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, 4 insertions, 4 deletions

diff --git a/src/nvim/api/win_config.c b/src/nvim/api/win_config.c
index 63cf3bb701..a579b0dde5 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -16,7 +16,7 @@
 #include "nvim/drawscreen.h"
 #include "nvim/extmark_defs.h"
 #include "nvim/globals.h"
-#include "nvim/grid_defs.h"
+#include "nvim/grid.h"
 #include "nvim/highlight_group.h"
 #include "nvim/macros.h"
 #include "nvim/mbyte.h"
@@ -348,7 +348,7 @@ Dictionary nvim_win_get_config(Window window, Error *err)
       for (size_t i = 0; i < 8; i++) {
         Array tuple = ARRAY_DICT_INIT;
 
-        String s = cstrn_to_string(config->border_chars[i], sizeof(schar_T));
+        String s = cstrn_to_string(config->border_chars[i], MAX_SCHAR_SIZE);
 
         int hi_id = config->border_hl_ids[i];
         char *hi_name = syn_id2name(hi_id);
@@ -520,7 +520,7 @@ static void parse_border_style(Object style,  FloatConfig *fconfig, Error *err)
 {
   struct {
     const char *name;
-    schar_T chars[8];
+    char chars[8][MAX_SCHAR_SIZE];
     bool shadow_color;
   } defaults[] = {
     { "double", { "╔", "═", "╗", "║", "╝", "═", "╚", "║" }, false },
@@ -531,7 +531,7 @@ static void parse_border_style(Object style,  FloatConfig *fconfig, Error *err)
     { NULL, { { NUL } }, false },
   };
 
-  schar_T *chars = fconfig->border_chars;
+  char (*chars)[MAX_SCHAR_SIZE] = fconfig->border_chars;
   int *hl_ids = fconfig->border_hl_ids;
 
   fconfig->border = true;