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Real arrays rarely divide evenly into tiles. The last tile at each border may run off the end of your data.

A thread whose global index exceeds the array size will read garbage or crash. Edge tiles must guard every access.

The simplest fix is an if (gid < n) guard so threads past the end skip their load and do no harmful work.

At the array start, the left halo points before index zero. There is no real neighbor there, so you must decide what to load.

One common choice is zero padding: treat missing neighbors as 0. It is simple and works well for sums and many filters.

Another option is to clamp the index to the nearest valid cell, repeating the border value. This avoids dark fringes in images.

The right boundary rule depends on the math. Choose zero, clamp, or wrap so the edge result matches what your problem expects.

Even after computing, a thread must check before writing so it never stores a result past the end of the output array.

Threads beyond the data still must reach __syncthreads. Put the bounds check around the load, not around the barrier itself.

Most tiling bugs hide at the boundaries. Always test sizes that are not multiples of the tile, like n equals 1000 with tile 256.

Edge handling adds a little code but keeps results correct for every input size. Get it right, then chase performance.

How should a thread handle a global index past the array end?

Edge tiles need bounds checks and a boundary rule (zero, clamp, or wrap) so every array size stays correct. Test the borders. ✅