nir: Add a faster lowest common ancestor algorithm

On a fossil from the blender 4.5.0 vulkan backend, this improves compile
times in nak by about 17%. Compile time of other shaders improves by a
more modest 1.2%.

No stat changes on shader-db.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36184>

src/compiler/nir/meson.build

@@ -114,6 +114,7 @@ else
   'nir_deref.h',
   'nir_divergence_analysis.c',
   'nir_dominance.c',
+  'nir_dominance_lca.c',
   'nir_fixup_is_exported.c',
   'nir_format_convert.c',
   'nir_format_convert.h',

src/compiler/nir/nir.c

@@ -692,6 +692,8 @@ nir_function_impl_create_bare(nir_shader *shader)
    impl->num_blocks = 0;
    impl->valid_metadata = nir_metadata_none;
    impl->structured = true;
+   range_minimum_query_table_init(&impl->dom_lca_info.table);
+   impl->dom_lca_info.block_from_idx = NULL;
 
    /* create start & end blocks */
    nir_block *start_block = nir_block_create(shader);

src/compiler/nir/nir.h

@@ -41,6 +41,7 @@
 #include "util/log.h"
 #include "util/macros.h"
 #include "util/ralloc.h"
+#include "util/range_minimum_query.h"
 #include "util/set.h"
 #include "util/u_math.h"
 #include "nir_defines.h"
@@ -3476,6 +3477,17 @@ typedef enum {
     */
    nir_metadata_divergence = 0x40,
 
+   /** Indicates that block dominance lca information is valid
+    *
+    * This includes:
+    *
+    *   - nir_function_impl::dom_lca_info
+    *
+    * A pass can preserve this metadata type if it preserves
+    * nir_metadata_dominance.
+    */
+   nir_metadata_dominance_lca = 0x80,
+
    /** All control flow metadata
     *
     * This includes all metadata preserved by a pass that preserves control flow
@@ -3486,7 +3498,8 @@ typedef enum {
     * This is the most common metadata set to preserve, so it has its own alias.
     */
    nir_metadata_control_flow = nir_metadata_block_index |
-                               nir_metadata_dominance,
+                               nir_metadata_dominance |
+                               nir_metadata_dominance_lca,
 
    /** All metadata
     *
@@ -3524,6 +3537,12 @@ typedef struct nir_function_impl {
    /* total number of basic blocks, only valid when block_index_dirty = false */
    unsigned num_blocks;
 
+   /** Information used for LCA queries */
+   struct nir_dom_lca_info {
+      struct range_minimum_query_table table;
+      nir_block **block_from_idx;
+   } dom_lca_info;
+
    /** True if this nir_function_impl uses structured control-flow
     *
     * Structured nir_function_impls have different validation rules.
@@ -4921,8 +4940,18 @@ bool nir_shader_lower_instructions(nir_shader *shader,
 
 void nir_calc_dominance_impl(nir_function_impl *impl);
 void nir_calc_dominance(nir_shader *shader);
+void nir_calc_dominance_lca_impl(nir_function_impl *impl);
 
+/**
+ * Computes the lowest common ancestor of two blocks in the dominator tree.
+ *
+ * If one of the blocks is null or unreachable, the other block is returned or
+ * NULL if it's unreachable.
+ *
+ * Requires nir_metadata_dominance_lca
+ */
 nir_block *nir_dominance_lca(nir_block *b1, nir_block *b2);
+
 bool nir_block_dominates(nir_block *parent, nir_block *child);
 bool nir_block_is_unreachable(nir_block *block);
 

src/compiler/nir/nir_dominance.c

@@ -212,35 +212,6 @@ nir_calc_dominance(nir_shader *shader)
    }
 }
 
-static nir_block *
-block_return_if_reachable(nir_block *b)
-{
-   return (b && nir_block_is_reachable(b)) ? b : NULL;
-}
-
-/**
- * Computes the least common ancestor of two blocks.  If one of the blocks
- * is null or unreachable, the other block is returned or NULL if it's
- * unreachable.
- */
-nir_block *
-nir_dominance_lca(nir_block *b1, nir_block *b2)
-{
-   if (b1 == NULL || !nir_block_is_reachable(b1))
-      return block_return_if_reachable(b2);
-
-   if (b2 == NULL || !nir_block_is_reachable(b2))
-      return block_return_if_reachable(b1);
-
-   assert(nir_cf_node_get_function(&b1->cf_node) ==
-          nir_cf_node_get_function(&b2->cf_node));
-
-   assert(nir_cf_node_get_function(&b1->cf_node)->valid_metadata &
-          nir_metadata_dominance);
-
-   return intersect(b1, b2);
-}
-
 /**
  * Returns true if parent dominates child according to the following
  * definition:

src/compiler/nir/nir_dominance_lca.c

@@ -0,0 +1,177 @@
+/*
+ * Copyright 2025 Valve Corporation
+ * SPDX-License-Identifier: MIT
+ */
+
+#include "nir.h"
+
+/**
+ * Find the lowest common ancestor in the dominance tree.
+ *
+ * We reduce the LCA problem to range minimum query using the standard euler
+ * tour method (see eg. Bender and Colton section 2). From there, we use the
+ * simple RMQ algorithm that uses O(n log n) preprcessing time and O(1) query
+ * time (Bender and Colton section 3).
+ *
+ * As a slight modification, we store the block index instead of the block
+ * depth. We can do this because the lower tree depth is always at a lower block
+ * index and we use an RMQ algorithm that doesn't rely on the -1/+1 property.
+ *
+ * Bender, M.A., Farach-Colton, M. (2000). The LCA Problem Revisited. In:
+ *     Gonnet, G.H., Viola, A. (eds) LATIN 2000: Theoretical Informatics. LATIN
+ *     2000. Lecture Notes in Computer Science, vol 1776. Springer, Berlin,
+ *     Heidelberg. https://doi.org/10.1007/10719839_9
+ */
+
+static void
+realloc_info(nir_function_impl *impl)
+{
+   struct nir_dom_lca_info *info = &impl->dom_lca_info;
+   const uint32_t euler_tour_size = impl->num_blocks * 2 - 1;
+
+   void *mem_ctx = ralloc_parent(impl);
+   range_minimum_query_table_resize(&info->table, mem_ctx, euler_tour_size);
+   info->block_from_idx = reralloc_array_size(mem_ctx, info->block_from_idx,
+                                              sizeof(nir_block *),
+                                              impl->num_blocks);
+}
+
+static uint32_t
+dom_lca_representative(nir_block *block)
+{
+   /* The dom_pre_index is 1-indexed so we need to subtract one to match our
+    * indices
+    */
+   return block->dom_pre_index - 1;
+}
+
+static void
+generate_euler_tour(nir_function_impl *impl)
+{
+   uint32_t *table = impl->dom_lca_info.table.table;
+   nir_block **block_from_idx = impl->dom_lca_info.block_from_idx;
+   if (impl->num_blocks == 1) {
+      nir_block *block = nir_start_block(impl);
+      table[0] = 0;
+      block_from_idx[0] = block;
+      return;
+   }
+
+   /* By definition, the first row of the table contains range minimum query
+    * lookups for each single-element block, meaning it is just the array that
+    * we will perform RMQs on. Therefore, when generating the Euler tour, we
+    * store results in the first row and are free to use the rest of the table
+    * as scratch memory for the depth-first search.
+    *
+    * The stack contains the index of the node's next child to visit.
+    */
+   assert(impl->dom_lca_info.table.height >= 2);
+   STATIC_ASSERT(sizeof(uint32_t) <= sizeof(nir_block *));
+   uint32_t *dfs_stack = (uint32_t *)&table[impl->dom_lca_info.table.width];
+
+   nir_block *cur_block = nir_start_block(impl);
+   uint32_t *cur_stack = dfs_stack;
+
+   bool first_visit = true;
+   uint32_t i;
+   for (i = 0; i < impl->dom_lca_info.table.width; i++) {
+      if (cur_block == NULL) {
+         /* This can happen earlier than expected if some blocks are
+          * unreachable
+          */
+         break;
+      }
+
+      assert(cur_stack >= dfs_stack);
+      table[i] = cur_block->index;
+
+      if (first_visit) {
+         /* First visit. Place it on the stack. */
+         *cur_stack = 0;
+         assert(i == dom_lca_representative(cur_block));
+         block_from_idx[cur_block->index] = cur_block;
+      }
+
+      if (*cur_stack < cur_block->num_dom_children) {
+         cur_block = cur_block->dom_children[*cur_stack];
+         *cur_stack += 1;
+         cur_stack += 1;
+         first_visit = true;
+      } else {
+         assert(*cur_stack == cur_block->num_dom_children);
+         cur_block = cur_block->imm_dom;
+         cur_stack -= 1;
+         first_visit = false;
+      }
+   }
+
+   assert(cur_block == NULL);
+
+   if (i != impl->dom_lca_info.table.width) {
+      void *mem_ctx = ralloc_parent(impl);
+      range_minimum_query_table_resize(&impl->dom_lca_info.table, mem_ctx, i);
+   }
+}
+
+void
+nir_calc_dominance_lca_impl(nir_function_impl *impl)
+{
+   if (impl->valid_metadata & nir_metadata_dominance_lca)
+      return;
+
+   nir_metadata_require(impl, nir_metadata_block_index |
+                                 nir_metadata_dominance);
+
+   realloc_info(impl);
+   generate_euler_tour(impl);
+   range_minimum_query_table_preprocess(&impl->dom_lca_info.table);
+}
+
+static nir_block *
+block_return_if_reachable(nir_block *b)
+{
+   return (b && nir_block_is_reachable(b)) ? b : NULL;
+}
+
+static bool
+is_lca(nir_block *result, nir_block *b1, nir_block *b2)
+{
+   if (!nir_block_dominates(result, b1) || !nir_block_dominates(result, b2))
+      return false;
+
+   for (int i = 0; i < result->num_dom_children; i++) {
+      nir_block *child = result->dom_children[i];
+      if (nir_block_dominates(child, b1) &&
+          nir_block_dominates(child, b2))
+         return false;
+   }
+
+   return true;
+}
+
+nir_block *
+nir_dominance_lca(nir_block *b1, nir_block *b2)
+{
+   if (b1 == NULL || !nir_block_is_reachable(b1))
+      return block_return_if_reachable(b2);
+
+   if (b2 == NULL || !nir_block_is_reachable(b2))
+      return block_return_if_reachable(b1);
+
+   assert(nir_cf_node_get_function(&b1->cf_node) ==
+          nir_cf_node_get_function(&b2->cf_node));
+
+   nir_function_impl *impl = nir_cf_node_get_function(&b1->cf_node);
+   assert(impl->valid_metadata & nir_metadata_dominance_lca);
+
+   uint32_t i1 = dom_lca_representative(b1);
+   uint32_t i2 = dom_lca_representative(b2);
+   if (i1 > i2)
+      SWAP(i1, i2);
+   uint32_t index = range_minimum_query(&impl->dom_lca_info.table, i1, i2 + 1);
+   nir_block *result = impl->dom_lca_info.block_from_idx[index];
+
+   assert(is_lca(result, b1, b2));
+
+   return result;
+}

src/compiler/nir/nir_metadata.c

@@ -38,6 +38,8 @@ nir_metadata_require(nir_function_impl *impl, nir_metadata required, ...)
       nir_index_instrs(impl);
    if (NEEDS_UPDATE(nir_metadata_dominance))
       nir_calc_dominance_impl(impl);
+   if (NEEDS_UPDATE(nir_metadata_dominance_lca))
+      nir_calc_dominance_lca_impl(impl);
    if (NEEDS_UPDATE(nir_metadata_live_defs))
       nir_live_defs_impl(impl);
    if (NEEDS_UPDATE(nir_metadata_divergence))
@@ -73,6 +75,9 @@ nir_progress(bool progress, nir_function_impl *impl, nir_metadata preserved)
    if (!progress)
       preserved = nir_metadata_all;
 
+   if (!(preserved & nir_metadata_dominance))
+      assert(!(preserved & nir_metadata_dominance_lca));
+
    /* If we discard valid liveness information, immediately free the
     * liveness information for each block. For large shaders, it can
     * consume a huge amount of memory, and it's usually not immediately

src/compiler/nir/nir_opt_gcm.c

@@ -796,8 +796,9 @@ weak_gvn(const nir_instr *a, const nir_instr *b)
 static bool
 opt_gcm_impl(nir_shader *shader, nir_function_impl *impl, bool value_number)
 {
-   nir_metadata_require(impl,
-                        nir_metadata_block_index | nir_metadata_dominance);
+   nir_metadata_require(impl, nir_metadata_block_index |
+                              nir_metadata_dominance |
+                              nir_metadata_dominance_lca);
    nir_metadata_require(impl, nir_metadata_loop_analysis,
                         shader->options->force_indirect_unrolling,
                         shader->options->force_indirect_unrolling_sampler);

src/compiler/nir/nir_opt_sink.c

@@ -361,11 +361,12 @@ nir_opt_sink(nir_shader *shader, nir_move_options options)
    bool progress = false;
 
    nir_foreach_function_impl(impl, shader) {
-      nir_metadata_require(impl,
-                           nir_metadata_block_index | nir_metadata_dominance |
-                           (options & (nir_move_only_convergent |
-                                       nir_move_only_divergent) ?
-                               nir_metadata_divergence : 0));
+      nir_metadata required = nir_metadata_block_index |
+                              nir_metadata_dominance |
+                              nir_metadata_dominance_lca;
+      if (options & (nir_move_only_convergent | nir_move_only_divergent))
+         required |= nir_metadata_divergence;
+      nir_metadata_require(impl, required);
 
       nir_foreach_block_reverse(block, impl) {
          nir_foreach_instr_reverse_safe(instr, block) {

src/intel/compiler/brw_nir_lower_rt_intrinsics_pre_trace.c

@@ -64,7 +64,8 @@ brw_nir_lower_rt_intrinsics_pre_trace(nir_shader *nir)
 
    if (intrinsics->entries > 0) {
       nir_foreach_function_with_impl(func, impl, nir) {
-         nir_metadata_require(impl, nir_metadata_dominance);
+         nir_metadata_require(impl, nir_metadata_dominance |
+                                    nir_metadata_dominance_lca);
 
          /* Going in reverse order of blocks, move the intrinsics gather above
           * in the LCA block to trace calls.