Decrease block size from 256 to 64

1M has register pressure that is so high that
even a small decrease in occupancy has a big impact:

Author: imreddyTeja

ext/cuda/data_layouts_copyto.jl

@@ -30,7 +30,7 @@ indicating if the column is padded (true for 63, false for 64).
 function knl_copyto_VIJFH_64!(dest, src, ::Val{P}) where {P}
     # P is a boolean, indicating if the column is padded
     P && threadIdx().x == 64 && return nothing
-    I = CartesianIndex(threadIdx().y, blockIdx().x, 1, threadIdx().x, blockIdx().y)
+    I = CartesianIndex(blockIdx().x, blockIdx().y, 1, threadIdx().x, blockIdx().z)
     @inbounds dest[I] = src[I]
     return nothing
 end
@@ -133,8 +133,8 @@ function Base.copyto!(
     auto_launch!(
         knl_copyto_VIJFH_64!,
         args;
-        threads_s = (64, Ni, 1),
-        blocks_s = (Nj, Nh, 1),
+        threads_s = (64, 1, 1),
+        blocks_s = (Ni, Nj, Nh),
     )
     return dest
 end
@@ -150,8 +150,8 @@ function Base.copyto!(
     auto_launch!(
         knl_copyto_VIJFH_64!,
         args;
-        threads_s = (64, Ni, 1),
-        blocks_s = (Nj, Nh, 1),
+        threads_s = (64, 1, 1),
+        blocks_s = (Ni, Nj, Nh),
     )
     return dest
 end

ext/cuda/operators_finite_difference.jl

@@ -92,8 +92,8 @@ function Base.copyto!(
             auto_launch!(
                 copyto_stencil_kernel_64!,
                 args;
-                threads_s = (64, Ni, 1),
-                blocks_s = (Nj, Nh, 1),
+                threads_s = (64, 1, 1),
+                blocks_s = (Ni, Nj, Nh),
             )
             return out
         end
@@ -161,10 +161,10 @@ function copyto_stencil_kernel_64!(
     @inbounds begin
         # P is a boolean, indicating if the column is padded
         P && threadIdx().x == 64 && return nothing
-        i = threadIdx().y
-        j = blockIdx().x
+        i = blockIdx().x
+        j = blockIdx().y
         v = threadIdx().x
-        h = blockIdx().y
+        h = blockIdx().z
         hidx = (i, j, h)
         (li, lw, rw, ri) = bds
         idx = v - 1 + li