 @@ void SpirvEmitter::doFunctionDecl(const FunctionDecl *decl) {
         spvBuilder.createReturn(returnLoc);
       } else {
         // If the source code does not provide a proper return value for some
-        // control flow path, it's undefined behavior. We just return null
-        // value here.
-        spvBuilder.createReturnValue(spvBuilder.getConstantNull(retType),
-                                     returnLoc);
+        // control flow path, it's undefined behavior. We just return an
+        // undefined value here.
+        spvBuilder.createReturnValue(spvBuilder.getUndef(retType), returnLoc);
       }
     }
   }

tools/clang/lib/SPIRV/SpirvInstruction.cpp

@@ -57,6 +57,7 @@ DEFINE_INVOKE_VISITOR_FOR_CLASS(SpirvConstantInteger)
 DEFINE_INVOKE_VISITOR_FOR_CLASS(SpirvConstantFloat)
 DEFINE_INVOKE_VISITOR_FOR_CLASS(SpirvConstantComposite)
 DEFINE_INVOKE_VISITOR_FOR_CLASS(SpirvConstantNull)
+DEFINE_INVOKE_VISITOR_FOR_CLASS(SpirvUndef)
 DEFINE_INVOKE_VISITOR_FOR_CLASS(SpirvCompositeConstruct)
 DEFINE_INVOKE_VISITOR_FOR_CLASS(SpirvCompositeExtract)
 DEFINE_INVOKE_VISITOR_FOR_CLASS(SpirvCompositeInsert)
@@ -540,6 +541,11 @@ bool SpirvConstant::operator==(const SpirvConstant &that) const {
     if (thatNullInst == nullptr)
       return false;
     return *nullInst == *thatNullInst;
+  } else if (auto *nullInst = dyn_cast<SpirvUndef>(this)) {
+    auto *thatNullInst = dyn_cast<SpirvUndef>(&that);
+    if (thatNullInst == nullptr)
+      return false;
+    return *nullInst == *thatNullInst;
   }
 
   assert(false && "operator== undefined for SpirvConstant subclass");
@@ -613,6 +619,15 @@ bool SpirvConstantNull::operator==(const SpirvConstantNull &that) const {
          astResultType == that.astResultType;
 }
 
+SpirvUndef::SpirvUndef(QualType type)
+    : SpirvInstruction(IK_Undef, spv::Op::OpUndef, type,
+                       /*SourceLocation*/ {}) {}
+
+bool SpirvUndef::operator==(const SpirvUndef &that) const {
+  return opcode == that.opcode && resultType == that.resultType &&
+         astResultType == that.astResultType;
+}
+
 SpirvCompositeExtract::SpirvCompositeExtract(QualType resultType,
                                              SourceLocation loc,
                                              SpirvInstruction *compositeInst,

tools/clang/lib/SPIRV/SpirvModule.cpp

@@ -17,8 +17,8 @@ namespace spirv {
 SpirvModule::SpirvModule()
     : capabilities({}), extensions({}), extInstSets({}), memoryModel(nullptr),
       entryPoints({}), executionModes({}), moduleProcesses({}), decorations({}),
-      constants({}), variables({}), functions({}), debugInstructions({}),
-      perVertexInterp(false) {}
+      constants({}), undefs({}), variables({}), functions({}),
+      debugInstructions({}), perVertexInterp(false) {}
 
 SpirvModule::~SpirvModule() {
   for (auto *cap : capabilities)
@@ -43,6 +43,8 @@ SpirvModule::~SpirvModule() {
     decoration->releaseMemory();
   for (auto *constant : constants)
     constant->releaseMemory();
+  for (auto *undef : undefs)
+    undef->releaseMemory();
   for (auto *var : variables)
     var->releaseMemory();
   for (auto *di : debugInstructions)
@@ -91,6 +93,12 @@ bool SpirvModule::invokeVisitor(Visitor *visitor, bool reverseOrder) {
         return false;
     }
 
+    for (auto iter = undefs.rbegin(); iter != undefs.rend(); ++iter) {
+      auto *undef = *iter;
+      if (!undef->invokeVisitor(visitor))
+        return false;
+    }
+
     // Since SetVector doesn't have 'rbegin()' and 'rend()' methods, we use
     // manual indexing.
     for (auto decorIndex = decorations.size(); decorIndex > 0; --decorIndex) {
@@ -203,6 +211,10 @@ bool SpirvModule::invokeVisitor(Visitor *visitor, bool reverseOrder) {
       if (!constant->invokeVisitor(visitor))
         return false;
 
+    for (auto undef : undefs)
+      if (!undef->invokeVisitor(visitor))
+        return false;
+
     for (auto var : variables)
       if (!var->invokeVisitor(visitor))
         return false;
@@ -334,6 +346,11 @@ void SpirvModule::addConstant(SpirvConstant *constant) {
   constants.push_back(constant);
 }
 
+void SpirvModule::addUndef(SpirvUndef *undef) {
+  assert(undef);
+  undefs.push_back(undef);
+}
+
 void SpirvModule::addString(SpirvString *str) {
   assert(str);
   constStrings.push_back(str);

tools/clang/test/CodeGenSPIRV/spirv.cf.ret-missing-resource.hlsl

@@ -0,0 +1,27 @@
+// RUN: %dxc -T vs_6_0 -E main -fcgl %s -spirv | FileCheck %s
+
+// CHECK: [[undef:%[0-9]+]] = OpUndef %type_2d_image
+
+Texture2D texA;
+Texture2D texB;
+
+// CHECK:      %select = OpFunction
+Texture2D select(bool lhs) {
+
+// CHECK:      %if_true = OpLabel
+  if (lhs)
+    return texA;
+// CHECK:      %if_false = OpLabel
+  else
+    return texB;
+
+  // no return for dead branch.
+// CHECK:      %if_merge = OpLabel
+// CHECK-NEXT: OpReturnValue [[undef]]
+}
+// CHECK-NEXT: OpFunctionEnd
+
+float main(bool a: A) : B {
+    Texture2D tex = select(true);
+    return 1.0;
+}

tools/clang/test/CodeGenSPIRV/spirv.cf.ret-missing.hlsl

@@ -1,12 +1,12 @@
 // RUN: %dxc -T vs_6_0 -E main -Wno-return-type -fcgl  %s -spirv | FileCheck %s
 
-// CHECK:[[null:%[0-9]+]] = OpConstantNull %float
+// CHECK:[[undef:%[0-9]+]] = OpUndef %float
 
 float main(bool a: A) : B {
     if (a) return 1.0;
     // No return value for else
 
 // CHECK:      %if_merge = OpLabel
-// CHECK-NEXT: OpReturnValue [[null]]
+// CHECK-NEXT: OpReturnValue [[undef]]
 // CHECK-NEXT: OpFunctionEnd
 }

tools/clang/test/CodeGenSPIRV/spirv.cf.ret-undef-shared.hlsl

@@ -0,0 +1,31 @@
+// RUN: %dxc -T vs_6_0 -E main -fcgl %s -spirv | FileCheck %s
+
+// CHECK:     [[undef:%[0-9]+]] = OpUndef %type_2d_image
+// CHECK-NOT:                     OpUndef %type_2d_image
+
+
+Texture2D texA;
+Texture2D texB;
+
+Texture2D select1(bool lhs) {
+  if (lhs)
+    return texA;
+  else
+    return texB;
+  // no return for dead branch.
+}
+
+Texture2D select2(bool lhs) {
+  if (lhs)
+    return texA;
+  else
+    return texB;
+  // no return for dead branch.
+}
+
+float main(bool a: A) : B {
+    Texture2D x = select1(true);
+    Texture2D y = select2(true);
+    return 1.0;
+}
+