Fix spurious re-compilations of `rotary_kernel` (#911)

All integer parameters are specialized by default, so the two parameters
removed in this commit could lead to kernel re-compilation, even if
they were completely unused.

flash_attn/ops/triton/rotary.py

@@ -8,15 +8,6 @@ import triton
 import triton.language as tl
 
 
-# @triton.autotune(
-#     configs=[
-#         triton.Config({"BLOCK_M": 2}),
-#         triton.Config({"BLOCK_M": 4}),
-#         triton.Config({"BLOCK_M": 8}),
-#         triton.Config({"BLOCK_M": 16}),
-#     ],
-#     key=["CACHE_KEY_SEQLEN", "BLOCK_K", "INTERLEAVED"],
-# )
 @triton.jit
 def rotary_kernel(
     OUT,  # Pointers to matrices
@@ -27,10 +18,8 @@ def rotary_kernel(
     SEQLEN_OFFSETS,  # this could be int or a pointer
     # Matrix dimensions
     seqlen,
-    nheads,
     rotary_dim,
     seqlen_ro,
-    CACHE_KEY_SEQLEN,
     # strides
     stride_out_batch,
     stride_out_seqlen,
@@ -218,10 +207,8 @@ def apply_rotary(
             cu_seqlens,
             seqlen_offsets,
             seqlen,  # shapes
-            nheads,
             rotary_dim,
             seqlen_ro,
-            seqlen // 128,  # key for triton cache (limit number of compilations)
             output.stride(0) if not is_varlen else 0,  # batch_strides if not varlen else 0
             output.stride(-3),  # seqlen_stride or total_seqlen_stride
             output.stride(-2),  # nheads_stride

tests/test_rotary.py

@@ -252,3 +252,40 @@ def test_rotary_emb_varlen_func(inplace, interleaved, rotary_fraction, seqlen_of
     assert torch.allclose(out, out_pt, rtol=rtol, atol=2 * atol)
     atol = ((x_pt.grad + 0.3 - 0.3) - x_pt.grad).abs().max().item()
     assert torch.allclose(x_grad, x_pt.grad, rtol=rtol, atol=2 * atol)
+
+
+def test_compilation_count():
+    batch_size = 1
+    headdim = 128
+    device = "cuda"
+    dtype = torch.float16
+    torch.manual_seed(42)
+
+    from triton.runtime.jit import JITFunction
+    from flash_attn.ops.triton.rotary import rotary_kernel
+    compilation_count = 0
+
+    def count_compilations(*args, **kwargs):
+        nonlocal compilation_count
+        compilation_count += 1
+
+    old_cache_func = JITFunction.cache_hook
+
+    try:
+        rotary_kernel.cache.clear()
+        JITFunction.cache_hook = count_compilations
+
+        for seqlen in (128, 256):
+            for nheads in (4, 32):
+                x = torch.randn(batch_size, seqlen, nheads, headdim, dtype=dtype, device=device)
+                x.requires_grad_()
+                cos, sin = generate_cos_sin(seqlen, headdim, device, dtype)
+                out = apply_rotary_emb(x, cos, sin)
+                out.backward(torch.randn_like(out))
+
+        # Only two kernels are expected to be compiled:
+        #   * for the forward pass (conjugate=False)
+        #   * for the backward pass (conjugate=True)
+        assert compilation_count == 2
+    finally:
+        JITFunction.cache_hook = old_cache_func