From aa8902a6ef88ae3eab4d6d039618d7fb4b383409 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Sun, 14 Mar 2021 14:32:00 +0100
Subject: [PATCH] term: cache currently selected charset
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

vt.c::action_print(), which is in the hot path, needs to lookup the
currently selected charset.

This currently requires two loads; first to load the current
charset *index*, and then another to get _which_ charset is currently
mapped to that index.

By caching the current charset, we only need one load.

We need to update the cache when:

* A different charset index is selected (i.e. SO/SI)
* The charset for the current index is changed (e.g. ‘\E(0’, ‘\E(B’
  etc)
* We restore saved charsets
---
 terminal.c |  1 +
 terminal.h |  1 +
 vt.c       | 16 ++++++++++------
 3 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/terminal.c b/terminal.c
index 88599094..9b1eb159 100644
--- a/terminal.c
+++ b/terminal.c
@@ -2335,6 +2335,7 @@ term_restore_cursor(struct terminal *term, const struct cursor *cursor)
 
     term->vt.attrs = term->vt.saved_attrs;
     term->charsets = term->saved_charsets;
+    term->charset = term->charsets.set[term->charsets.selected];
 }
 
 void
diff --git a/terminal.h b/terminal.h
index 0fcd97ea..f112376a 100644
--- a/terminal.h
+++ b/terminal.h
@@ -280,6 +280,7 @@ struct terminal {
 
     struct charsets charsets;
     struct charsets saved_charsets; /* For save/restore cursor + attributes */
+    enum charset charset;  /* cached copy of charsets.set[charsets.selected] */
 
     bool auto_margin;
     bool insert_mode;
diff --git a/vt.c b/vt.c
index d94420da..17cc6a3e 100644
--- a/vt.c
+++ b/vt.c
@@ -184,11 +184,13 @@ action_execute(struct terminal *term, uint8_t c)
     case '\x0e':
         /* SO - shift out */
         term->charsets.selected = 1; /* G1 */
+        term->charset = term->charsets.set[1];
         break;
 
     case '\x0f':
         /* SI - shift in */
         term->charsets.selected = 0; /* G0 */
+        term->charset = term->charsets.set[0];
         break;
 
         /*
@@ -238,13 +240,10 @@ action_print(struct terminal *term, uint8_t c)
         L'│', L'≤', L'≥', L'π', L'≠', L'£', L'·',       /* x - ~ */
     };
 
-    if (unlikely(term->charsets.set[term->charsets.selected] == CHARSET_GRAPHIC) &&
-        c >= 0x60 && c <= 0x7e)
-    {
+    if (unlikely(term->charset == CHARSET_GRAPHIC) && c >= 0x60 && c <= 0x7e)
         term_print(term, vt100_0[c - 0x60], 1);
-    } else {
+    else
         term_print(term, c, 1);
-    }
 }
 
 static void
@@ -414,11 +413,13 @@ action_esc_dispatch(struct terminal *term, uint8_t final)
         case 'N':
             /* SS2 - Single Shift 2 */
             term->charsets.selected = 2; /* G2 */
+            term->charset = term->charsets.set[2];
             break;
 
         case 'O':
             /* SS3 - Single Shift 3 */
             term->charsets.selected = 3; /* G3 */
+            term->charset = term->charsets.set[3];
             break;
 
         case '\\':
@@ -453,6 +454,8 @@ action_esc_dispatch(struct terminal *term, uint8_t final)
                 '+' ? 3 : -1;
             xassert(idx != -1);
             term->charsets.set[idx] = CHARSET_GRAPHIC;
+            if (idx == term->charsets.selected)
+                term->charset = CHARSET_GRAPHIC;
             break;
         }
 
@@ -465,7 +468,8 @@ action_esc_dispatch(struct terminal *term, uint8_t final)
                 '+' ? 3 : -1;
             xassert(idx != -1);
             term->charsets.set[idx] = CHARSET_ASCII;
-
+            if (idx == term->charsets.selected)
+                term->charset = CHARSET_ASCII;
             break;
         }
         }