jax-ml/jax · error · ValueError

LHS must be a TMEM/SMEM Ref.

Error message

LHS must be a TMEM/SMEM Ref.

What it means

The LHS operand of tcgen05.mma must be a ref in either SMEM or TMEM; a GMEM ref or other memory space fails this check.

Source

Thrown at jax/_src/pallas/mosaic_gpu/primitives.py:2570

@tcgen05_mma_p.def_effectful_abstract_eval
def _tcgen05_mma_abstract_eval(acc, a, b, accumulate,
                               *barrier_scales_and_transforms_leaves,
                               acc_transforms_tree, a_transforms_tree,
                               b_transforms_tree,
                               barrier_transforms_tree,
                               a_scale_transforms_tree,
                               b_scale_transforms_tree,
                               a_sparse_metadata_transforms_tree,
                               collective_axis,
                               arrive,
                               scaled,
                               sparse):
  del accumulate, acc_transforms_tree, a_transforms_tree, b_transforms_tree, barrier_transforms_tree

  if acc.memory_space != gpu_core.TMEM:
    raise ValueError("Accumulator must be a TMEM Ref.")
  if a.memory_space not in (gpu_core.SMEM, gpu_core.TMEM):
    raise ValueError("LHS must be a TMEM/SMEM Ref.")
  if b.memory_space != gpu_core.SMEM:
    raise ValueError("RHS must be an SMEM Ref.")

  if collective_axis is not None:
    # TODO(justinfu): If under a core_map, the avals for acc/a
    # become normal MemRefs so we cannot check if they are collective.
    # Figure out a way to fix this.
    if isinstance(acc, gpu_core.AbstractTMEMRef) and not acc.collective:
      raise ValueError(
          "Accumulator Ref must be collective if collective_axis is set.")
    if isinstance(a, gpu_core.AbstractTMEMRef) and not a.collective:
      raise ValueError(
          "LHS Ref must be collective if collective_axis is set.")

  scales_and_transforms_leaves = barrier_scales_and_transforms_leaves
  if arrive:
    barrier, *scales_and_transforms_leaves = barrier_scales_and_transforms_leaves
    orders_tensor_core = getattr(

View on GitHub (pinned to 1e1c6a8fc0)

Solutions

1. Load the LHS tile into an SMEM buffer with proper swizzle before the MMA
2. Or keep A in TMEM if using a TMEM-resident LHS layout
3. Ensure the ref's memory_space attribute is SMEM or TMEM

Example fix

# before
tcgen05.mma(a_gmem_ref, b_smem, acc, k_dim=k)
# after
a_smem = load_to_smem(a_gmem_ref, swizzle=128)
tcgen05.mma(a_smem, b_smem, acc, k_dim=k)

assert a.memory_space in (gpu_core.SMEM, gpu_core.TMEM)

def lhs_space_ok(ref):
    return getattr(ref, 'memory_space', None) in (gpu_core.SMEM, gpu_core.TMEM)

When it happens

Trigger: Passing a raw kernel input (GMEM) directly as the LHS without first loading/swizzling it into SMEM or TMEM.

Common situations: Forgetting to stage the A tile through an SMEM scratch buffer with the required swizzle/tiling; treating tcgen05.mma like a regular dot that accepts GMEM operands.

Related errors

• Accumulator must be a TMEM Ref.
• RHS must be an SMEM Ref.
• packed, collective and layout arguments are only supported f
• All aliased Refs must have the same memory space (SMEM or TM
• Only workgroup memory is supported but got {ref}.

AI-assisted analysis of jax-ml/jax@1e1c6a8fc0 (2026-08-27). Data as JSON: /api/errors/9df1e62d61bca43f. Report an issue: GitHub.