panvk-bifrost/mesa-panvk-bifrost/iter16/applied_state/panvk_vX_winding.c

/*
 * Copyright © 2026 mfritsche / claude-noether
 * SPDX-License-Identifier: MIT
 *
 * iter16: primitive-decomposition tables for transform_feedback capture
 * on PanVk-Bifrost (PAN_ARCH < 9 only). When XFB is active and the bound
 * topology is a strip/fan/adjacency variant, the Vulkan spec requires
 * vertices to be captured AS IF the primitive sequence were decomposed
 * into a list of independent primitives. iter13's pan_nir_lower_xfb
 * captures one entry per VS invocation, which gives one output per input
 * vertex — wrong for non-LIST topologies.
 *
 * This file holds the seven decomposition tables (one per affected
 * topology). Caller (jm/panvk_vX_cmd_draw.c CmdDraw) walks the table to
 * build a synthetic index buffer, overrides the bound topology to the
 * equivalent LIST, and dispatches as an indexed draw — the existing
 * pan_nir_lower_xfb formula then writes the right number of entries in
 * the right order.
 *
 * See ~/src/panvk-bifrost/iter16/phase2_design.md for the design lock.
 */

#include "panvk_macros.h"

#if PAN_ARCH < 9

#include "panvk_cmd_draw.h"

#include <vulkan/vulkan_core.h>

/* TRIANGLE_STRIP: 3*(N-2) outputs.
 *   Even prim i: {i, i+1, i+2}
 *   Odd  prim i: {i, i+2, i+1}   ← winding reverses, hence "winding" tests
 */
static uint32_t
prim_count_tri_strip(uint32_t n)
{
   return (n >= 2) ? (n - 2) : 0;
}

static void
expected_tri_strip(uint32_t i, uint32_t *out)
{
   uint32_t iMod2 = i & 1u;
   out[0] = i;
   out[1] = i + 1 + iMod2;
   out[2] = i + 2 - iMod2;
}

/* LINE_STRIP: 2*(N-1) outputs. Each prim i: {i, i+1} */
static uint32_t
prim_count_line_strip(uint32_t n)
{
   return (n >= 1) ? (n - 1) : 0;
}

static void
expected_line_strip(uint32_t i, uint32_t *out)
{
   out[0] = i;
   out[1] = i + 1u;
}

/* TRIANGLE_FAN: 3*(N-2) outputs. Each prim i: {i+1, i+2, 0} */
static uint32_t
prim_count_tri_fan(uint32_t n)
{
   return (n >= 2) ? (n - 2) : 0;
}

static void
expected_tri_fan(uint32_t i, uint32_t *out)
{
   out[0] = i + 1u;
   out[1] = i + 2u;
   out[2] = 0u;
}

/* LINE_LIST_WITH_ADJACENCY: N/4 primitives, each emits {i+1, i+2} from
 * the 4-vertex input window (i, i+1, i+2, i+3). N must be a multiple of 4. */
static uint32_t
prim_count_line_list_adj(uint32_t n)
{
   return n / 4u;
}

static void
expected_line_list_adj(uint32_t i, uint32_t *out)
{
   out[0] = 4 * i + 1u;
   out[1] = 4 * i + 2u;
}

/* LINE_STRIP_WITH_ADJACENCY: 2*(N-3) outputs. Each prim i: {i+1, i+2} */
static uint32_t
prim_count_line_strip_adj(uint32_t n)
{
   return (n >= 3) ? (n - 3) : 0;
}

static void
expected_line_strip_adj(uint32_t i, uint32_t *out)
{
   out[0] = i + 1u;
   out[1] = i + 2u;
}

/* TRIANGLE_LIST_WITH_ADJACENCY: N/2 inputs map to N/6 primitives, each emits
 * {6*i, 6*i+2, 6*i+4} from the 6-vertex input window. */
static uint32_t
prim_count_tri_list_adj(uint32_t n)
{
   return n / 6u;
}

static void
expected_tri_list_adj(uint32_t i, uint32_t *out)
{
   out[0] = 6 * i + 0u;
   out[1] = 6 * i + 2u;
   out[2] = 6 * i + 4u;
}

/* TRIANGLE_STRIP_WITH_ADJACENCY: 3*(N/2-2) outputs with winding flip on odd.
 *   Even prim i: {2i, 2i+2, 2i+4}
 *   Odd  prim i: {2i, 2i+4, 2i+2}
 */
static uint32_t
prim_count_tri_strip_adj(uint32_t n)
{
   return (n >= 6) ? (3u * (n / 2u - 2u) / 3u) : 0;
   /* That's just (n/2 - 2) primitives, each emitting 3. */
}

static void
expected_tri_strip_adj(uint32_t i, uint32_t *out)
{
   bool even = ((i & 1u) == 0u);
   out[0] = 2 * i + 0u;
   if (even) {
      out[1] = 2 * i + 2u;
      out[2] = 2 * i + 4u;
   } else {
      out[1] = 2 * i + 4u;
      out[2] = 2 * i + 2u;
   }
}

/* The table itself — gated to topologies that need decomposition.
 * LIST topologies (POINT_LIST, LINE_LIST, TRIANGLE_LIST) return NULL. */
const struct panvk_winding_table *
panvk_per_arch(get_winding_table)(VkPrimitiveTopology topo)
{
   static const struct panvk_winding_table TABLES[] = {
      [VK_PRIMITIVE_TOPOLOGY_LINE_STRIP] = {
         .verts_per_prim = 2,
         .prim_count = prim_count_line_strip,
         .decompose = expected_line_strip,
         .list_equiv = VK_PRIMITIVE_TOPOLOGY_LINE_LIST,
         .name = "LINE_STRIP",
      },
      [VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP] = {
         .verts_per_prim = 3,
         .prim_count = prim_count_tri_strip,
         .decompose = expected_tri_strip,
         .list_equiv = VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST,
         .name = "TRIANGLE_STRIP",
      },
      [VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN] = {
         .verts_per_prim = 3,
         .prim_count = prim_count_tri_fan,
         .decompose = expected_tri_fan,
         .list_equiv = VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST,
         .name = "TRIANGLE_FAN",
      },
      [VK_PRIMITIVE_TOPOLOGY_LINE_LIST_WITH_ADJACENCY] = {
         .verts_per_prim = 2,
         .prim_count = prim_count_line_list_adj,
         .decompose = expected_line_list_adj,
         .list_equiv = VK_PRIMITIVE_TOPOLOGY_LINE_LIST,
         .name = "LINE_LIST_WITH_ADJ",
      },
      [VK_PRIMITIVE_TOPOLOGY_LINE_STRIP_WITH_ADJACENCY] = {
         .verts_per_prim = 2,
         .prim_count = prim_count_line_strip_adj,
         .decompose = expected_line_strip_adj,
         .list_equiv = VK_PRIMITIVE_TOPOLOGY_LINE_LIST,
         .name = "LINE_STRIP_WITH_ADJ",
      },
      [VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST_WITH_ADJACENCY] = {
         .verts_per_prim = 3,
         .prim_count = prim_count_tri_list_adj,
         .decompose = expected_tri_list_adj,
         .list_equiv = VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST,
         .name = "TRIANGLE_LIST_WITH_ADJ",
      },
      [VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_WITH_ADJACENCY] = {
         .verts_per_prim = 3,
         .prim_count = prim_count_tri_strip_adj,
         .decompose = expected_tri_strip_adj,
         .list_equiv = VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST,
         .name = "TRIANGLE_STRIP_WITH_ADJ",
      },
   };

   if (topo >= ARRAY_SIZE(TABLES))
      return NULL;
   const struct panvk_winding_table *t = &TABLES[topo];
   /* Slots not in our table list above have verts_per_prim==0 (zero-init) */
   return t->verts_per_prim ? t : NULL;
}

#endif /* PAN_ARCH < 9 */