Tweak cmap functions for speed.

In looking at the Advertising-PowerPoint-A4.pdf file, I noted that
cmap_gray_direct and cmap_gray_halftoned were taking a noticable
amount of time. This commit attempts to alleviate that.

Various observations:

1) Avoid unnecessary loop in cmap_gray_halftoned (and similar functions).

In some of the code, we do:

 for (i = 0; i < n; i++)
    if (i == k)
       do_something_with(i)

why not just use:

  if (k < n)
    do_something_with(k)

2) Typically functions like cmap_gray_direct do a load of work,
culminating in trying to encode a color value. If that encoding fails,
it would fallback to trying to use a cmap_gray_halftoned, which would
do all the work a second time.

Tweak the code to avoid the call, and hence the repetition.

The downside to this is that the work is typically of the form:

  for (i = 0; i < n; i++)
    cv[i] = frac2cv(some_calculation_involving(cm_comps[i]));

and in order to be able to avoid the call to cmap_gray_halftoned, we
need to make it:

  for (i = 0; i < n; i++) {
     cm_comps[i] = some_calculation_involving(cm_comps[i]);
     cv[i] = frac2cv(cm_comps[i]);
  }

i.e. we have to do more stores than before. This can hurt us in some
cases, but it seems like a worthwhile win, especially in light of 3).

3) A lot of the work in these functions involves mapping colors through
the effective_transfer functions. This happens in code of the form:

  for (i = 0; i < n; i++) {
     cm_comps[i] = gx_map_color_frac(pgs, cm_comps[i], effective_transfer[i]);
  }

If effective_transfer[i] is identity (as it almost always is), then
this whole loop is a nop.

We make some steps to optimise for this case by having the
gx_map_color_frac macro check for effective_transfer[i] being
gs_identity_transfer before calling it, but this doesn't help us
avoid the loop/load/store.

We therefore extend pgs with a count of the number of
'effective_transfer's that are non identity (essentially a flag
that enables us to know if we can skip this loop or not),
and use that to optimise our work.

1 files changed, 1 insertions, 0 deletions

diff --git a/base/gxgstate.h b/base/gxgstate.h
index 7f0176eec..6d5663065 100644
--- a/base/gxgstate.h
+++ b/base/gxgstate.h
@@ -107,6 +107,7 @@ typedef struct gx_transfer_s {
                 /* dictionaries.  (In Level 1 systems, set_transfer and */\
                 /* effective_transfer are always the same.) */\
         gx_transfer set_transfer;		/* members are (RC) */\
+        int effective_transfer_non_identity_count;\
         gx_transfer_map *effective_transfer[GX_DEVICE_COLOR_MAX_COMPONENTS]; /* see below */\
 \
                 /* Color caches: */\