[Frontend] Drop stale single-reduction-axis guard breaking test_diffusion#250
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The guard at mlir_codegen_backend.py:909-910
if len(reductions.loops) > 1:
raise NotImplementedError("Not support multiple reduction axis..")
was a leftover from when the codegen was deliberately limited to a
single reduction axis, but multi-axis support was restored months ago
and the guard was never removed.
Timeline:
- 8eacf27 ("Optimize reduce/elementwise code", 2025): replaced
`for reduction in reductions.loops` with `reductions.loops[0]`,
added this guard (no `raise` — silently dead).
- c61f67d ("Support multi reduction dim + Add Diffusion model test",
2025-08-14): restored the `for reduction_loop in reductions.loops`
iteration explicitly to support multi-axis reduction. test_diffusion
was added in this same commit and passed. The guard was left in
place — harmless because still `raise`-less.
- 5045837 ("Fix four codegen correctness issues surfaced by review",
2026-05-26, issue #237): added the missing `raise`, framing it as a
"silently fell through to incorrect codegen" fix. In fact the
codegen below the guard (which iterates all reduction loops) was
correct — c61f67d had made it so — and adding `raise` broke the
very test (test_diffusion) that c61f67d added to validate multi-axis
support.
Right fix is to delete the guard, not add `raise` to it. The other
three items in #237 are unrelated and stand.
Verified: file py_compiles. CI test_diffusion is the empirical proof
that the post-guard codegen handles multi-axis correctly within the
allclose tolerance the test uses.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Summary
Removes the dead guard at `mlir_codegen_backend.py:909-910`. The guard claimed multi-axis reduction was unsupported, but the codegen below it actually iterates all reduction loops and produces correct output — `test_diffusion`, added specifically to validate multi-axis support, passed for ~9 months while this guard had no `raise` and was effectively a no-op.
PR #237 (5045837) added a `raise` to the guard as part of a four-item correctness fix. The intent ("missing raise") was correct in form but wrong in substance: the guard itself was stale. With the raise added, every diffusion CI run now fails at this guard.
Timeline
Evidence the codegen handles multi-axis
Scope
Fixes the diffusion failures observed on PR CI runs since `5045837` landed on develop (e.g. run #404, #418).
Test plan
🤖 Generated with Claude Code