[mlir][gpu][nvptx] Remove null terminator when outputting PTX (#133019)

PTX source files are expected to only contain ASCII text
(https://docs.nvidia.com/cuda/parallel-thread-execution/#source-format) and no null terminators.

`ptxas` has so far not enforced this but is moving towards doing so.
This revealed a problem where the null terminator is getting printed out
in the output file in MLIR path when outputting ptx directly. Only add the null on the assembly output path for JIT instead of in output of `moduleToObject `.

Author: modiking

mlir/lib/Target/LLVM/NVVM/Target.cpp

@@ -722,12 +722,8 @@ NVPTXSerializer::moduleToObject(llvm::Module &llvmModule) {
 #undef DEBUG_TYPE
 
   // Return PTX if the compilation target is `assembly`.
-  if (targetOptions.getCompilationTarget() ==
-      gpu::CompilationTarget::Assembly) {
-    // Make sure to include the null terminator.
-    StringRef bin(serializedISA->c_str(), serializedISA->size() + 1);
-    return SmallVector<char, 0>(bin.begin(), bin.end());
-  }
+  if (targetOptions.getCompilationTarget() == gpu::CompilationTarget::Assembly)
+    return SmallVector<char, 0>(serializedISA->begin(), serializedISA->end());
 
   std::optional<SmallVector<char, 0>> result;
   moduleToObjectTimer.startTimer();

mlir/lib/Target/LLVMIR/Dialect/GPU/SelectObjectAttr.cpp

@@ -116,8 +116,11 @@ LogicalResult SelectObjectAttrImpl::embedBinary(
   llvm::Module *module = moduleTranslation.getLLVMModule();
 
   // Embed the object as a global string.
+  // Add null for assembly output for JIT paths that expect null-terminated
+  // strings.
+  bool addNull = (object.getFormat() == gpu::CompilationTarget::Assembly);
   llvm::Constant *binary = llvm::ConstantDataArray::getString(
-      builder.getContext(), object.getObject().getValue(), false);
+      builder.getContext(), object.getObject().getValue(), addNull);
   llvm::GlobalVariable *serializedObj =
       new llvm::GlobalVariable(*module, binary->getType(), true,
                                llvm::GlobalValue::LinkageTypes::InternalLinkage,

mlir/unittests/Target/LLVM/SerializeNVVMTarget.cpp

@@ -130,6 +130,7 @@ TEST_F(MLIRTargetLLVMNVVM, SKIP_WITHOUT_NVPTX(SerializeNVVMToPTX)) {
 
     ASSERT_TRUE(
         StringRef(object->data(), object->size()).contains("nvvm_kernel"));
+    ASSERT_TRUE(StringRef(object->data(), object->size()).count('\0') == 0);
   }
 }