Emit error if WordCount exceeds 16-bit max value

According to the SPIR-V specification(Table 2) the 16 high-order bits of every
instruction encode word count(length) of the instruction. This fundamental
limitation doesn't allow to have instruction longer than 65535 words.

Signed-off-by: Alexey Sotkin <[email protected]>

Author: AlexeySotkin

llvm-spirv/lib/SPIRV/SPIRVWriter.cpp

@@ -1029,6 +1029,21 @@ SPIRVValue *LLVMToSPIRV::transValueWithoutDecoration(Value *V,
       } else
         BVarInit = I->second;
     } else if (Init && !isa<UndefValue>(Init)) {
+      if (auto ArrTy = dyn_cast_or_null<ArrayType>(Init->getType())) {
+        // First 3 words of OpConstantComposite encode: 1) word count & opcode,
+        // 2) Result Type and 3) Result Id.
+        // Max length of SPIRV instruction = 65535 words.
+        const int MaxNumElements = 65535 - 3;
+        if (ArrTy->getNumElements() > MaxNumElements &&
+            !isa<ConstantAggregateZero>(Init)) {
+          std::stringstream SS;
+          SS << "Global variable has a constant array initializer with a number"
+             << " of elements greater than OpConstantComposite can have "
+             << "(65532). Should the array be split?\n Original LLVM value:\n"
+             << toString(GV);
+          getErrorLog().checkError(false, SPIRVEC_InvalidWordCount, SS.str());
+        }
+      }
       BVarInit = transValue(Init, nullptr);
     }
 

llvm-spirv/lib/SPIRV/libSPIRV/SPIRVEntry.cpp

@@ -195,7 +195,9 @@ void SPIRVEntry::encodeWordCountOpCode(spv_ostream &O) const {
     return;
   }
 #endif
-  getEncoder(O) << mkWord(WordCount, OpCode);
+  assert(WordCount < 65536 && "WordCount must fit into 16-bit value");
+  SPIRVWord WordCountOpCode = (WordCount << WordCountShift) | OpCode;
+  getEncoder(O) << WordCountOpCode;
 }
 // Read words from SPIRV binary and create members for SPIRVEntry.
 // The word count and op code has already been read before calling this

llvm-spirv/lib/SPIRV/libSPIRV/SPIRVEntry.h

@@ -381,6 +381,12 @@ class SPIRVEntry {
     assert(Module && "Invalid module");
     assert(OpCode != OpNop && "Invalid op code");
     assert((!hasId() || isValidId(Id)) && "Invalid Id");
+    if (WordCount > 65535) {
+      std::stringstream SS;
+      SS << "Id: " << Id << ", OpCode: " << OpCodeNameMap::map(OpCode)
+         << ", Name: \"" << Name << "\"\n";
+      getErrorLog().checkError(false, SPIRVEC_InvalidWordCount, SS.str());
+    }
   }
   void validateFunctionControlMask(SPIRVWord FCtlMask) const;
   void validateValues(const std::vector<SPIRVId> &) const;

llvm-spirv/lib/SPIRV/libSPIRV/SPIRVEnum.h

@@ -56,10 +56,6 @@ typedef uint32_t SPIRVId;
 
 inline bool isValidId(SPIRVId Id) { return Id != SPIRVID_INVALID && Id != 0; }
 
-inline SPIRVWord mkWord(unsigned WordCount, Op OpCode) {
-  return (WordCount << 16) | OpCode;
-}
-
 const static unsigned KSpirvMemOrderSemanticMask = 0x1F;
 
 enum SPIRVGeneratorKind {

llvm-spirv/lib/SPIRV/libSPIRV/SPIRVErrorEnum.h

@@ -13,3 +13,5 @@ _SPIRV_OP(InvalidModule, "Invalid SPIR-V module:")
 _SPIRV_OP(UnimplementedOpCode, "Unimplemented opcode")
 _SPIRV_OP(FunctionPointers, "Can't translate function pointer:\n")
 _SPIRV_OP(InvalidInstruction, "Can't translate llvm instruction:\n")
+_SPIRV_OP(InvalidWordCount,
+          "Can't encode instruction with word count greater than 65535:\n")