drivers/llvmpipe/lp_bld_depth.c

/**************************************************************************
 *
 * Copyright 2009 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
 * Depth/stencil testing to LLVM IR translation.
 *
 * To be done accurately/efficiently the depth/stencil test must be done with
 * the same type/format of the depth/stencil buffer, which implies massaging
 * the incoming depths to fit into place. Using a more straightforward
 * type/format for depth/stencil values internally and only convert when
 * flushing would avoid this, but it would most likely result in depth fighting
 * artifacts.
 *
 * We are free to use a different pixel layout though. Since our basic
 * processing unit is a quad (2x2 pixel block) we store the depth/stencil
 * values tiled, a quad at time. That is, a depth buffer containing
 *
 *  Z11 Z12 Z13 Z14 ...
 *  Z21 Z22 Z23 Z24 ...
 *  Z31 Z32 Z33 Z34 ...
 *  Z41 Z42 Z43 Z44 ...
 *  ... ... ... ... ...
 *
 * will actually be stored in memory as
 *
 *  Z11 Z12 Z21 Z22 Z13 Z14 Z23 Z24 ...
 *  Z31 Z32 Z41 Z42 Z33 Z34 Z43 Z44 ...
 *  ... ... ... ... ... ... ... ... ...
 *
 *
 * Stencil test:
 * Two-sided stencil test is supported but probably not as efficient as
 * it could be.  Currently, we use if/then/else constructs to do the
 * operations for front vs. back-facing polygons.  We could probably do
 * both the front and back arithmetic then use a Select() instruction to
 * choose the result depending on polyon orientation.  We'd have to
 * measure performance both ways and see which is better.
 *
 * @author Jose Fonseca <jfonseca@vmware.com>
 */

#include "pipe/p_state.h"
#include "util/u_format.h"

#include "gallivm/lp_bld_type.h"
#include "gallivm/lp_bld_arit.h"
#include "gallivm/lp_bld_bitarit.h"
#include "gallivm/lp_bld_const.h"
#include "gallivm/lp_bld_conv.h"
#include "gallivm/lp_bld_logic.h"
#include "gallivm/lp_bld_flow.h"
#include "gallivm/lp_bld_intr.h"
#include "gallivm/lp_bld_debug.h"
#include "gallivm/lp_bld_swizzle.h"

#include "lp_bld_depth.h"


/** Used to select fields from pipe_stencil_state */
enum stencil_op {
   S_FAIL_OP,
   Z_FAIL_OP,
   Z_PASS_OP
};


/**
 * Do the stencil test comparison (compare FB stencil values against ref value).
 * This will be used twice when generating two-sided stencil code.
 * \param stencil  the front/back stencil state
 * \param stencilRef  the stencil reference value, replicated as a vector
 * \param stencilVals  vector of stencil values from framebuffer
 * \return vector mask of pass/fail values (~0 or 0)
 */
static LLVMValueRef
lp_build_stencil_test_single(struct lp_build_context *bld,
                             const struct pipe_stencil_state *stencil,
                             LLVMValueRef stencilRef,
                             LLVMValueRef stencilVals)
{
   const unsigned stencilMax = 255; /* XXX fix */
   struct lp_type type = bld->type;
   LLVMValueRef res;

   assert(type.sign);

   assert(stencil->enabled);

   if (stencil->valuemask != stencilMax) {
      /* compute stencilRef = stencilRef & valuemask */
      LLVMValueRef valuemask = lp_build_const_int_vec(type, stencil->valuemask);
      stencilRef = LLVMBuildAnd(bld->builder, stencilRef, valuemask, "");
      /* compute stencilVals = stencilVals & valuemask */
      stencilVals = LLVMBuildAnd(bld->builder, stencilVals, valuemask, "");
   }

   res = lp_build_cmp(bld, stencil->func, stencilRef, stencilVals);

   return res;
}


/**
 * Do the one or two-sided stencil test comparison.
 * \sa lp_build_stencil_test_single
 * \param front_facing  an integer vector mask, indicating front (~0) or back
 *                      (0) facing polygon. If NULL, assume front-facing.
 */
static LLVMValueRef
lp_build_stencil_test(struct lp_build_context *bld,
                      const struct pipe_stencil_state stencil[2],
                      LLVMValueRef stencilRefs[2],
                      LLVMValueRef stencilVals,
                      LLVMValueRef front_facing)
{
   LLVMValueRef res;

   assert(stencil[0].enabled);

   /* do front face test */
   res = lp_build_stencil_test_single(bld, &stencil[0],
                                      stencilRefs[0], stencilVals);

   if (stencil[1].enabled && front_facing) {
      /* do back face test */
      LLVMValueRef back_res;

      back_res = lp_build_stencil_test_single(bld, &stencil[1],
                                              stencilRefs[1], stencilVals);

      res = lp_build_select(bld, front_facing, res, back_res);
   }

   return res;
}


/**
 * Apply the stencil operator (add/sub/keep/etc) to the given vector
 * of stencil values.
 * \return  new stencil values vector
 */
static LLVMValueRef
lp_build_stencil_op_single(struct lp_build_context *bld,
                           const struct pipe_stencil_state *stencil,
                           enum stencil_op op,
                           LLVMValueRef stencilRef,
                           LLVMValueRef stencilVals)

{
   struct lp_type type = bld->type;
   LLVMValueRef res;
   LLVMValueRef max = lp_build_const_int_vec(type, 0xff);
   unsigned stencil_op;

   assert(type.sign);

   switch (op) {
   case S_FAIL_OP:
      stencil_op = stencil->fail_op;
      break;
   case Z_FAIL_OP:
      stencil_op = stencil->zfail_op;
      break;
   case Z_PASS_OP:
      stencil_op = stencil->zpass_op;
      break;
   default:
      assert(0 && "Invalid stencil_op mode");
      stencil_op = PIPE_STENCIL_OP_KEEP;
   }

   switch (stencil_op) {
   case PIPE_STENCIL_OP_KEEP:
      res = stencilVals;
      /* we can return early for this case */
      return res;
   case PIPE_STENCIL_OP_ZERO:
      res = bld->zero;
      break;
   case PIPE_STENCIL_OP_REPLACE:
      res = stencilRef;
      break;
   case PIPE_STENCIL_OP_INCR:
      res = lp_build_add(bld, stencilVals, bld->one);
      res = lp_build_min(bld, res, max);
      break;
   case PIPE_STENCIL_OP_DECR:
      res = lp_build_sub(bld, stencilVals, bld->one);
      res = lp_build_max(bld, res, bld->zero);
      break;
   case PIPE_STENCIL_OP_INCR_WRAP:
      res = lp_build_add(bld, stencilVals, bld->one);
      res = LLVMBuildAnd(bld->builder, res, max, "");
      break;
   case PIPE_STENCIL_OP_DECR_WRAP:
      res = lp_build_sub(bld, stencilVals, bld->one);
      res = LLVMBuildAnd(bld->builder, res, max, "");
      break;
   case PIPE_STENCIL_OP_INVERT:
      res = LLVMBuildNot(bld->builder, stencilVals, "");
      res = LLVMBuildAnd(bld->builder, res, max, "");
      break;
   default:
      assert(0 && "bad stencil op mode");
      res = bld->undef;
   }

   return res;
}


/**
 * Do the one or two-sided stencil test op/update.
 */
static LLVMValueRef
lp_build_stencil_op(struct lp_build_context *bld,
                    const struct pipe_stencil_state stencil[2],
                    enum stencil_op op,
                    LLVMValueRef stencilRefs[2],
                    LLVMValueRef stencilVals,
                    LLVMValueRef mask,
                    LLVMValueRef front_facing)

{
   LLVMValueRef res;

   assert(stencil[0].enabled);

   /* do front face op */
   res = lp_build_stencil_op_single(bld, &stencil[0], op,
                                     stencilRefs[0], stencilVals);

   if (stencil[1].enabled && front_facing) {
      /* do back face op */
      LLVMValueRef back_res;

      back_res = lp_build_stencil_op_single(bld, &stencil[1], op,
                                            stencilRefs[1], stencilVals);

      res = lp_build_select(bld, front_facing, res, back_res);
   }

   if (stencil->writemask != 0xff) {
      /* mask &= stencil->writemask */
      LLVMValueRef writemask = lp_build_const_int_vec(bld->type, stencil->writemask);
      mask = LLVMBuildAnd(bld->builder, mask, writemask, "");
      /* res = (res & mask) | (stencilVals & ~mask) */
      res = lp_build_select_bitwise(bld, writemask, res, stencilVals);
   }
   else {
      /* res = mask ? res : stencilVals */
      res = lp_build_select(bld, mask, res, stencilVals);
   }

   return res;
}


/**
 * Return a type appropriate for depth/stencil testing.
 */
struct lp_type
lp_depth_type(const struct util_format_description *format_desc,
              unsigned length)
{
   struct lp_type type;
   unsigned swizzle;

   assert(format_desc->colorspace == UTIL_FORMAT_COLORSPACE_ZS);
   assert(format_desc->block.width == 1);
   assert(format_desc->block.height == 1);

   swizzle = format_desc->swizzle[0];
   assert(swizzle < 4);

   memset(&type, 0, sizeof type);
   type.width = format_desc->block.bits;

   if(format_desc->channel[swizzle].type == UTIL_FORMAT_TYPE_FLOAT) {
      type.floating = TRUE;
      assert(swizzle == 0);
      assert(format_desc->channel[swizzle].size == format_desc->block.bits);
   }
   else if(format_desc->channel[swizzle].type == UTIL_FORMAT_TYPE_UNSIGNED) {
      assert(format_desc->block.bits <= 32);
      if(format_desc->channel[swizzle].normalized)
         type.norm = TRUE;
   }
   else
      assert(0);

   assert(type.width <= length);
   type.length = length / type.width;

   return type;
}


/**
 * Compute bitmask and bit shift to apply to the incoming fragment Z values
 * and the Z buffer values needed before doing the Z comparison.
 *
 * Note that we leave the Z bits in the position that we find them
 * in the Z buffer (typically 0xffffff00 or 0x00ffffff).  That lets us
 * get by with fewer bit twiddling steps.
 */
static boolean
get_z_shift_and_mask(const struct util_format_description *format_desc,
                     unsigned *shift, unsigned *mask)
{
   const unsigned total_bits = format_desc->block.bits;
   unsigned z_swizzle;
   unsigned chan;
   unsigned padding_left, padding_right;

   assert(format_desc->colorspace == UTIL_FORMAT_COLORSPACE_ZS);
   assert(format_desc->block.width == 1);
   assert(format_desc->block.height == 1);

   z_swizzle = format_desc->swizzle[0];

   if (z_swizzle == UTIL_FORMAT_SWIZZLE_NONE)
      return FALSE;

   padding_right = 0;
   for (chan = 0; chan < z_swizzle; ++chan)
      padding_right += format_desc->channel[chan].size;

   padding_left =
      total_bits - (padding_right + format_desc->channel[z_swizzle].size);

   if (padding_left || padding_right) {
      unsigned long long mask_left = (1ULL << (total_bits - padding_left)) - 1;
      unsigned long long mask_right = (1ULL << (padding_right)) - 1;
      *mask = mask_left ^ mask_right;
   }
   else {
      *mask = 0xffffffff;
   }

   *shift = padding_left;

   return TRUE;
}


/**
 * Compute bitmask and bit shift to apply to the framebuffer pixel values
 * to put the stencil bits in the least significant position.
 * (i.e. 0x000000ff)
 */
static boolean
get_s_shift_and_mask(const struct util_format_description *format_desc,
                     unsigned *shift, unsigned *mask)
{
   unsigned s_swizzle;
   unsigned chan, sz;

   s_swizzle = format_desc->swizzle[1];

   if (s_swizzle == UTIL_FORMAT_SWIZZLE_NONE)
      return FALSE;

   *shift = 0;
   for (chan = 0; chan < s_swizzle; chan++)
      *shift += format_desc->channel[chan].size;

   sz = format_desc->channel[s_swizzle].size;
   *mask = (1U << sz) - 1U;

   return TRUE;
}


/**
 * Perform the occlusion test and increase the counter.
 * Test the depth mask. Add the number of channel which has none zero mask
 * into the occlusion counter. e.g. maskvalue is {-1, -1, -1, -1}.
 * The counter will add 4.
 *
 * \param type holds element type of the mask vector.
 * \param maskvalue is the depth test mask.
 * \param counter is a pointer of the uint32 counter.
 */
static void
lp_build_occlusion_count(LLVMBuilderRef builder,
                         struct lp_type type,
                         LLVMValueRef maskvalue,
                         LLVMValueRef counter)
{
   LLVMValueRef countmask = lp_build_const_int_vec(type, 1);
   LLVMValueRef countv = LLVMBuildAnd(builder, maskvalue, countmask, "countv");
   LLVMTypeRef i8v16 = LLVMVectorType(LLVMInt8Type(), 16);
   LLVMValueRef counti = LLVMBuildBitCast(builder, countv, i8v16, "counti");
   LLVMValueRef maskarray[4] = {
      LLVMConstInt(LLVMInt32Type(), 0, 0),
      LLVMConstInt(LLVMInt32Type(), 4, 0),
      LLVMConstInt(LLVMInt32Type(), 8, 0),
      LLVMConstInt(LLVMInt32Type(), 12, 0),
   };
   LLVMValueRef shufflemask = LLVMConstVector(maskarray, 4);
   LLVMValueRef shufflev =  LLVMBuildShuffleVector(builder, counti, LLVMGetUndef(i8v16), shufflemask, "shufflev");
   LLVMValueRef shuffle = LLVMBuildBitCast(builder, shufflev, LLVMInt32Type(), "shuffle");
   LLVMValueRef count = lp_build_intrinsic_unary(builder, "llvm.ctpop.i32", LLVMInt32Type(), shuffle);
   LLVMValueRef orig = LLVMBuildLoad(builder, counter, "orig");
   LLVMValueRef incr = LLVMBuildAdd(builder, orig, count, "incr");
   LLVMBuildStore(builder, incr, counter);
}


/**
 * Generate code for performing depth and/or stencil tests.
 * We operate on a vector of values (typically a 2x2 quad).
 *
 * \param depth  the depth test state
 * \param stencil  the front/back stencil state
 * \param type  the data type of the fragment depth/stencil values
 * \param format_desc  description of the depth/stencil surface
 * \param mask  the alive/dead pixel mask for the quad (vector)
 * \param stencil_refs  the front/back stencil ref values (scalar)
 * \param z_src  the incoming depth/stencil values (a 2x2 quad, float32)
 * \param zs_dst_ptr  pointer to depth/stencil values in framebuffer
 * \param facing  contains float value indicating front/back facing polygon
 */
void
lp_build_depth_stencil_test(LLVMBuilderRef builder,
                            const struct pipe_depth_state *depth,
                            const struct pipe_stencil_state stencil[2],
                            struct lp_type z_src_type,
                            const struct util_format_description *format_desc,
                            struct lp_build_mask_context *mask,
                            LLVMValueRef stencil_refs[2],
                            LLVMValueRef z_src,
                            LLVMValueRef zs_dst_ptr,
                            LLVMValueRef face,
                            LLVMValueRef counter,
                            boolean do_branch)
{
   struct lp_type type;
   struct lp_build_context bld;
   struct lp_build_context sbld;
   struct lp_type s_type;
   LLVMValueRef zs_dst, z_dst = NULL;
   LLVMValueRef stencil_vals = NULL;
   LLVMValueRef z_bitmask = NULL, stencil_shift = NULL;
   LLVMValueRef z_pass = NULL, s_pass_mask = NULL;
   LLVMValueRef orig_mask = mask->value;
   LLVMValueRef front_facing = NULL;

   /* Prototype a simpler path:
    */
   if (z_src_type.floating &&
       format_desc->format == PIPE_FORMAT_X8Z24_UNORM &&
       depth->enabled)
   {
      LLVMValueRef zscaled;
      LLVMValueRef const_ffffff_float;
      LLVMValueRef const_8_int;
      LLVMTypeRef int32_vec_type;

      /* We know the values in z_dst are all >= 0, so allow
       * lp_build_compare to use signed compare intrinsics:
       */
      type.floating = 0;
      type.fixed = 0;
      type.sign = 1;
      type.norm = 1;
      type.width = 32;
      type.length = z_src_type.length;

      int32_vec_type = LLVMVectorType(LLVMInt32Type(), z_src_type.length);

      const_8_int = lp_build_const_int_vec(type, 8);
      const_ffffff_float = lp_build_const_vec(z_src_type, (float)0xffffff);

      zscaled = LLVMBuildFMul(builder, z_src, const_ffffff_float, "zscaled");
      z_src = LLVMBuildFPToSI(builder, zscaled, int32_vec_type, "z_src");

      /* Load current z/stencil value from z/stencil buffer */
      z_dst = LLVMBuildLoad(builder, zs_dst_ptr, "zsbufval");
      z_dst = LLVMBuildLShr(builder, z_dst, const_8_int, "z_dst");

      /* compare src Z to dst Z, returning 'pass' mask */
      z_pass = lp_build_compare(builder,
                                type,
                                depth->func, z_src, z_dst);

      lp_build_mask_update(mask, z_pass);

      if (do_branch)
         lp_build_mask_check(mask);

      /* No need to worry about old stencil contents, just blend the
       * old and new values and shift into the correct position for
       * storage.
       */
      if (depth->writemask) {
         type.sign = 0;
         lp_build_context_init(&bld, builder, type);

         z_dst = lp_build_select(&bld, mask->value, z_src, z_dst);
         z_dst = LLVMBuildShl(builder, z_dst, const_8_int, "z_dst");
         LLVMBuildStore(builder, z_dst, zs_dst_ptr);
      }

      if (counter)
         lp_build_occlusion_count(builder, type, mask->value, counter);

      return;
   }

   /*
    * Depths are expected to be between 0 and 1, even if they are stored in
    * floats. Setting these bits here will ensure that the lp_build_conv() call
    * below won't try to unnecessarily clamp the incoming values.
    */
   if(z_src_type.floating) {
      z_src_type.sign = FALSE;
      z_src_type.norm = TRUE;
   }
   else {
      assert(!z_src_type.sign);
      assert(z_src_type.norm);
   }

   /* Pick the depth type. */
   type = lp_depth_type(format_desc, z_src_type.width*z_src_type.length);

   /* FIXME: Cope with a depth test type with a different bit width. */
   assert(type.width == z_src_type.width);
   assert(type.length == z_src_type.length);

   /* Convert fragment Z from float to integer */
   lp_build_conv(builder, z_src_type, type, &z_src, 1, &z_src, 1);

   zs_dst_ptr = LLVMBuildBitCast(builder,
                                 zs_dst_ptr,
                                 LLVMPointerType(lp_build_vec_type(type), 0), "");


   /* Sanity checking */
   {
      const unsigned z_swizzle = format_desc->swizzle[0];
      const unsigned s_swizzle = format_desc->swizzle[1];

      assert(z_swizzle != UTIL_FORMAT_SWIZZLE_NONE ||
             s_swizzle != UTIL_FORMAT_SWIZZLE_NONE);

      assert(depth->enabled || stencil[0].enabled);

      assert(format_desc->colorspace == UTIL_FORMAT_COLORSPACE_ZS);
      assert(format_desc->block.width == 1);
      assert(format_desc->block.height == 1);

      if (stencil[0].enabled) {
         assert(format_desc->format == PIPE_FORMAT_Z24_UNORM_S8_USCALED ||
                format_desc->format == PIPE_FORMAT_S8_USCALED_Z24_UNORM);
      }

      assert(z_swizzle < 4);
      assert(format_desc->block.bits == type.width);
      if (type.floating) {
         assert(z_swizzle == 0);
         assert(format_desc->channel[z_swizzle].type ==
                UTIL_FORMAT_TYPE_FLOAT);
         assert(format_desc->channel[z_swizzle].size ==
                format_desc->block.bits);
      }
      else {
         assert(format_desc->channel[z_swizzle].type ==
                UTIL_FORMAT_TYPE_UNSIGNED);
         assert(format_desc->channel[z_swizzle].normalized);
         assert(!type.fixed);
         assert(!type.sign);
         assert(type.norm);
      }
   }


   /* Setup build context for Z vals */
   lp_build_context_init(&bld, builder, type);

   /* Setup build context for stencil vals */
   s_type = lp_type_int_vec(type.width);
   lp_build_context_init(&sbld, builder, s_type);

   /* Load current z/stencil value from z/stencil buffer */
   zs_dst = LLVMBuildLoad(builder, zs_dst_ptr, "");

   lp_build_name(zs_dst, "zsbufval");


   /* Compute and apply the Z/stencil bitmasks and shifts.
    */
   {
      unsigned z_shift, z_mask;
      unsigned s_shift, s_mask;

      if (get_z_shift_and_mask(format_desc, &z_shift, &z_mask)) {
         if (z_shift) {
            LLVMValueRef shift = lp_build_const_int_vec(type, z_shift);
            z_src = LLVMBuildLShr(builder, z_src, shift, "");
         }

         if (z_mask != 0xffffffff) {
            LLVMValueRef mask = lp_build_const_int_vec(type, z_mask);
            z_src = LLVMBuildAnd(builder, z_src, mask, "");
            z_dst = LLVMBuildAnd(builder, zs_dst, mask, "");
            z_bitmask = mask;  /* used below */
         }
         else {
            z_dst = zs_dst;
         }

         lp_build_name(z_dst, "zsbuf.z");
      }

      if (get_s_shift_and_mask(format_desc, &s_shift, &s_mask)) {
         if (s_shift) {
            LLVMValueRef shift = lp_build_const_int_vec(type, s_shift);
            stencil_vals = LLVMBuildLShr(builder, zs_dst, shift, "");
            stencil_shift = shift;  /* used below */
         }
         else {
            stencil_vals = zs_dst;
         }

         if (s_mask != 0xffffffff) {
            LLVMValueRef mask = lp_build_const_int_vec(type, s_mask);
            stencil_vals = LLVMBuildAnd(builder, stencil_vals, mask, "");
         }

         lp_build_name(stencil_vals, "stencil");
      }
   }

   if (stencil[0].enabled) {

      if (face) {
         LLVMValueRef zero = LLVMConstReal(LLVMFloatType(), 0.0);

         /* front_facing = face > 0.0 ? ~0 : 0 */
         front_facing = LLVMBuildFCmp(builder, LLVMRealUGT, face, zero, "");
         front_facing = LLVMBuildSExt(builder, front_facing,
                                      LLVMIntType(bld.type.length*bld.type.width),
                                      "");
         front_facing = LLVMBuildBitCast(builder, front_facing,
                                         bld.int_vec_type, "");
      }

      /* convert scalar stencil refs into vectors */
      stencil_refs[0] = lp_build_broadcast_scalar(&bld, stencil_refs[0]);
      stencil_refs[1] = lp_build_broadcast_scalar(&bld, stencil_refs[1]);

      s_pass_mask = lp_build_stencil_test(&sbld, stencil,
                                          stencil_refs, stencil_vals,
                                          front_facing);

      /* apply stencil-fail operator */
      {
         LLVMValueRef s_fail_mask = lp_build_andnot(&bld, orig_mask, s_pass_mask);
         stencil_vals = lp_build_stencil_op(&sbld, stencil, S_FAIL_OP,
                                            stencil_refs, stencil_vals,
                                            s_fail_mask, front_facing);
      }
   }

   if (depth->enabled) {
      /* compare src Z to dst Z, returning 'pass' mask */
      z_pass = lp_build_cmp(&bld, depth->func, z_src, z_dst);

      if (!stencil[0].enabled) {
         /* We can potentially skip all remaining operations here, but only
          * if stencil is disabled because we still need to update the stencil
          * buffer values.  Don't need to update Z buffer values.
          */
         lp_build_mask_update(mask, z_pass);

         if (do_branch) {
            lp_build_mask_check(mask);
            do_branch = FALSE;
         }
      }

      if (depth->writemask) {
         LLVMValueRef zselectmask = mask->value;

         /* mask off bits that failed Z test */
         zselectmask = LLVMBuildAnd(builder, zselectmask, z_pass, "");

         /* mask off bits that failed stencil test */
         if (s_pass_mask) {
            zselectmask = LLVMBuildAnd(builder, zselectmask, s_pass_mask, "");
         }

         /* if combined Z/stencil format, mask off the stencil bits */
         if (z_bitmask) {
            zselectmask = LLVMBuildAnd(builder, zselectmask, z_bitmask, "");
         }

         /* Mix the old and new Z buffer values.
          * z_dst[i] = (zselectmask[i] & z_src[i]) | (~zselectmask[i] & z_dst[i])
          */
         z_dst = lp_build_select_bitwise(&bld, zselectmask, z_src, z_dst);
      }

      if (stencil[0].enabled) {
         /* update stencil buffer values according to z pass/fail result */
         LLVMValueRef z_fail_mask, z_pass_mask;

         /* apply Z-fail operator */
         z_fail_mask = lp_build_andnot(&bld, orig_mask, z_pass);
         stencil_vals = lp_build_stencil_op(&sbld, stencil, Z_FAIL_OP,
                                            stencil_refs, stencil_vals,
                                            z_fail_mask, front_facing);

         /* apply Z-pass operator */
         z_pass_mask = LLVMBuildAnd(bld.builder, orig_mask, z_pass, "");
         stencil_vals = lp_build_stencil_op(&sbld, stencil, Z_PASS_OP,
                                            stencil_refs, stencil_vals,
                                            z_pass_mask, front_facing);
      }
   }
   else {
      /* No depth test: apply Z-pass operator to stencil buffer values which
       * passed the stencil test.
       */
      s_pass_mask = LLVMBuildAnd(bld.builder, orig_mask, s_pass_mask, "");
      stencil_vals = lp_build_stencil_op(&sbld, stencil, Z_PASS_OP,
                                         stencil_refs, stencil_vals,
                                         s_pass_mask, front_facing);
   }

   /* The Z bits are already in the right place but we may need to shift the
    * stencil bits before ORing Z with Stencil to make the final pixel value.
    */
   if (stencil_vals && stencil_shift)
      stencil_vals = LLVMBuildShl(bld.builder, stencil_vals,
                                  stencil_shift, "");

   /* Finally, merge/store the z/stencil values */
   if ((depth->enabled && depth->writemask) ||
       (stencil[0].enabled && stencil[0].writemask)) {

      if (z_dst && stencil_vals)
         zs_dst = LLVMBuildOr(bld.builder, z_dst, stencil_vals, "");
      else if (z_dst)
         zs_dst = z_dst;
      else
         zs_dst = stencil_vals;

      LLVMBuildStore(builder, zs_dst, zs_dst_ptr);
   }

   if (s_pass_mask)
      lp_build_mask_update(mask, s_pass_mask);

   if (depth->enabled && stencil[0].enabled)
      lp_build_mask_update(mask, z_pass);

   if (do_branch)
      lp_build_mask_check(mask);

   if (counter)
      lp_build_occlusion_count(builder, type, mask->value, counter);
}