build: eliminate GCC9 warning from sift.simd.hpp

modules/features2d/src/sift.simd.hpp

@@ -807,11 +807,18 @@ if( dstMat.type() == CV_32F )
         __dst = v_min(v_max(v_cvt_f32(v_round(__dst * __nrm2)), __min), __max);
         v_store(dst + k, __dst);
     }
+#endif
+#if defined(__GNUC__) && __GNUC__ >= 9
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Waggressive-loop-optimizations"  // iteration XX invokes undefined behavior
 #endif
     for( ; k < len; k++ )
     {
         dst[k] = saturate_cast<uchar>(rawDst[k]*nrm2);
     }
+#if defined(__GNUC__) && __GNUC__ >= 9
+#pragma GCC diagnostic pop
+#endif
 }
 else // CV_8U
 {
@@ -831,9 +838,8 @@ else // CV_8U
 #endif
 
 #if defined(__GNUC__) && __GNUC__ >= 9
-// avoid warning "iteration 7 invokes undefined behavior" on Linux ARM64
 #pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Waggressive-loop-optimizations"
+#pragma GCC diagnostic ignored "-Waggressive-loop-optimizations"  // iteration XX invokes undefined behavior
 #endif
     for( ; k < len; k++ )
     {