fix clang warning for GPU (#2807)

* try the fix

* fix 2

* try clang

* Apply suggestions from code review

* get back to gcc
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

src/treelearner/gpu_tree_learner.cpp

@@ -700,17 +700,20 @@ void GPUTreeLearner::InitGPU(int platform_id, int device_id) {
   }
   // determine which kernel to use based on the max number of bins
   if (max_num_bin_ <= 16) {
-    kernel_source_ = kernel16_src_;
+    // the +9 skips extra characters ")", newline, "#endif" and newline at the beginning
+    kernel_source_ = kernel16_src_ + 9;
     kernel_name_ = "histogram16";
     device_bin_size_ = 16;
     dword_features_ = 8;
   } else if (max_num_bin_ <= 64) {
-    kernel_source_ = kernel64_src_;
+    // the +9 skips extra characters ")", newline, "#endif" and newline at the beginning
+    kernel_source_ = kernel64_src_ + 9;
     kernel_name_ = "histogram64";
     device_bin_size_ = 64;
     dword_features_ = 4;
   } else if (max_num_bin_ <= 256) {
-    kernel_source_ = kernel256_src_;
+    // the +9 skips extra characters ")", newline, "#endif" and newline at the beginning
+    kernel_source_ = kernel256_src_ + 9;
     kernel_name_ = "histogram256";
     device_bin_size_ = 256;
     dword_features_ = 4;

src/treelearner/ocl/histogram16.cl

@@ -762,6 +762,5 @@ R""()
 }
 
 // The following line ends the string literal, adds an extra #endif at the end
-// the +9 skips extra characters ")", newline, "#endif" and newline at the beginning
-// )"" "\n#endif" + 9
+// )"" "\n#endif"
 #endif

src/treelearner/ocl/histogram256.cl

@@ -787,7 +787,5 @@ R""()
 }
 
 // The following line ends the string literal, adds an extra #endif at the end
-// the +9 skips extra characters ")", newline, "#endif" and newline at the beginning
-// )"" "\n#endif" + 9
+// )"" "\n#endif"
 #endif
-

src/treelearner/ocl/histogram64.cl

@@ -738,7 +738,5 @@ R""()
 }
 
 // The following line ends the string literal, adds an extra #endif at the end
-// the +9 skips extra characters ")", newline, "#endif" and newline at the beginning
-// )"" "\n#endif" + 9
+// )"" "\n#endif"
 #endif
-