gallivm: handle srgb-to-linear and linear-to-srgb conversions

srgb-to-linear is using 3rd degree polynomial for now which should be _just_
good enough. Reverse is using some rational polynomials and is quite accurate,
though not hooked into llvmpipe's blend code yet and hence unused (untested).
Using a table might also be an option (for srgb-to-linear especially).
This does not enable any new features yet because EXT_texture_srgb was already
supported via util_format fallbacks, but performance was lacking probably due
to the external function call (the table used by the util_format_srgb code may
not be all that much slower on its own).
Some performance figures (taken from modified gloss, replaced both base and
sphere texture to use GL_SRGB instead of GL_RGB, measured on 1Ghz Sandy Bridge,
the numbers aren't terribly accurate):

normal gloss, aos, 8-wide: 47 fps
normal gloss, aos, 4-wide: 48 fps

normal gloss, forced to soa, 8-wide: 48 fps
normal gloss, forced to soa, 4-wide: 47 fps

patched gloss, old code, soa, 8-wide: 21 fps
patched gloss, old code, soa, 4-wide: 24 fps

patched gloss, new code, soa, 8-wide: 41 fps
patched gloss, new code, soa, 4-wide: 38 fps

So there's a performance hit but it seems acceptable, certainly better
than using the fallback.
Note the new code only works for 4x8bit srgb formats, others (L8/L8A8) will
continue to use the old util_format fallback, because I can't be bothered
to write code for formats noone uses anyway (as decoding is done as part of
lp_build_unpack_rgba_soa which can only handle block type width of 32).
Compressed srgb formats should get their own path though eventually (it is
going to be expensive in any case, first decompress, then convert).
No piglit regressions.

v2: use lp_build_polynomial instead of ad-hoc polynomial construction, also
since keeping both linear to srgb functions for now make sure both are
compiled (since they share quite some code just integrate into the same
function).

v3: formatting fixes and bugfix in the complicated (disabled) linear-to-srgb
path.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

6 files changed, 332 insertions, 7 deletions

diff --git a/src/gallium/auxiliary/Makefile.sources b/src/gallium/auxiliary/Makefile.sources
index c989584..acbcef7 100644
--- a/src/gallium/auxiliary/Makefile.sources
+++ b/src/gallium/auxiliary/Makefile.sources
@@ -173,6 +173,7 @@ GALLIVM_SOURCES := \
         gallivm/lp_bld_format_aos.c \
         gallivm/lp_bld_format_aos_array.c \
 	gallivm/lp_bld_format_float.c \
+        gallivm/lp_bld_format_srgb.c \
         gallivm/lp_bld_format_soa.c \
         gallivm/lp_bld_format_yuv.c \
         gallivm/lp_bld_gather.c \
diff --git a/src/gallium/auxiliary/gallivm/lp_bld_arit.c b/src/gallium/auxiliary/gallivm/lp_bld_arit.c
index 7d6fe04..e7955aa 100644
--- a/src/gallium/auxiliary/gallivm/lp_bld_arit.c
+++ b/src/gallium/auxiliary/gallivm/lp_bld_arit.c
@@ -2896,7 +2896,7 @@ lp_build_log(struct lp_build_context *bld,
  * Generate polynomial.
  * Ex:  coeffs[0] + x * coeffs[1] + x^2 * coeffs[2].
  */
-static LLVMValueRef
+LLVMValueRef
 lp_build_polynomial(struct lp_build_context *bld,
                     LLVMValueRef x,
                     const double *coeffs,
diff --git a/src/gallium/auxiliary/gallivm/lp_bld_arit.h b/src/gallium/auxiliary/gallivm/lp_bld_arit.h
index 920e339..04e180c 100644
--- a/src/gallium/auxiliary/gallivm/lp_bld_arit.h
+++ b/src/gallium/auxiliary/gallivm/lp_bld_arit.h
@@ -239,6 +239,12 @@ lp_build_fast_rsqrt(struct lp_build_context *bld,
                     LLVMValueRef a);
 
 LLVMValueRef
+lp_build_polynomial(struct lp_build_context *bld,
+                    LLVMValueRef x,
+                    const double *coeffs,
+                    unsigned num_coeffs);
+
+LLVMValueRef
 lp_build_cos(struct lp_build_context *bld,
              LLVMValueRef a);
 
diff --git a/src/gallium/auxiliary/gallivm/lp_bld_format.h b/src/gallium/auxiliary/gallivm/lp_bld_format.h
index 12a0318..744d002 100644
--- a/src/gallium/auxiliary/gallivm/lp_bld_format.h
+++ b/src/gallium/auxiliary/gallivm/lp_bld_format.h
@@ -158,4 +158,15 @@ lp_build_rgb9e5_to_float(struct gallivm_state *gallivm,
                          LLVMValueRef src,
                          LLVMValueRef *dst);
 
+LLVMValueRef
+lp_build_linear_to_srgb(struct gallivm_state *gallivm,
+                        struct lp_type src_type,
+                        LLVMValueRef src);
+
+LLVMValueRef
+lp_build_srgb_to_linear(struct gallivm_state *gallivm,
+                        struct lp_type src_type,
+                        LLVMValueRef src);
+
+
 #endif /* !LP_BLD_FORMAT_H */
diff --git a/src/gallium/auxiliary/gallivm/lp_bld_format_soa.c b/src/gallium/auxiliary/gallivm/lp_bld_format_soa.c
index 4c6bd81..114ce03 100644
--- a/src/gallium/auxiliary/gallivm/lp_bld_format_soa.c
+++ b/src/gallium/auxiliary/gallivm/lp_bld_format_soa.c
@@ -163,11 +163,23 @@ lp_build_unpack_rgba_soa(struct gallivm_state *gallivm,
           */
 
          if (type.floating) {
-            if(format_desc->channel[chan].normalized)
-               input = lp_build_unsigned_norm_to_float(gallivm, width, type, input);
-            else
-               input = LLVMBuildSIToFP(builder, input,
-                                       lp_build_vec_type(gallivm, type), "");
+            if (format_desc->colorspace == UTIL_FORMAT_COLORSPACE_SRGB) {
+               assert(width == 8);
+               if (format_desc->swizzle[3] == chan) {
+                  input = lp_build_unsigned_norm_to_float(gallivm, width, type, input);
+               }
+               else {
+                  struct lp_type conv_type = lp_uint_type(type);
+                  input = lp_build_srgb_to_linear(gallivm, conv_type, input);
+               }
+            }
+            else {
+               if(format_desc->channel[chan].normalized)
+                  input = lp_build_unsigned_norm_to_float(gallivm, width, type, input);
+               else
+                  input = LLVMBuildSIToFP(builder, input,
+                                          lp_build_vec_type(gallivm, type), "");
+            }
          }
          else if (format_desc->channel[chan].pure_integer) {
             /* Nothing to do */
@@ -344,6 +356,7 @@ lp_build_fetch_rgba_soa(struct gallivm_state *gallivm,
 
    if (format_desc->layout == UTIL_FORMAT_LAYOUT_PLAIN &&
        (format_desc->colorspace == UTIL_FORMAT_COLORSPACE_RGB ||
+        format_desc->colorspace == UTIL_FORMAT_COLORSPACE_SRGB ||
         format_desc->colorspace == UTIL_FORMAT_COLORSPACE_ZS) &&
        format_desc->block.width == 1 &&
        format_desc->block.height == 1 &&
@@ -394,7 +407,7 @@ lp_build_fetch_rgba_soa(struct gallivm_state *gallivm,
       packed = lp_build_gather(gallivm, type.length,
                                format_desc->block.bits,
                                type.width, base_ptr, offset,
-			       FALSE);
+                               FALSE);
       if (format_desc->format == PIPE_FORMAT_R11G11B10_FLOAT) {
          lp_build_r11g11b10_to_float(gallivm, packed, rgba_out);
       }
diff --git a/src/gallium/auxiliary/gallivm/lp_bld_format_srgb.c b/src/gallium/auxiliary/gallivm/lp_bld_format_srgb.c
new file mode 100644
index 0000000..217aaa9
--- /dev/null
+++ b/src/gallium/auxiliary/gallivm/lp_bld_format_srgb.c
@@ -0,0 +1,294 @@
+/**************************************************************************
+ *
+ * Copyright 2013 VMware, Inc.
+ * All Rights Reserved.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sub license, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the
+ * next paragraph) shall be included in all copies or substantial portions
+ * of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT.
+ * IN NO EVENT SHALL VMWARE AND/OR ITS SUPPLIERS BE LIABLE FOR
+ * ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+ * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+ * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ *
+ **************************************************************************/
+
+
+/**
+ * @file
+ * Format conversion code for srgb formats.
+ *
+ * Functions for converting from srgb to linear and vice versa.
+ * From http://www.opengl.org/registry/specs/EXT/texture_sRGB.txt:
+ *
+ * srgb->linear:
+ * cl = cs / 12.92,                 cs <= 0.04045
+ * cl = ((cs + 0.055)/1.055)^2.4,   cs >  0.04045
+ *
+ * linear->srgb:
+ * if (isnan(cl)) {
+ *    Map IEEE-754 Not-a-number to zero.
+ *    cs = 0.0;
+ * } else if (cl > 1.0) {
+ *    cs = 1.0;
+ * } else if (cl < 0.0) {
+ *    cs = 0.0;
+ * } else if (cl < 0.0031308) {
+ *    cs = 12.92 * cl;
+ * } else {
+ *    cs = 1.055 * pow(cl, 0.41666) - 0.055;
+ * }
+ *
+ * This does not need to be accurate, however at least for d3d10
+ * (http://msdn.microsoft.com/en-us/library/windows/desktop/dd607323%28v=vs.85%29.aspx):
+ * 1) For srgb->linear, it is required that the error on the srgb side is
+ *    not larger than 0.5f, which I interpret that if you map the value back
+ *    to srgb from linear using the ideal conversion, it would not be off by
+ *    more than 0.5f (that is, it would map to the same 8-bit integer value
+ *    as it was before conversion to linear).
+ * 2) linear->srgb is permitted 0.6f which luckily looks like quite a large
+ *    error is allowed.
+ * 3) Additionally, all srgb values converted to linear and back must result
+ *    in the same value as they were originally.
+ *
+ * @author Roland Scheidegger <sroland@vmware.com>
+ */
+
+
+#include "util/u_debug.h"
+
+#include "lp_bld_type.h"
+#include "lp_bld_const.h"
+#include "lp_bld_arit.h"
+#include "lp_bld_bitarit.h"
+#include "lp_bld_logic.h"
+#include "lp_bld_format.h"
+
+
+
+/**
+ * Convert srgb int values to linear float values.
+ * Several possibilities how to do this, e.g.
+ * - table
+ * - doing the pow() with int-to-float and float-to-int tricks
+ *   (http://stackoverflow.com/questions/6475373/optimizations-for-pow-with-const-non-integer-exponent)
+ * - just using standard polynomial approximation
+ *   (3rd order polynomial is required for crappy but just sufficient accuracy)
+ *
+ * @param src   integer (vector) value(s) to convert
+ *              (8 bit values unpacked to 32 bit already).
+ */
+LLVMValueRef
+lp_build_srgb_to_linear(struct gallivm_state *gallivm,
+                        struct lp_type src_type,
+                        LLVMValueRef src)
+{
+   struct lp_type f32_type = lp_type_float_vec(32, src_type.length * 32);
+   struct lp_build_context f32_bld;
+   LLVMValueRef srcf, part_lin, part_pow, is_linear, lin_const, lin_thresh;
+   double coeffs[4] = {0.0023f,
+                       0.0030f / 255.0f,
+                       0.6935f / (255.0f * 255.0f),
+                       0.3012f / (255.0f * 255.0f * 255.0f)
+   };
+
+   assert(src_type.width == 32);
+
+   lp_build_context_init(&f32_bld, gallivm, f32_type);
+
+   /*
+    * using polynomial: (src * (src * (src * 0.3012 + 0.6935) + 0.0030) + 0.0023)
+    * ( poly =  0.3012*x^3 + 0.6935*x^2 + 0.0030*x + 0.0023)
+    * (found with octave polyfit and some magic as I couldn't get the error
+    * function right). Using the above mentioned error function, the values stay
+    * within +-0.35, except for the lowest values - hence tweaking linear segment
+    * to cover the first 16 instead of the first 11 values (the error stays
+    * just about acceptable there too).
+    * Hence: lin = src > 15 ? poly : src / 12.6
+    * This function really only makes sense for vectors, should use LUT otherwise.
+    * All in all (including float conversion) 11 instructions (with sse4.1),
+    * 6 constants (polynomial could be done with 1 instruction less at the cost
+    * of slightly worse dependency chain, fma should also help).
+    */
+   /* doing the 1/255 mul as part of the approximation */
+   srcf = lp_build_int_to_float(&f32_bld, src);
+   lin_const = lp_build_const_vec(gallivm, f32_type, 1.0f / (12.6f * 255.0f));
+   part_lin = lp_build_mul(&f32_bld, srcf, lin_const);
+
+   part_pow = lp_build_polynomial(&f32_bld, srcf, coeffs, 4);
+
+   lin_thresh = lp_build_const_vec(gallivm, f32_type, 15.0f);
+   is_linear = lp_build_compare(gallivm, f32_type, PIPE_FUNC_LEQUAL, srcf, lin_thresh);
+   return lp_build_select(&f32_bld, is_linear, part_lin, part_pow);
+}
+
+
+/**
+ * Convert linear float values to srgb int values.
+ * Several possibilities how to do this, e.g.
+ * - use table (based on exponent/highest order mantissa bits) and do
+ *   linear interpolation (https://gist.github.com/rygorous/2203834)
+ * - Chebyshev polynomial
+ * - Approximation using reciprocals
+ * - using int-to-float and float-to-int tricks for pow()
+ *   (http://stackoverflow.com/questions/6475373/optimizations-for-pow-with-const-non-integer-exponent)
+ *
+ * @param src   float (vector) value(s) to convert.
+ */
+LLVMValueRef
+lp_build_linear_to_srgb(struct gallivm_state *gallivm,
+                        struct lp_type src_type,
+                        LLVMValueRef src)
+{
+   LLVMBuilderRef builder = gallivm->builder;
+   struct lp_build_context f32_bld;
+   LLVMValueRef lin_thresh, lin, lin_const, is_linear, tmp, pow_final;
+
+   lp_build_context_init(&f32_bld, gallivm, src_type);
+
+   src = lp_build_clamp(&f32_bld, src, f32_bld.zero, f32_bld.one);
+
+   if (0) {
+      /*
+       * using int-to-float and float-to-int trick for pow().
+       * This is much more accurate than necessary thanks to the correction,
+       * but it most certainly makes no sense without rsqrt available.
+       * Bonus points if you understand how this works...
+       * All in all (including min/max clamp, conversion) 19 instructions.
+       */
+
+      float exp_f = 2.0f / 3.0f;
+      float coeff_f = 0.62996f;
+      LLVMValueRef pow_approx, coeff, x2, exponent, pow_1, pow_2;
+      struct lp_type int_type = lp_int_type(src_type);
+
+      /*
+       * First calculate approx x^8/12
+       */
+      exponent = lp_build_const_vec(gallivm, src_type, exp_f);
+      coeff = lp_build_const_vec(gallivm, src_type,
+                                 exp2f(127.0f / exp_f - 127.0f) *
+                                 powf(coeff_f, 1.0f / exp_f));
+
+      /* premultiply src */
+      tmp = lp_build_mul(&f32_bld, coeff, src);
+      /* "log2" */
+      tmp = LLVMBuildBitCast(builder, tmp, lp_build_vec_type(gallivm, int_type), "");
+      tmp = lp_build_int_to_float(&f32_bld, tmp);
+      /* multiply for pow */
+      tmp = lp_build_mul(&f32_bld, tmp, exponent);
+      /* "exp2" */
+      pow_approx = lp_build_itrunc(&f32_bld, tmp);
+      pow_approx = LLVMBuildBitCast(builder, pow_approx,
+                                    lp_build_vec_type(gallivm, src_type), "");
+
+      /*
+       * Since that pow was inaccurate (like 3 bits, though each sqrt step would
+       * give another bit), compensate the error (which is why we chose another
+       * exponent in the first place).
+       */
+      /* x * x^(8/12) = x^(20/12) */
+      pow_1 = lp_build_mul(&f32_bld, pow_approx, src);
+
+      /* x * x * x^(-4/12) = x^(20/12) */
+      /* Should avoid using rsqrt if it's not available, but
+       * using x * x^(4/12) * x^(4/12) instead will change error weight */
+      tmp = lp_build_fast_rsqrt(&f32_bld, pow_approx);
+      x2 = lp_build_mul(&f32_bld, src, src);
+      pow_2 = lp_build_mul(&f32_bld, x2, tmp);
+
+      /* average the values so the errors cancel out, compensate bias,
+       * we also squeeze the 1.055 mul of the srgb conversion plus the 255.0 mul
+       * for conversion to int in here */
+      tmp = lp_build_add(&f32_bld, pow_1, pow_2);
+      coeff = lp_build_const_vec(gallivm, src_type,
+                                 1.0f / (3.0f * coeff_f) * 0.999852f *
+                                 powf(1.055f * 255.0f, 4.0f));
+      pow_final = lp_build_mul(&f32_bld, tmp, coeff);
+
+      /* x^(5/12) = rsqrt(rsqrt(x^20/12)) */
+      if (lp_build_fast_rsqrt_available(src_type)) {
+         pow_final = lp_build_fast_rsqrt(&f32_bld,
+                        lp_build_fast_rsqrt(&f32_bld, pow_final));
+      }
+      else {
+         pow_final = lp_build_sqrt(&f32_bld, lp_build_sqrt(&f32_bld, pow_final));
+      }
+      pow_final = lp_build_add(&f32_bld, pow_final,
+                               lp_build_const_vec(gallivm, src_type, -0.055f * 255.0f));
+   }
+
+   else {
+      /*
+       * using "rational polynomial" approximation here.
+       * Essentially y = a*x^0.375 + b*x^0.5 + c, with also
+       * factoring in the 255.0 mul and the scaling mul.
+       * (a is closer to actual value so has higher weight than b.)
+       * Note: the constants are magic values. They were found empirically,
+       * possibly could be improved but good enough (be VERY careful with
+       * error metric if you'd want to tweak them, they also MUST fit with
+       * the crappy polynomial above for srgb->linear since it is required
+       * that each srgb value maps back to the same value).
+       * This function has an error of max +-0.17 (and we'd only require +-0.6),
+       * for the approximated srgb->linear values the error is naturally larger
+       * (+-0.42) but still accurate enough (required +-0.5 essentially).
+       * All in all (including min/max clamp, conversion) 15 instructions.
+       * FMA would help (minus 2 instructions).
+       */
+
+      LLVMValueRef x05, x0375, a_const, b_const, c_const, tmp2;
+
+      if (lp_build_fast_rsqrt_available(src_type)) {
+         tmp = lp_build_fast_rsqrt(&f32_bld, src);
+         x05 = lp_build_mul(&f32_bld, src, tmp);
+      }
+      else {
+         /*
+          * I don't really expect this to be practical without rsqrt
+          * but there's no reason for triple punishment so at least
+          * save the otherwise resulting division and unnecessary mul...
+          */
+         x05 = lp_build_sqrt(&f32_bld, src);
+      }
+
+      tmp = lp_build_mul(&f32_bld, x05, src);
+      if (lp_build_fast_rsqrt_available(src_type)) {
+         x0375 = lp_build_fast_rsqrt(&f32_bld, lp_build_fast_rsqrt(&f32_bld, tmp));
+      }
+      else {
+         x0375 = lp_build_sqrt(&f32_bld, lp_build_sqrt(&f32_bld, tmp));
+      }
+
+      a_const = lp_build_const_vec(gallivm, src_type, 0.675f * 1.0622 * 255.0f);
+      b_const = lp_build_const_vec(gallivm, src_type, 0.325f * 1.0622 * 255.0f);
+      c_const = lp_build_const_vec(gallivm, src_type, -0.0620f * 255.0f);
+
+      tmp = lp_build_mul(&f32_bld, a_const, x0375);
+      tmp2 = lp_build_mul(&f32_bld, b_const, x05);
+      tmp2 = lp_build_add(&f32_bld, tmp2, c_const);
+      pow_final = lp_build_add(&f32_bld, tmp, tmp2);
+   }
+
+   /* linear part is easy */
+   lin_const = lp_build_const_vec(gallivm, src_type, 12.92f * 255.0f);
+   lin = lp_build_mul(&f32_bld, src, lin_const);
+
+   lin_thresh = lp_build_const_vec(gallivm, src_type, 0.0031308f);
+   is_linear = lp_build_compare(gallivm, src_type, PIPE_FUNC_LEQUAL, src, lin_thresh);
+   tmp = lp_build_select(&f32_bld, is_linear, lin, pow_final);
+
+   f32_bld.type.sign = 0;
+   return lp_build_iround(&f32_bld, tmp);
+}