Don't attempt to cache glyph metrics for FONT_INVALID_CODE

This was causing massive slowdown in redisplay when eg #xfe0f
(VARIATION SELECTOR-16) was present, as the cache ended up very large,
unused, and being recreated on every call to font_fill_lglyph_metrics
(Bug#39133).

* src/composite.c (fill_gstring_body): Hoist FONT_OBJECT_P check out
of loop.  Calculate glyph code and check for FONT_INVALID_CODE before
calling font_fill_lglyph_metrics.  Pass glyph code to it.

* src/font.c (font_fill_lglyph_metrics): Add code parameter, move
glyph code calculation up the call stack into fill_gstring_body.

* src/font.h: Adjust font_fill_lglyph_metrics prototype.

1 files changed, 14 insertions, 4 deletions

diff --git a/src/composite.c b/src/composite.c
index 53e6930b5f2..364d5c9316e 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -818,6 +818,11 @@ fill_gstring_body (Lisp_Object gstring)
   Lisp_Object header = AREF (gstring, 0);
   ptrdiff_t len = LGSTRING_CHAR_LEN (gstring);
   ptrdiff_t i;
+  struct font *font = NULL;
+  unsigned int code;
+
+  if (FONT_OBJECT_P (font_object))
+    font = XFONT_OBJECT (font_object);
 
   for (i = 0; i < len; i++)
     {
@@ -832,10 +837,15 @@ fill_gstring_body (Lisp_Object gstring)
       LGLYPH_SET_FROM (g, i);
       LGLYPH_SET_TO (g, i);
       LGLYPH_SET_CHAR (g, c);
-      if (FONT_OBJECT_P (font_object))
-        {
-          font_fill_lglyph_metrics (g, font_object);
-        }
+
+      if (font != NULL)
+        code = font->driver->encode_char (font, LGLYPH_CHAR (g));
+      else
+        code = FONT_INVALID_CODE;
+      if (code != FONT_INVALID_CODE)
+        {
+          font_fill_lglyph_metrics (g, font, code);
+        }
       else
         {
           int width = XFIXNAT (CHAR_TABLE_REF (Vchar_width_table, c));