Merge pull request opencv#19486 from fpetrogalli:dotprod_fast-3.4

* [hal][neon] Optimize the v_dotprod_fast intrinsics for aarch64.

On Armv8 in AArch64 execution mode, we can skip the sequence

   v<op>_<ty>(vget_high_<ty>(x), vget_high_<ty>(y))

in favour of

   v<op>_high_<ty>(x, y)

This has better changes for recent compilers to use less data movement
operations and better register allocation. See for example:

   https://godbolt.org/z/bPq7vd

* [hal][neon] Fix build failure on armv7.

* [hal][neon] Address review comments in PR.

PR: opencv#19486

* [hal][neon] Define macro to check for the AArch64 execution state of Armv8.

* [hal][neon] Fix macro definition for AArch64.

The fix is needed to prevent warnings when building for Armv7.

Author: fpetrogalli

modules/core/include/opencv2/core/hal/intrin_neon.hpp

@@ -62,6 +62,22 @@ CV_CPU_OPTIMIZATION_HAL_NAMESPACE_BEGIN
 #define CV_SIMD128_64F 0
 #endif
 
+// The following macro checks if the code is being compiled for the
+// AArch64 execution state of Armv8, to enable the 128-bit
+// intrinsics. The macro `__ARM_64BIT_STATE` is the one recommended by
+// the Arm C Language Extension (ACLE) specifications [1] to check the
+// availability of 128-bit intrinsics, and it is supporrted by clang
+// and gcc. The macro `_M_ARM64` is the equivalent one for Microsoft
+// Visual Studio [2] .
+//
+// [1] https://developer.arm.com/documentation/101028/0012/13--Advanced-SIMD--Neon--intrinsics
+// [2] https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros
+#if defined(__ARM_64BIT_STATE) || defined(_M_ARM64)
+#define CV_NEON_AARCH64 1
+#else
+#define CV_NEON_AARCH64 0
+#endif
+
 // TODO
 #define CV_NEON_DOT 0
 
@@ -726,41 +742,61 @@ inline v_float64x2 v_dotprod_expand(const v_int32x4& a,   const v_int32x4& b,
 // 16 >> 32
 inline v_int32x4 v_dotprod_fast(const v_int16x8& a, const v_int16x8& b)
 {
+#if CV_NEON_AARCH64
+    int32x4_t p = vmull_s16(vget_low_s16(a.val), vget_low_s16(b.val));
+    return v_int32x4(vmlal_high_s16(p, a.val, b.val));
+#else
     int16x4_t a0 = vget_low_s16(a.val);
     int16x4_t a1 = vget_high_s16(a.val);
     int16x4_t b0 = vget_low_s16(b.val);
     int16x4_t b1 = vget_high_s16(b.val);
     int32x4_t p = vmull_s16(a0, b0);
     return v_int32x4(vmlal_s16(p, a1, b1));
+#endif
 }
 inline v_int32x4 v_dotprod_fast(const v_int16x8& a, const v_int16x8& b, const v_int32x4& c)
 {
+#if CV_NEON_AARCH64
+    int32x4_t p = vmlal_s16(c.val, vget_low_s16(a.val), vget_low_s16(b.val));
+    return v_int32x4(vmlal_high_s16(p, a.val, b.val));
+#else
     int16x4_t a0 = vget_low_s16(a.val);
     int16x4_t a1 = vget_high_s16(a.val);
     int16x4_t b0 = vget_low_s16(b.val);
     int16x4_t b1 = vget_high_s16(b.val);
     int32x4_t p = vmlal_s16(c.val, a0, b0);
     return v_int32x4(vmlal_s16(p, a1, b1));
+#endif
 }
 
 // 32 >> 64
 inline v_int64x2 v_dotprod_fast(const v_int32x4& a, const v_int32x4& b)
 {
+#if CV_NEON_AARCH64
+    int64x2_t p = vmull_s32(vget_low_s32(a.val), vget_low_s32(b.val));
+    return v_int64x2(vmlal_high_s32(p, a.val, b.val));
+#else
     int32x2_t a0 = vget_low_s32(a.val);
     int32x2_t a1 = vget_high_s32(a.val);
     int32x2_t b0 = vget_low_s32(b.val);
     int32x2_t b1 = vget_high_s32(b.val);
     int64x2_t p = vmull_s32(a0, b0);
     return v_int64x2(vmlal_s32(p, a1, b1));
+#endif
 }
 inline v_int64x2 v_dotprod_fast(const v_int32x4& a, const v_int32x4& b, const v_int64x2& c)
 {
+#if CV_NEON_AARCH64
+    int64x2_t p = vmlal_s32(c.val, vget_low_s32(a.val), vget_low_s32(b.val));
+    return v_int64x2(vmlal_high_s32(p, a.val, b.val));
+#else
     int32x2_t a0 = vget_low_s32(a.val);
     int32x2_t a1 = vget_high_s32(a.val);
     int32x2_t b0 = vget_low_s32(b.val);
     int32x2_t b1 = vget_high_s32(b.val);
     int64x2_t p = vmlal_s32(c.val, a0, b0);
     return v_int64x2(vmlal_s32(p, a1, b1));
+#endif
 }
 
 // 8 >> 32
@@ -1292,7 +1328,7 @@ inline int64 v_reduce_sum(const v_int64x2& a)
 #if CV_SIMD128_64F
 inline double v_reduce_sum(const v_float64x2& a)
 {
-    return vgetq_lane_f64(a.val, 0) + vgetq_lane_f64(a.val, 1);
+    return vaddvq_f64(a.val);
 }
 #endif