refactor(grid): change schar_T representation to be more compact

Previously, a screen cell would occupy 28+4=32 bytes per cell
as we always made space for up to MAX_MCO+1 codepoints in a cell.

As an example, even a pretty modest 50*80 screen would consume

50*80*2*32 = 256000, i e a quarter megabyte

With the factor of two due to the TUI side buffer, and even more when
using msg_grid and/or ext_multigrid.

This instead stores a 4-byte union of either:
- a valid UTF-8 sequence up to 4 bytes
- an escape char which is invalid UTF-8 (0xFF) plus a 24-bit index to a
  glyph cache

This avoids allocating space for huge composed glyphs _upfront_, while
still keeping rendering such glyphs reasonably fast (1 hash table lookup
+ one plain index lookup). If the same large glyphs are using repeatedly
on the screen, this is still a net reduction of memory/cache
consumption. The only case which really gets worse is if you blast
the screen full with crazy emojis and zalgo text and even this case
only leads to 4 extra bytes per char.

When only <= 4-byte glyphs are used, plus the 4-byte attribute code,
i e 8 bytes in total there is a factor of four reduction of memory use.
Memory which will be quite hot in cache as the screen buffer is scanned
over in win_line() buffer text drawing

A slight complication is that the representation depends on host byte
order. I've tested this manually by compling and running this
in qemu-s390x and it works fine. We might add a qemu based solution
to CI at some point.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/nvim/ugrid.c b/src/nvim/ugrid.c
index be1746983c..d3a1097aba 100644
--- a/src/nvim/ugrid.c
+++ b/src/nvim/ugrid.c
@@ -4,6 +4,7 @@
 #include <assert.h>
 #include <string.h>
 
+#include "nvim/grid.h"
 #include "nvim/memory.h"
 #include "nvim/ugrid.h"
 
@@ -79,8 +80,7 @@ static void clear_region(UGrid *grid, int top, int bot, int left, int right, sat
 {
   for (int row = top; row <= bot; row++) {
     UGRID_FOREACH_CELL(grid, row, left, right + 1, {
-      cell->data[0] = ' ';
-      cell->data[1] = 0;
+      cell->data = schar_from_ascii(' ');
       cell->attr = attr;
     });
   }