nir/lower_tex: Rework invalid implicit LOD lowering

Only fragment and some compute shaders support implicit derivatives.
They're totally meaningless without helper invocations and some
understanding of the dispatch pattern.  We've got code to lower
nir_texop_tex in these shader stages to use an explicit derivative of 0
but it was pretty badly broken:

 1. It only handled nir_texop_tex, not nir_texop_txb or nir_texop_lod.

 2. It didn't take min_lod into account

 3. It was conflated with adding a missing LOD parameter to opcodes
    which expect one such as nir_texop_txf.  While not really a bug,
    this does make it way harder to reason about the code.

 4. Unless you set a flag (which most drivers don't), it left the
    opcode nir_texop_tex instead of nir_texop_txl which it should have
    been.

This reworks it to go through roughly the same path as other LOD
lowering only with a constant lod of 0 instead of calling out to
nir_texop_lod.  We also get rid of the lower_tex_without_implicit_lod
flag because most drivers set it and those that don't are probably
subtly broken.  If someone really wants to get nir_texop_tex in their
vertex shaders, they can write a new patch to add the flag back in.

Fixes: e382890e25 "nir: set default lod to texture opcodes that..."
Fixes: d5ac5d6e83 "nir: Add option to lower tex to txl when..."
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11775>

src/compiler/nir/nir.h

@@ -5019,12 +5019,6 @@ typedef struct nir_lower_tex_options {
     */
    unsigned lower_srgb;
 
-   /**
-    * If true, lower nir_texop_tex on shaders that doesn't support implicit
-    * LODs to nir_texop_txl.
-    */
-   bool lower_tex_without_implicit_lod;
-
    /**
     * If true, lower nir_texop_txd on cube maps with nir_texop_txl.
     */

src/compiler/nir/nir_lower_tex.c

@@ -230,17 +230,13 @@ lower_rect_tex_scale(nir_builder *b, nir_tex_instr *tex)
 }
 
 static void
-lower_implicit_lod(nir_builder *b, nir_tex_instr *tex)
+lower_lod(nir_builder *b, nir_tex_instr *tex, nir_ssa_def *lod)
 {
    assert(tex->op == nir_texop_tex || tex->op == nir_texop_txb);
    assert(nir_tex_instr_src_index(tex, nir_tex_src_lod) < 0);
    assert(nir_tex_instr_src_index(tex, nir_tex_src_ddx) < 0);
    assert(nir_tex_instr_src_index(tex, nir_tex_src_ddy) < 0);
 
-   b->cursor = nir_before_instr(&tex->instr);
-
-   nir_ssa_def *lod = nir_get_texture_lod(b, tex);
-
    int bias_idx = nir_tex_instr_src_index(tex, nir_tex_src_bias);
    if (bias_idx >= 0) {
       /* If we have a bias, add it in */
@@ -259,6 +255,27 @@ lower_implicit_lod(nir_builder *b, nir_tex_instr *tex)
    tex->op = nir_texop_txl;
 }
 
+static void
+lower_implicit_lod(nir_builder *b, nir_tex_instr *tex)
+{
+   b->cursor = nir_before_instr(&tex->instr);
+   lower_lod(b, tex, nir_get_texture_lod(b, tex));
+}
+
+static void
+lower_zero_lod(nir_builder *b, nir_tex_instr *tex)
+{
+   b->cursor = nir_before_instr(&tex->instr);
+
+   if (tex->op == nir_texop_lod) {
+      nir_ssa_def_rewrite_uses(&tex->dest.ssa, nir_imm_int(b, 0));
+      nir_instr_remove(&tex->instr);
+      return;
+   }
+
+   lower_lod(b, tex, nir_imm_int(b, 0));
+}
+
 static nir_ssa_def *
 sample_plane(nir_builder *b, nir_tex_instr *tex, int plane,
              const nir_lower_tex_options *options)
@@ -1340,26 +1357,33 @@ nir_lower_tex_block(nir_block *block, nir_builder *b,
          continue;
       }
 
-      bool shader_supports_implicit_lod =
-         b->shader->info.stage == MESA_SHADER_FRAGMENT ||
-         (b->shader->info.stage == MESA_SHADER_COMPUTE &&
-          b->shader->info.cs.derivative_group != DERIVATIVE_GROUP_NONE);
-
       /* TXF, TXS and TXL require a LOD but not everything we implement using those
        * three opcodes provides one.  Provide a default LOD of 0.
        */
       if ((nir_tex_instr_src_index(tex, nir_tex_src_lod) == -1) &&
           (tex->op == nir_texop_txf || tex->op == nir_texop_txs ||
-           tex->op == nir_texop_txl || tex->op == nir_texop_query_levels ||
-           (tex->op == nir_texop_tex && !shader_supports_implicit_lod))) {
+           tex->op == nir_texop_txl || tex->op == nir_texop_query_levels)) {
          b->cursor = nir_before_instr(&tex->instr);
          nir_tex_instr_add_src(tex, nir_tex_src_lod, nir_src_for_ssa(nir_imm_int(b, 0)));
-         if (tex->op == nir_texop_tex && options->lower_tex_without_implicit_lod)
-            tex->op = nir_texop_txl;
          progress = true;
          continue;
       }
 
+      /* Only fragment and compute (in some cases) support implicit
+       * derivatives.  Lower those opcodes which use implicit derivatives to
+       * use an explicit LOD of 0.
+       */
+      bool shader_supports_implicit_lod =
+         b->shader->info.stage == MESA_SHADER_FRAGMENT ||
+         (b->shader->info.stage == MESA_SHADER_COMPUTE &&
+          b->shader->info.cs.derivative_group != DERIVATIVE_GROUP_NONE);
+
+      if (nir_tex_instr_has_implicit_derivative(tex) &&
+          !shader_supports_implicit_lod) {
+         lower_zero_lod(b, tex);
+         progress = true;
+      }
+
       if (options->lower_txs_lod && tex->op == nir_texop_txs) {
          progress |= nir_lower_txs_lod(b, tex);
          continue;

src/gallium/auxiliary/gallivm/lp_bld_nir.c

@@ -2349,7 +2349,7 @@ void lp_build_opt_nir(struct nir_shader *nir)
       NIR_PASS(progress, nir, nir_opt_algebraic);
       NIR_PASS(progress, nir, nir_lower_pack);
 
-      nir_lower_tex_options options = { .lower_tex_without_implicit_lod = true };
+      nir_lower_tex_options options = { 0, };
       NIR_PASS_V(nir, nir_lower_tex, &options);
 
       const nir_lower_subgroups_options subgroups_options = {

src/intel/compiler/brw_nir.c

@@ -831,7 +831,6 @@ brw_preprocess_nir(const struct brw_compiler *compiler, nir_shader *nir,
       .lower_txp = ~0,
       .lower_txf_offset = true,
       .lower_rect_offset = true,
-      .lower_tex_without_implicit_lod = true,
       .lower_txd_cube_map = true,
       .lower_txd_3d = devinfo->verx10 >= 125,
       .lower_txb_shadow_clamp = true,

src/panfrost/bifrost/bifrost_compile.c

@@ -3190,7 +3190,6 @@ bi_optimize_nir(nir_shader *nir, unsigned gpu_id, bool is_blend)
         nir_lower_tex_options lower_tex_options = {
                 .lower_txs_lod = true,
                 .lower_txp = ~0,
-                .lower_tex_without_implicit_lod = true,
                 .lower_tg4_broadcom_swizzle = true,
                 .lower_txd = true,
         };