Made the tensor dot() function use cuBLAS.

dlib/dnn/cublas_dlibapi.cpp

@@ -7,6 +7,7 @@
 
 #include "cublas_dlibapi.h"
 #include "cuda_utils.h"
+#include "tensor.h"
 
 #include <cublas_v2.h>
 
@@ -88,6 +89,23 @@ namespace dlib
             return c.get_handle();
         }
 
+    // -----------------------------------------------------------------------------------
+
+        float dot (
+            const tensor& a,
+            const tensor& b
+        )
+        {
+            DLIB_CASSERT(a.size() == b.size(), "");
+            float result = 0;
+            CHECK_CUBLAS(cublasSdot(context(), 
+                                    a.size(),
+                                    a.device(), 1,
+                                    b.device(), 1,
+                                    &result));
+            return result;
+        }
+
     // -----------------------------------------------------------------------------------
 
         void gemm (

dlib/dnn/cublas_dlibapi.h

@@ -5,14 +5,30 @@
 
 #ifdef DLIB_USE_CUDA
 
-#include "tensor.h"
 #include "cuda_errors.h"
 
 namespace dlib
 {
+    class tensor;
+
     namespace cuda 
     {
 
+    // -----------------------------------------------------------------------------------
+
+        float dot (
+            const tensor& a,
+            const tensor& b
+        );
+        /*!
+            requires
+                - a.size() == b.size()
+            ensures
+                - returns the dot product between a and b when they are both treated as
+                  a.size() dimensional vectors.  That is, this function pointwise
+                  multiplies the vectors together, then sums the result and returns it.
+        !*/
+
     // -----------------------------------------------------------------------------------
 
         void gemm (

dlib/dnn/tensor.h

@@ -7,6 +7,7 @@
 #include <cstring>
 #include "../matrix.h"
 #include "cudnn_dlibapi.h"
+#include "cublas_dlibapi.h"
 #include "gpu_data.h"
 #include <memory>
 
@@ -405,7 +406,9 @@ namespace dlib
         const tensor& b
     )
     {
-        // TODO, do on GPU?
+#ifdef DLIB_USE_CUDA
+        return cuda::dot(a,b);
+#else
         DLIB_CASSERT(a.size() == b.size(), "");
         const float* da = a.host();
         const float* db = b.host();
@@ -413,6 +416,7 @@ namespace dlib
         for (size_t i = 0; i < a.size(); ++i)
             sum += da[i]*db[i];
         return sum;
+#endif
     }
 
 // ----------------------------------------------------------------------------------------