[Dev][Bugfix] Fix bug in ThreadTagChecker; Add WgmmaSync rewriter and add MHA WGMMA pipelined example (#128)

* [Dev] Add RetNet Linear Attention example

* [Dev] Add WgmmaSync rewriter for pipelined WGMMA operations and add MHA WGMMA pipelined example (FA3-like scheduling)

This commit introduces a new transformation pass `RewriteWgmmaSync` to optimize warp group matrix multiply accumulate (WGMMA) operations in the TileLang compiler:

- Implemented `WgmmaSyncRewriter` in `src/transform/wgmma_sync_rewriter.cc`
- Added pass registration for `RewriteWgmmaSync`
- Updated `tilelang/engine/phase.py` to include the new transformation pass
- Updated `tilelang/transform/__init__.py` to expose the new pass

The rewriter intelligently manages synchronization and dependencies between WGMMA operations, improving pipeline efficiency for complex matrix multiplication kernels.

* [Bugfix] Fix bug in ThreadTagChecker for warp specialization

Improve thread tag validation in warp specialized rewriter to prevent unintended transformations:
- Add more precise checks for threadIdx.y and threadIdx.z
- Validate thread extent to ensure only single-extent thread bindings are allowed
- Prevent warp specialization for multi-extent thread bindings in y and z dimensions

* lint

* [CI] Add TMA descriptor attribute to transformed module in test case

Author: chengyupku

examples/flash_attention/example_mha_fwd_bshd_wgmma_pipelined.py

@@ -0,0 +1,232 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+import torch
+import torch.nn.functional as F
+import tilelang
+from tilelang import Profiler
+from tilelang.autotuner import *
+import tilelang.language as T
+import itertools
+import argparse
+from functools import partial
+
+
+def get_configs():
+    block_M = [128]
+    block_N = [128]
+    num_stages = [2]
+    threads = [256]
+    _configs = list(itertools.product(block_M, block_N, num_stages, threads))
+
+    configs = [{
+        'block_M': c[0],
+        'block_N': c[1],
+        'num_stages': c[2],
+        'threads': c[3]
+    } for c in _configs]
+    return configs
+
+
+def flashattn(batch, heads, seq_len, dim, is_causal, tune=False):
+    scale = (1.0 / dim)**0.5 * 1.44269504  # log2(e)
+    shape = [batch, seq_len, heads, dim]
+    dtype = "float16"
+    accum_dtype = "float"
+
+    def kernel_func(block_M, block_N, num_stages, threads):
+
+        @T.macro
+        def MMA0(
+            K: T.Buffer(shape, dtype),
+            Q_shared: T.Buffer([block_M, dim], dtype),
+            K_shared: T.Buffer([block_N, dim], dtype),
+            acc_s: T.Buffer([block_M, block_N], accum_dtype),
+            k: T.int32,
+            bx: T.int32,
+            by: T.int32,
+            bz: T.int32,
+        ):
+            T.copy(K[bz, k * block_N:(k + 1) * block_N, by, :], K_shared)
+            if is_causal:
+                for i, j in T.Parallel(block_M, block_N):
+                    acc_s[i, j] = T.if_then_else(bx * block_M + i >= k * block_N + j, 0,
+                                                 -T.infinity(acc_s.dtype))
+            else:
+                T.clear(acc_s)
+            T.gemm(Q_shared, K_shared, acc_s, transpose_B=True, policy=T.GemmWarpPolicy.FullRow)
+
+        @T.macro
+        def MMA1(
+                V: T.Buffer(shape, dtype),
+                V_shared: T.Buffer([block_M, dim], dtype),
+                acc_s_cast: T.Buffer([block_M, block_N], dtype),
+                acc_o: T.Buffer([block_M, dim], accum_dtype),
+                k: T.int32,
+                by: T.int32,
+                bz: T.int32,
+        ):
+            T.copy(V[bz, k * block_N:(k + 1) * block_N, by, :], V_shared)
+            T.gemm(acc_s_cast, V_shared, acc_o, policy=T.GemmWarpPolicy.FullRow)
+
+        @T.macro
+        def Softmax(
+                acc_s: T.Buffer([block_M, block_N], accum_dtype),
+                acc_s_cast: T.Buffer([block_M, block_N], dtype),
+                scores_max: T.Buffer([block_M], accum_dtype),
+                scores_max_prev: T.Buffer([block_M], accum_dtype),
+                scores_scale: T.Buffer([block_M], accum_dtype),
+                scores_sum: T.Buffer([block_M], accum_dtype),
+                logsum: T.Buffer([block_M], accum_dtype),
+        ):
+            T.copy(scores_max, scores_max_prev)
+            T.fill(scores_max, -T.infinity(accum_dtype))
+            T.reduce_max(acc_s, scores_max, dim=1, clear=False)
+            # To do causal softmax, we need to set the scores_max to 0 if it is -inf
+            # This process is called Check_inf in FlashAttention3 code, and it only need to be done
+            # in the first ceil_div(kBlockM, kBlockN) steps.
+            # for i in T.Parallel(block_M):
+            #     scores_max[i] = T.if_then_else(scores_max[i] == -T.infinity(accum_dtype), 0, scores_max[i])
+            for i in T.Parallel(block_M):
+                scores_scale[i] = T.exp2(scores_max_prev[i] * scale - scores_max[i] * scale)
+            for i, j in T.Parallel(block_M, block_N):
+                # Instead of computing exp(x - max), we compute exp2(x * log_2(e) -
+                # max * log_2(e)) This allows the compiler to use the ffma
+                # instruction instead of fadd and fmul separately.
+                acc_s[i, j] = T.exp2(acc_s[i, j] * scale - scores_max[i] * scale)
+            T.reduce_sum(acc_s, scores_sum, dim=1)
+            for i in T.Parallel(block_M):
+                logsum[i] = logsum[i] * scores_scale[i] + scores_sum[i]
+            T.copy(acc_s, acc_s_cast)
+
+        @T.macro
+        def Rescale(
+                acc_o: T.Buffer([block_M, dim], accum_dtype),
+                scores_scale: T.Buffer([block_M], accum_dtype),
+        ):
+            for i, j in T.Parallel(block_M, dim):
+                acc_o[i, j] *= scores_scale[i]
+
+        @T.prim_func
+        def main(
+                Q: T.Buffer(shape, dtype),
+                K: T.Buffer(shape, dtype),
+                V: T.Buffer(shape, dtype),
+                Output: T.Buffer(shape, dtype),
+        ):
+            with T.Kernel(
+                    T.ceildiv(seq_len, block_M), heads, batch, threads=threads) as (bx, by, bz):
+                Q_shared = T.alloc_shared([block_M, dim], dtype)
+                K_shared = T.alloc_shared([block_N, dim], dtype)
+                V_shared = T.alloc_shared([block_N, dim], dtype)
+                O_shared = T.alloc_shared([block_M, dim], dtype)
+                acc_s = T.alloc_fragment([block_M, block_N], accum_dtype)
+                acc_s_cast = T.alloc_fragment([block_M, block_N], dtype)
+                acc_o = T.alloc_fragment([block_M, dim], accum_dtype)
+                scores_max = T.alloc_fragment([block_M], accum_dtype)
+                scores_max_prev = T.alloc_fragment([block_M], accum_dtype)
+                scores_scale = T.alloc_fragment([block_M], accum_dtype)
+                scores_sum = T.alloc_fragment([block_M], accum_dtype)
+                logsum = T.alloc_fragment([block_M], accum_dtype)
+
+                T.copy(Q[bz, bx * block_M:(bx + 1) * block_M, by, :], Q_shared)
+                T.fill(acc_o, 0)
+                T.fill(logsum, 0)
+                T.fill(scores_max, -T.infinity(accum_dtype))
+
+                loop_range = (
+                    T.min(T.ceildiv(seq_len, block_N), T.ceildiv(
+                        (bx + 1) * block_M, block_N)) if is_causal else T.ceildiv(seq_len, block_N))
+
+                for k in T.Pipelined(
+                        loop_range,
+                        num_stages=num_stages,
+                        order=[-1, 0, 3, 1, -1, 2],
+                        stage=[-1, 0, 0, 1, -1, 1],
+                        group=[[0], [1, 2], [3, 4, 5, 6, 7, 8, 9, 10], [11], [12], [13]]):
+                    MMA0(K, Q_shared, K_shared, acc_s, k, bx, by, bz)
+                    Softmax(acc_s, acc_s_cast, scores_max, scores_max_prev, scores_scale,
+                            scores_sum, logsum)
+                    Rescale(acc_o, scores_scale)
+                    MMA1(V, V_shared, acc_s_cast, acc_o, k, by, bz)
+                for i, j in T.Parallel(block_M, dim):
+                    acc_o[i, j] /= logsum[i]
+                T.copy(acc_o, O_shared)
+                T.copy(O_shared, Output[bz, bx * block_M:(bx + 1) * block_M, by, :])
+
+        return main
+
+    if tune:
+
+        @autotune(
+            configs=get_configs(),
+            keys=["block_M", "block_N", "num_stages", "threads"],
+            warmup=10,
+            rep=10)
+        @jit(
+            out_idx=[3],
+            supply_type=tilelang.TensorSupplyType.Integer,
+            ref_prog=None,
+            profiler="auto")
+        def kernel(block_M=None, block_N=None, num_stages=None, threads=None):
+            return kernel_func(block_M, block_N, num_stages, threads)
+
+        return kernel()
+    else:
+
+        def kernel(block_M, block_N, num_stages, threads):
+            return kernel_func(block_M, block_N, num_stages, threads)
+
+        return kernel
+
+
+def ref_program(Q, K, V, is_causal):
+    dim = Q.size(-1)
+    scores = torch.einsum('bqhd,bkhd->bhqk', Q, K)
+    scores = scores / torch.sqrt(torch.tensor(dim, dtype=scores.dtype))
+    if is_causal:
+        seq_len = Q.size(1)
+        mask = torch.tril(torch.ones(seq_len, seq_len, device=scores.device))
+        mask = mask.unsqueeze(0).unsqueeze(0)
+        scores = scores.masked_fill(mask == 0, float('-inf'))
+    attention_weights = F.softmax(scores, dim=-1)
+    output = torch.einsum('bhqk,bkhd->bqhd', attention_weights, V)
+    return output
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--batch', type=int, default=8, help='batch size')
+    parser.add_argument('--heads', type=int, default=32, help='heads')
+    parser.add_argument('--seq_len', type=int, default=4096, help='sequence length')
+    parser.add_argument('--dim', type=int, default=128, help='dim')
+    parser.add_argument('--is_causal', action='store_true', help='causal')
+    parser.add_argument('--tune', action='store_true', help='tune configs')
+    args = parser.parse_args()
+    batch, heads, seq_len, dim, is_causal = args.batch, args.heads, args.seq_len, args.dim, args.is_causal
+    flops_per_matmul = 2.0 * batch * heads * seq_len * seq_len * dim
+    total_flops = 2 * flops_per_matmul
+    if is_causal:
+        total_flops *= 0.5
+
+    if (not args.tune):
+        program = flashattn(
+            batch, heads, seq_len, dim, is_causal, tune=args.tune)(
+                block_M=128, block_N=128, num_stages=2, threads=256)
+        ref_program = partial(ref_program, is_causal=is_causal)
+        mod, params = tilelang.lower(program)
+        mod = Profiler(mod, params, [3], tilelang.TensorSupplyType.Normal)
+        mod.assert_allclose(ref_program, rtol=0.01, atol=0.01)
+        print("All checks pass.")
+        latency = mod.do_bench(ref_program, warmup=500)
+        print("Ref: {:.2f} ms".format(latency))
+        print("Ref: {:.2f} TFlops".format(total_flops / latency * 1e-9))
+        latency = mod.do_bench(mod.func, warmup=500)
+        print("Tile-lang: {:.2f} ms".format(latency))
+        print("Tile-lang: {:.2f} TFlops".format(total_flops / latency * 1e-9))
+    else:
+        best_latency, best_config, _ = flashattn(
+            batch, heads, seq_len, dim, is_causal, tune=args.tune)
+        print(f"Best latency: {best_latency}")
+        print(f"Best TFlops: {total_flops / best_latency * 1e-9}")
+        print(f"Best config: {best_config}")

src/transform/warp_specialized_rewriter.cc

@@ -878,9 +878,12 @@ class ThreadTagChecker : public StmtExprVisitor {
 private:
   void VisitStmt_(const AttrStmtNode *op) final {
     if (op->attr_key == tir::attr::thread_extent) {
-      auto iter_var = Downcast<IterVar>(op->node);
-      if (iter_var->thread_tag.length() > 0 &&
-          iter_var->thread_tag != "threadIdx.x") {
+      IterVar iter_var = Downcast<IterVar>(op->node);
+      String thread_tag = iter_var->thread_tag;
+      bool is_y_or_z =
+          thread_tag == "threadIdx.y" || thread_tag == "threadIdx.z";
+
+      if (!thread_tag.empty() && is_y_or_z && !is_one(iter_var->dom->extent)) {
         is_valid_ = false;
       }
     }
@@ -891,8 +894,14 @@ class ThreadTagChecker : public StmtExprVisitor {
     if (op->kind == ForKind::kThreadBinding) {
       ICHECK(op->thread_binding.defined());
       String thread_tag = op->thread_binding.value()->thread_tag;
-      if (thread_tag.length() > 0 && thread_tag != "threadIdx.x") {
-        is_valid_ = false;
+      bool is_y_or_z =
+          thread_tag == "threadIdx.y" || thread_tag == "threadIdx.z";
+      if (!thread_tag.empty() && is_y_or_z) {
+        auto iter_var = Downcast<IterVar>(op->thread_binding);
+        if (iter_var.defined() && iter_var->dom.defined() &&
+            !is_one(iter_var->dom->extent)) {
+          is_valid_ = false;
+        }
       }
     }
     StmtExprVisitor::VisitStmt_(op);