[Pallas/Mosaic GPU] Add an abstraction to obtain a slice of dynamic shared memory when using waprgroup semantics.

Explicitly make the assumption that `runtime_smem` starts at `0` in the Pallas
module context---which should be enforced by Mosaic GPU.

This is in preparation of changes implementing transform inference.

PiperOrigin-RevId: 732091266

jax/_src/pallas/mosaic_gpu/lowering.py

@@ -237,7 +237,7 @@ class ModuleContext:
   program_ids: Sequence[ir.Value] | None
   approx_math: bool
   single_wg_lane_predicate: ir.Value
-  runtime_smem: ir.Value  # ir.MemRefType
+  smem_requested_bytes: int
   smem_used_bytes: int
   runtime_barriers: MutableMapping[
       mgpu.Barrier, MutableSequence[mgpu.BarrierRef]
@@ -279,25 +279,38 @@ class ModuleContext:
       and the second element is a sequence of memref views into the
       runtime scratch buffer.
     """
-    smem_scratch_bytes = math.prod(ir.MemRefType(self.runtime_smem.type).shape)
-
+    smem_base = None
+    smem = ir.Attribute.parse("#gpu.address_space<workgroup>")
+    i8 = ir.IntegerType.get_signless(8)
+    i32 = ir.IntegerType.get_signless(32)
+    if self.thread_semantics == mgpu.ThreadSemantics.Lane:
+      smem_base = gpu_dialect.dynamic_shared_memory(
+          ir.MemRefType.get((mgpu_utils.DYNAMIC,), i8, memory_space=smem)
+      )
     views = []
     off = initial_used_bytes = self.smem_used_bytes
     assert off % _SMEM_ALIGNMENT == 0
-    smem = ir.Attribute.parse("#gpu.address_space<workgroup>")
     for s in structs:
       scratch_ty = ir.MemRefType.get(
           s.shape,
           mgpu_utils.dtype_to_ir_type(s.dtype),
           memory_space=smem,
       )
-      views.append(
-          memref_dialect.view(scratch_ty, self.runtime_smem, _as_index(off), [])
-      )
+      # The below code emission relies on the assumption that the first scratch
+      # operand provided by Mosaic GPU always begins at the beginning of
+      # dynamic SMEM. Mosaic GPU is expected to uphold that invariant.
+      if self.thread_semantics == mgpu.ThreadSemantics.Lane:
+        view = memref_dialect.view(
+            scratch_ty, smem_base, _as_index(off), []
+        )
+      else:
+        view = mgpu.dialect.slice_smem(scratch_ty, mgpu_utils.c(off, i32))
+      views.append(view)
+
       off += _align_to(
           math.prod(s.shape) * jnp.dtype(s.dtype).itemsize, _SMEM_ALIGNMENT
       )
-    assert off <= smem_scratch_bytes, "Ran out of scoped SMEM"
+    assert off <= self.smem_requested_bytes, "Ran out of scoped SMEM"
     assert off % _SMEM_ALIGNMENT == 0
 
     self.smem_used_bytes = off
@@ -596,7 +609,7 @@ def lower_jaxpr_to_module(
         [_program_id(axis, squashed_dims) for axis in range(len(grid))],
         approx_math,
         mgpu.single_thread_predicate(per_block=False),
-        runtime_smem,
+        smem_requested_bytes=math.prod(ir.MemRefType(runtime_smem.type).shape),
         smem_used_bytes=0,
         runtime_barriers=grouped_barriers,
         name_stack=source_info_util.NameStack(),

jax/experimental/mosaic/gpu/dialect_lowering.py

@@ -28,6 +28,7 @@ from jax._src.lib.mlir.dialects import builtin
 from jax._src.lib.mlir.dialects import func
 from jax._src.lib.mlir.dialects import gpu
 from jax._src.lib.mlir.dialects import llvm
+from jax._src.lib.mlir.dialects import memref
 from jax._src.lib.mlir.dialects import nvvm
 from jax._src.lib.mlir.dialects import scf
 from jax._src.lib.mlir.dialects import vector
@@ -598,15 +599,25 @@ def _mgpu_wait_op_lowering_rule(
   return []
 
 
-@_register_lowering(WaitOp)
-def _for_op_lowering_rule(
-    _: LoweringContext, wait_op: scf.ForOp
+# TODO(bchetioui): remove this once jaxlib minimum version >= 0.5.2.
+SliceSMEMOp = getattr(mgpu, "SliceSMEMOp", None)
+
+
+@_register_lowering(SliceSMEMOp)
+def _mgpu_slice_smem_op_lowering_rule(
+    ctx: LoweringContext, op: SliceSMEMOp
 ) -> Sequence[ir.Value]:
+  del ctx
+  i8 = ir.IntegerType.get_signless(8)
+  smem = ir.Attribute.parse("#gpu.address_space<workgroup>")
 
-  barrier = utils.BarrierRef.from_dialect_barrier_memref(wait_op.barrier)
-  barrier.wait_parity(wait_op.parity)
+  smem_base = gpu.dynamic_shared_memory(
+      ir.MemRefType.get((utils.DYNAMIC,), i8, memory_space=smem)
+  )
 
-  return []
+  offset = arith.index_cast(ir.IndexType.get(), op.offset)
+
+  return [memref.view(op.result.type, smem_base, offset, [])]
 
 
 @_register_lowering(scf.ForOp)

jaxlib/mosaic/dialect/gpu/mosaic_gpu.td

@@ -372,6 +372,14 @@ def MosaicGPU_WGMMALayout :
   let cppNamespace = "::mosaic_gpu";
 }
 
+
+def MosaicGPU_SliceSMEMOp : Op<MosaicGPU_Dialect, "slice_smem", []> {
+  let summary = "Constructs an SMEM MemRef with the requested type that begins at the specified SMEM offset address.";
+
+  let arguments = (ins I32:$offset);
+  let results = (outs MemRefOf<[AnyType]>);
+}
+
 def MosaicGPU_WGMMAOp : Op<MosaicGPU_Dialect, "wgmma", [InferTypeOpInterface]> {
   let summary = "Multiply two matrices asyncronously using warpgroup level matrix multiply operations.";
   let description = [{

tests/mosaic/gpu_dialect_test.py

@@ -802,6 +802,28 @@ class DialectLoweringTest(MosaicGpuTest):
     reg_vec_ty = ir.VectorType.get((2,), i32)
     self.assertSequenceEqual(result_types, [i32, reg_vec_ty, reg_vec_ty])
 
+  def test_lowering_slice_smem_op(self):
+    shift = 1234
+    offset = None
+
+    def body():
+      nonlocal offset
+      i32 = ir.IntegerType.get_signless(32)
+      offset = arith.constant(i32, shift)
+      mgpu.dialect.slice_smem(i32, offset)
+
+    with ir.InsertionPoint(self.module.body):
+      func.FuncOp.from_py_func()(body)
+
+    mgpu.lower_mgpu_dialect(self.module, None)
+    # Avoid making a change detector, only validate that lowering runs as
+    # expected.
+    self.assertEmpty(
+        find_if(
+            self.module, lambda op: isinstance(op, mgpu.dialect.SliceSMEMOp)
+        )
+    )
+
 
 if __name__ == "__main__":
   parameterized.absltest.main(testLoader=jtu.JaxTestLoader())