[buffer] Add flags to disable unsafe-to-concat and unsafe-to-break calculations

[behdad]

In my measurements, disabling those can speedup shaping by up to 17%. Vast majority of clients won't use those flags. Should have been opt-in from the beginning. Oh well. We can allow opt-out now, to allow those browsers, or Android, that don't use it to opt out and save the planet...

Note that most of that overhead is from unsafe-to-concat. unsafe-to-break is much lower-overhead. Maybe unsafe-to-concat can become opt-in?

[behdad]

unsafe-to-concat is new enough that we can still change it.

[behdad]

cc @drott @jfkthame

[behdad]

This patch is enough to regain much of the lost performance:

diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index ee72dfafb..12daeba16 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -377,7 +377,8 @@ typedef enum { /*< flags >*/
   HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES   = 0x00000004u,
   HB_BUFFER_FLAG_REMOVE_DEFAULT_IGNORABLES     = 0x00000008u,
   HB_BUFFER_FLAG_DO_NOT_INSERT_DOTTED_CIRCLE   = 0x00000010u,
-  HB_BUFFER_FLAG_VERIFY                                = 0x00000020u
+  HB_BUFFER_FLAG_VERIFY                                = 0x00000020u,
+  HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT      = 0x00000040u
 } hb_buffer_flags_t;
 
 HB_EXTERN void
diff --git a/src/hb-buffer.hh b/src/hb-buffer.hh
index cc20f3aef..bc6992905 100644
--- a/src/hb-buffer.hh
+++ b/src/hb-buffer.hh
@@ -460,6 +460,8 @@ struct hb_buffer_t
   }
   void unsafe_to_concat (unsigned int start = 0, unsigned int end = -1)
   {
+    if (likely ((flags & HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT) == 0))
+      return;
     _set_glyph_flags (HB_GLYPH_FLAG_UNSAFE_TO_CONCAT,
                      start, end,
                      true);
@@ -472,6 +474,8 @@ struct hb_buffer_t
   }
   void unsafe_to_concat_from_outbuffer (unsigned int start = 0, unsigned int end = -1)
   {
+    if (likely ((flags & HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT) == 0))
+      return;
     _set_glyph_flags (HB_GLYPH_FLAG_UNSAFE_TO_CONCAT,
                      start, end,
                      false, true);

I suggest I put this in. As I said, unsafe-to-break seems to be much lower overhead and also water-under-the-bridge already.