add notes

include/ck_tile/ops/fmha/block/block_masking.hpp

@@ -243,6 +243,20 @@ namespace impl {
 // this version only have 2 variation: masking and non-masking
 // This is more friendly to codegen (e.g. need generate less kernel)
 // ... with the trade-off that may have more instruction in causal mode
+
+// clang-format off
+/*  y_ratio is used to describe the step length of y-direction changes
+    in certain performance optimization scenarios like merging seqlen 
+    and qk_head_ratio, for example:
+x=1/y=6/y_ratio=2(top-left)
+1 * * * * * * * 
+1 * * * * * * *
+1 1 * * * * * * 
+1 1 * * * * * *
+1 1 1 * * * * *
+1 1 1 * * * * *
+*/
+// clang-format on
 template <bool IsMasking_ = true>
 struct SimplifiedGenericAttentionMask
 {
@@ -254,9 +268,11 @@ struct SimplifiedGenericAttentionMask
         : SimplifiedGenericAttentionMask(0, 0, y_total_, x_total_)
     {
     }
-
+    // TODO: Y or i_y cannot be negative if y_ratio is not equal to 1,
+    // because integer_divide_floor do not support negative numbers.
     CK_TILE_HOST_DEVICE
-    SimplifiedGenericAttentionMask(index_t y_, index_t x_, index_t y_total_, index_t x_total_, index_t y_ratio_=1)
+    SimplifiedGenericAttentionMask(
+        index_t y_, index_t x_, index_t y_total_, index_t x_total_, index_t y_ratio_ = 1)
         : y(y_), x(x_), y_total(y_total_), x_total(x_total_), y_ratio(y_ratio_)
     {
     }
@@ -284,14 +300,14 @@ struct SimplifiedGenericAttentionMask
         {
             // get the tile start/end range assum we loop over along X tile by tile
             index_t x_start = [&]() {
-                index_t tmp = max((-y + i_y + y_ratio) / y_ratio, 0);
+                index_t tmp = max(integer_divide_floor(-y + i_y + y_ratio, y_ratio), 0);
                 return (tmp / XTile) * XTile; // round to tile aligned
             }();
 
             // TODO: end could be negative, we ignore clamp here, and let caller to check
             //      ... in which case end-start is negative
             index_t x_end = [&]() {
-                index_t tmp = min((i_y + YTile - 1) / y_ratio + x, x_total);
+                index_t tmp = min(integer_divide_floor(i_y + YTile - 1, y_ratio) + x, x_total);
                 return ((tmp + XTile - 1) / XTile) * XTile;
             }();
 
@@ -357,8 +373,10 @@ struct SimplifiedGenericAttentionMask
         }
         else
         {
-            index_t x_start = (-y + i_y + y_ratio) / y_ratio;          // this could be negative, but it's fine
-            index_t x_end   = min(i_y / y_ratio + x, x_total); // need min in case x is padded
+            index_t x_start = integer_divide_floor(
+                -y + i_y + y_ratio, y_ratio); // this could be negative, but it's fine
+            index_t x_end = min(integer_divide_floor(i_y, y_ratio) + x,
+                                x_total); // need min in case x is padded
 
             return i_x < x_start || i_x >= x_end || i_y >= y_total;
         }
@@ -388,8 +406,10 @@ struct SimplifiedGenericAttentionMask
             index_t i_y_end = i_y + TileHeight;
             // index_t x_end    = min(i_y + x, x_total);
 
-            bool top_right_edge   = i_x_end > min(i_y / y_ratio + x, x_total); // consider right pad
-            bool bottom_left_edge = i_y_end > min(i_x * y_ratio + y, y_total); // consider bottom pad
+            bool top_right_edge = i_x_end > min(integer_divide_floor(i_y, y_ratio) + x,
+                                                x_total); // consider right pad
+            bool bottom_left_edge =
+                i_y_end > min(i_x * y_ratio + y, y_total); // consider bottom pad
             // bool is_partial_out_of_bound = i_x_end > x_end; // only consider right-pad for now
 
             return top_right_edge || bottom_left_edge;