[XLA:GPU] Enable dots with block_n=8 in triton and autotuner

This change utilizes recently added Triton support for smaller block sizes.

Skipping occupancy optimization for some configs is essentially a workaround for incompatible split_k values. The impact of these configs is limited however because they are only present in non-exhaustive mode, so they mostly get filtered out anyway.

PiperOrigin-RevId: 820617352

third_party/xla/xla/backends/gpu/codegen/triton/fusion_emitter_device_legacy_test.cc

@@ -1889,6 +1889,39 @@ ENTRY e  {
                    .status());
 }
 
+class RhsLayoutParameterizedTritonGemmTest
+    : public TritonGemmTest,
+      public ::testing::WithParamInterface<absl::string_view> {};
+
+TEST_P(RhsLayoutParameterizedTritonGemmTest,
+       BF16WithSmallRHSOuterDimDoesNotCrash) {
+  std::string hlo_text = absl::Substitute(R"(
+  triton_dot {
+    p0 = bf16[64,32] parameter(0)
+    p1 = bf16[32,8]$0 parameter(1)
+    ROOT dot = f32[64,8] dot(p0, p1),
+      lhs_contracting_dims={1},
+      rhs_contracting_dims={0}
+  }
+
+  ENTRY e {
+    p0 = bf16[64,32] parameter(0)
+    p1 = bf16[32,8]$0 parameter(1)
+    ROOT _ = f32[64,8] fusion(p0, p1), kind=kCustom, calls=triton_dot,
+      backend_config={"fusion_backend_config": {kind: "__triton_gemm", triton_gemm_config:
+        {"block_m":64,"block_n":8,"block_k":32,
+        "split_k":1,"num_stages":1,"num_warps":4,
+        "num_ctas":1}}}
+  })",
+                                          GetParam());
+
+  EXPECT_TRUE(RunAndCompare(hlo_text, ErrorSpec{/*aabs=*/1e-1, /*arel=*/1e-2}));
+}
+
+INSTANTIATE_TEST_SUITE_P(RhsLayoutParameterizedTritonGemmTestSuite,
+                         RhsLayoutParameterizedTritonGemmTest,
+                         ::testing::Values("", "{0, 1}", "{1, 0}"));
+
 TEST_F(TritonGemmTest, BinaryOperationWithSmallInputsIsFused) {
   constexpr absl::string_view kHloText = R"(
 HloModule m

third_party/xla/xla/backends/gpu/codegen/triton/fusion_emitter_device_test.cc

@@ -4148,6 +4148,55 @@ ENTRY entry {
       kHloText, ErrorSpec{/*aabs=*/1e-4, /*arel=*/1e-6}));
 }
 
+TEST_F(TritonEmitterTest, BF16WithSmallRHSOuterDimDoesNotCrash) {
+  const std::string kHloText = R"(
+flhs {
+  ROOT flhs.p0 = bf16[64,32] parameter(0)
+}
+
+frhs {
+  ROOT frhs.p0 = bf16[32,8] parameter(0)
+}
+
+fdot {
+  fdot.p0 = bf16[64,32] parameter(0)
+  fdot.p1 = bf16[32,8] parameter(1)
+  fdot.lhs = bf16[64,32] fusion(fdot.p0), kind=kCustom, calls=flhs, backend_config={
+    "fusion_backend_config":{
+      "kind":"__triton_nested_gemm_fusion", "block_level_fusion_config":{
+        "output_tiles":[{"sizes":["64", "32"]}]
+      }
+    }
+  }
+  fdot.rhs = bf16[32,8]{1,0} fusion(fdot.p1), kind=kCustom, calls=frhs, backend_config={
+    "fusion_backend_config":{
+      "kind":"__triton_nested_gemm_fusion", "block_level_fusion_config":{
+        "output_tiles":[{"sizes":["32", "8"]}]
+      }
+    }
+  }
+  ROOT fdot.root = bf16[64,8]{1,0} dot(fdot.lhs, fdot.rhs),
+    lhs_contracting_dims={1}, rhs_contracting_dims={0}
+}
+
+ENTRY entry {
+  entry.p0 = bf16[64,32] parameter(0)
+  entry.p1 = bf16[32,8] parameter(1)
+  ROOT fusion = bf16[64,8] fusion(entry.p0, entry.p1),
+    kind=kCustom, calls=fdot, backend_config={
+      "fusion_backend_config":{
+        "kind":"__triton_nested_gemm_fusion",
+        "block_level_fusion_config":{
+          "output_tiles":[{"sizes":["64","8"]}],
+          "num_warps":"4",
+          "num_ctas":"1",
+          "num_stages":"1"}}}
+})";
+
+  EXPECT_TRUE(RunAndCompareNoHloPasses(
+      kHloText, ErrorSpec{/*aabs=*/1e-1, /*arel=*/1e-2}));
+}
+
 struct ScaleDotTestParams {
   std::string lhs_type;
   std::string rhs_type;

third_party/xla/xla/backends/gpu/codegen/triton/fusion_emitter_legacy_matmul.cc

@@ -812,7 +812,7 @@ absl::Status ValidateMatMulConfig(const TritonGemmConfig& config,
   TF_RET_CHECK(config.split_k >= 1);
   TF_RET_CHECK(config.block_m >= 16);
   TF_RET_CHECK(config.block_k >= 16);
-  TF_RET_CHECK(config.block_n >= 16);
+  TF_RET_CHECK(config.block_n >= 8);
 
   const auto& dims = dot.dot_dimension_numbers();
   int num_batch_dims =

third_party/xla/xla/service/gpu/autotuning/dot_search_space.cc

@@ -329,12 +329,11 @@ bool TritonDotFusionSearchSpace::ShouldOptimizeForOccupancy() const {
 
 TritonDotFusionSearchSpace::OutputTile
 TritonDotFusionSearchSpace::GetMinOutputTile() const {
-  // Triton currently doesn't support tiles smaller than 16x16.
-  // TODO: b/395572776 - Lift this restriction, and calculate a smaller tile
-  // based on the requested algorithm (e.g., if we want to use wgmma vs mma
-  // vs fma, the minimal reasonable tile size is different).
-  constexpr OutputTile kMinSupportedTile = {16, 16};
-  constexpr OutputTile kMinWgmmaTile = {64, 16};
+  // TODO: b/395572776 - Calculate tile sizes based on the requested algorithm
+  // (e.g., if we want to use wgmma vs mma vs fma, the minimal reasonable tile
+  // size is different).
+  constexpr OutputTile kMinSupportedTile = {16, 8};
+  constexpr OutputTile kMinWgmmaTile = {64, 8};
   if (device_description_.cuda_compute_capability().IsAtLeastHopper() &&
       !should_optimize_for_occupancy_) {
     VLOG(5) << "Computing output_tile: Want to use wgmma, so output_tile >= "
@@ -656,6 +655,14 @@ void TritonDotFusionSearchSpace::EliminateLowOccupancyConfigs(
 
   ConfigWithNotes last_config = configs.back();  // Largest split.
   auto has_too_few_tiles = [](const ConfigWithNotes& config) {
+    // Small dots frequently lead to large split_k values that are not
+    // compatible with codegen. We skip occupancy optimization for them to be
+    // able to consider smaller splits in non-exhaustive mode.
+    // The value of 4 was found by running exhaustive autotuning and noting that
+    // the majority of optimal configs with block_n == 8 had split_k <= 4.
+    if (config.config.block_n == 8 && config.config.split_k <= 4) {
+      return false;
+    }
     if (config.not_enough_tiles) {
       VLOG(10) << "Skipping due to fewer tiles than cores, config = "
                << config.ToString();

third_party/xla/xla/service/gpu/autotuning/dot_search_space_test.cc

@@ -45,6 +45,7 @@ void PrintTo(const TritonGemmConfig& config, std::ostream* os) {
 namespace {
 
 using ::testing::AllOf;
+using ::testing::Contains;
 using ::testing::ElementsAre;
 using ::testing::ElementsAreArray;
 using ::testing::Eq;
@@ -195,7 +196,7 @@ TEST_F(DotSearchSpaceTest, SerializesSearchSpace) {
 
   EXPECT_EQ(search_space.ToString(),
             "problem_size_BxMxNxKxE: 1x1024x1024x1024x(16->16) "
-            "tile_range_SxMxNxK: [1-64]x[16-256]x[16-512]x[16-?] "
+            "tile_range_SxMxNxK: [1-64]x[16-256]x[8-512]x[16-?] "
             "desired_total_warps: 2640 occupancy_optimization: 1 "
             "warps_per_cta: [2-?]");
 }
@@ -306,16 +307,15 @@ TEST_F(DotSearchSpaceTest, FindsGoodDataReuseTilesForLowOccupancyProblem) {
               Contains(AllOf(BlockMIs(Ge(32)), SplitKIs(Ge(2)))));
 }
 
-TEST_F(DotSearchSpaceTest,
-       FindsUniqueOccupancyMaximizingTilingForSmallProblem) {
+TEST_F(DotSearchSpaceTest, FindsOccupancyMaximizingTilingForSmallProblem) {
   TF_ASSERT_OK_AND_ASSIGN(
       std::unique_ptr<VerifiedHloModule> module,
       GetDefaultDotModule(/*lhs_parallel_dim=*/64, /*rhs_parallel_dim=*/64,
                           /*contracting_dim=*/64));
   TritonDotFusionSearchSpace search_space = MakeSearchSpace(module.get());
-  EXPECT_THAT(search_space.GenerateConfigs(),
-              AllOf(SizeIs(1), Each(AllOf(BlockMIs(Eq(16)), BlockNIs(Eq(16)),
-                                          SplitKIs(Eq(4))))));
+  EXPECT_THAT(
+      search_space.GenerateConfigs(),
+      Contains(AllOf(BlockMIs(Eq(16)), BlockNIs(Eq(8)), SplitKIs(Eq(4)))));
 }
 
 TEST_F(DotSearchSpaceTest, FindsGoodDataReuseTilesForForcedHugeSplit) {
@@ -348,7 +348,7 @@ TEST_F(DotSearchSpaceTest, HonorsMinimumOutputTileSizeForTinyProblem) {
 
   EXPECT_THAT(
       search_space.GenerateConfigs(),
-      AllOf(Not(IsEmpty()), Each(BlockMIs(Ge(16))), Each(BlockNIs(Ge(16)))));
+      AllOf(Not(IsEmpty()), Each(BlockMIs(Ge(16))), Each(BlockNIs(Ge(8)))));
 }
 
 TEST_F(DotSearchSpaceTest, AssignsEnoughWarpsPerScheduler) {

third_party/xla/xla/service/gpu/autotuning/gemm_fusion_autotuner_test.cc

@@ -582,7 +582,7 @@ ENTRY e {
   MatchOptimizedHlo(kHloText, R"(
 ; CHECK: reduce
 ; CHECK: ENTRY
-; CHECK: f32[16,7,18]{2,1,0} fusion({{.*}})
+; CHECK: f32[{{.*}},7,18]{2,1,0} fusion({{.*}})
 ; CHECK: ROOT {{.*}} f16[7,18]{1,0} fusion({{.*}})
 )");
 

third_party/xla/xla/service/gpu/autotuning/triton_configs.h

@@ -65,7 +65,7 @@ static const std::vector<TritonGemmConfig>* const kHopperAmpereConfigs =
          Config(128, 16, 32, 8, 4, 2),   Config(128, 16, 64, 16, 3, 2),
          Config(128, 16, 64, 16, 1, 4),  Config(128, 32, 32, 8, 4, 2),
          Config(128, 128, 32, 8, 4, 8),  Config(128, 256, 32, 1, 4, 8),
-         Config(128, 256, 64, 1, 4, 8)});
+         Config(128, 256, 64, 1, 4, 8),  Config(64, 8, 128, 2, 3, 4, 1)});
 
 static const std::vector<TritonGemmConfig>* const kDefaultCudaConfigs =
     new std::vector<TritonGemmConfig>(