fix(ir): rebuild a WhileStmt's iter_args in Simplify so its uses stay one node - #2564
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fix(ir): rebuild a WhileStmt's iter_args in Simplify so its uses stay one node#2564Hzfengsy wants to merge 2 commits into
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… one node
## Summary
`Simplify` split a `while` loop's `IterArg` in two: the loop header kept the
original node while every reference in the condition and body pointed at an
undefined clone of it. `UseAfterDef` reported one error per reference.
An `IterArg` is not a plain `Var` — a *use* is the same node as its declaration,
and it carries `initValue_`. So `IRMutator::VisitExpr_(IterArgPtr)`
(`src/ir/transforms/mutator.cpp:264`) re-visits `initValue_` at every occurrence
and mints a fresh `IterArg` whenever it changed:
```cpp
auto new_init_value = ExprFunctor<ExprPtr>::VisitExpr(op->initValue_);
...
auto fresh = std::make_shared<const IterArg>(op->name_hint_, ..., new_init_value, ...);
var_remap_[op.get()] = fresh; // every later use gets `fresh`; the header does not
```
Simplify's own constant propagation is what makes the init change. A top-level
`i: pl.Scalar[pl.INDEX] = 0` has a constant RHS at `scope_depth_ == 0`, so
`VisitStmt_(AssignStmtPtr)` full-binds it; the first use of the loop variable —
the `while` condition — then folds `i__ssa_v0 -> 0` and trips the rebuild.
`ForStmt` is immune because it rebuilds `iter_args_` before its body, seeding
`var_remap_` so the header and every body reference resolve to one node. The
`WhileStmt` handler had no such call: it simplified the condition and body, then
copied the *original* `iter_args_` back in.
The reported case is `tests/st/runtime/control_flow/test_ctrl_flow.py::
TestCtrlFlowOperations::test_while_loop_add`, whose `kernel_while_add` has
exactly this shape. After Simplify its body reads:
```python
for i__iter_v1, out__iter_v0 in pl.while_(init_values=(i__ssa_v0, c__ssa_v0)):
pl.cond(i__iter_v1_1__FREE_VAR < 4) # printer's name for the orphan
offset_i__ssa_v0 = i__iter_v1_1__FREE_VAR * 64
i__ssa_v3 = i__iter_v1_1__FREE_VAR + 1
```
Nothing miscompiled: codegen resolves by name and both nodes are named
`i__iter_v1`, which is why the ST case passed on device. But the SSA edge was
dangling, and it made `UseAfterDef` unverifiable at every point after pass 5.
## Changes
- `src/ir/transforms/simplify_pass.cpp`: `VisitStmt_(WhileStmtPtr)` rebuilds
`iter_args_` before the condition and body. This is the fix.
It also rebuilds `return_vars_` after the body, matching `ForStmt`. That was
the same omission one field over — latent rather than active, since it only
bites when a return var's *type* simplifies and `MaybeRebuildVar` is a no-op
otherwise. Included for symmetry rather than left as a second copy of the gap.
- `tests/ut/ir/transforms/test_simplify_pass.py`: regression test in
`TestControlFlow`. It authors the `pl.while_` form directly so it exercises
`Simplify` alone, and asserts no `UseAfterDef` errors, that the init actually
folded to `0` (so it cannot pass vacuously), and that the condition operand
`same_as` the header's `IterArg`.
- `docs/{en,zh}/dev/passes/05-simplify.md`: `WhileStmt` was documented as merely
"visit the body with scoped scalar unbinding", sharing a bullet with
`SpmdScopeStmt`. Split into its own bullet documenting the rebuild order and
why it is required rather than cosmetic.
## Verification
- `cmake --build build --parallel 20`: exit 0
- `pytest tests/ut -n 8`: 10598 passed, 3 skipped, 3 xfailed
- `pytest tests/st/codegen -n 8`: 60 passed
- `pytest tests/st/runtime/control_flow`: 9 passed on device, including
`test_while_loop_add[a2a3]` — the case this issue was reported against
- `ctest --parallel 8`: 1/1 passed
- The new test was confirmed to fail on a build without the fix (4 errors, one
per reference) and pass with it.
- `UseAfterDef` on `kernel_while_add` now verifies clean after **every** pass of
the Default pipeline (52 passes); previously it broke at pass 5 and stayed
broken.
- lint: `check_headers`, `check_english_only`, `check_docs_en_zh_parity`,
`check_docs_nav`, `check_no_broad_raises`, `check_op_name_literals`,
`clang-format --dry-run --Werror`, `ruff check` / `ruff format --check`: exit 0
each.
## Reviewer notes
The trigger needs all three of: a `while` (a `for` is immune), the induction
variable carried as an `IterArg`, and a *constant* initializer. Change the seed
to a scalar parameter and no substitution happens, so the node never splits —
which is why this shape went unnoticed.
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📝 WalkthroughWalkthroughThe Simplify pass now rebuilds ChangesWhileStmt Simplification
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: 🟡 Moderate · up to The PR fixes malformed while-loop IterArg identities, but loop-local remapping may still leak beyond the loop and cause later sibling statements to reference invalid values. Merge should wait until that scoping concern is addressed or explicitly accepted; the Chinese documentation wording issue is minor. Poem
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Inline comments:
In `@docs/zh/dev/passes/05-simplify.md`:
- Line 66: Update the Chinese documentation sentence describing SpmdScopeStmt to
refer to visiting its statement body rather than its loop body, while preserving
the surrounding scalar unbinding and core_num_ folding details.
In `@src/ir/transforms/simplify_pass.cpp`:
- Line 487: In the WhileStmt transformation around VisitScopedBody(op->body_),
save var_remap_ before visiting the body, restore it immediately afterward, and
then rebuild return_vars_ to match the existing ForStmt behavior. Ensure
loop-private mappings cannot remain active for subsequent sibling statements.
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docs/en/dev/passes/05-simplify.mddocs/zh/dev/passes/05-simplify.mdsrc/ir/transforms/simplify_pass.cpptests/ut/ir/transforms/test_simplify_pass.py
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Addresses review feedback on hw-native-sys#2564. `VisitScopedBody` unbinds scalars but not `var_remap_`. A nested fold inside a while body records `outer_var -> body-local value`, and that mapping stayed active for everything after the loop. `ForStmt` has snapshotted and restored `var_remap_` around its body all along; `WhileStmt` did not. The leak is not merely a dangling reference — it silently rewrites a value. Pre-SSA, with a single-trip inner loop firing Fold B: i: pl.Scalar[pl.INDEX] = 0 acc_next: pl.Scalar[pl.INDEX] = 0 while i < 4: for j, (acc,) in pl.range(0, 1, init_values=(i,)): acc_next = pl.yield_(acc + 1) i = i + 1 pl.tensor.write(out, [0], acc_next) Fold B binds `acc_next -> acc + 1` with `acc` substituted by its init `i`, and the post-loop write became `pl.tensor.write(out, [0], i + 1)`. `acc_next` holds what the last iteration computed, which equals the post-loop `i`, so `i + 1` is off by one — and `i` is in scope in leak mode, so no verifier flags it. Also corrects the Chinese doc bullet for `SpmdScopeStmt`, which said `访问循环体` (visit the *loop* body) after the previous commit split it out of the shared `WhileStmt` bullet. A spmd scope is not a loop. ## Changes - `src/ir/transforms/simplify_pass.cpp`: snapshot `var_remap_` before the body visit and restore it after, before the `return_vars_` rebuild — the same placement `ForStmt` uses. The `MaybeRebuildIterArg` additions are captured in the baseline, so they stay valid in the body and after the loop. - `tests/ut/ir/transforms/test_simplify_pass.py`: regression test on the shape above, checked for *both* loop kinds so the two must agree. - `docs/{en,zh}/dev/passes/05-simplify.md`: document the snapshot; fix the zh `SpmdScopeStmt` wording. ## Note on the test's PassContext The test runs its two `passes.simplify()` calls under `with passes.PassContext([])`. Fold B lifts a body by *substitution* rather than by emitting `AssignStmt(rv, yielded)`, so in leak mode the surviving post-loop reference has no defining statement and trips `UseAfterDef`. That is a pre-existing Fold B limitation, **not** introduced here: the `for` half of the same test shows `ForStmt` behaves identically, and it has had this restore since before this PR. What changed for `WhileStmt` is silent-wrong-value -> loud-dangling-reference, which is the better failure. Unreachable in the real pipeline, since `Simplify` runs at positions 5 and 46, both after `ConvertToSSA`, and SSA form has no leak-mode reads. ## Verification - `cmake --build build --parallel 20`: exit 0 - `pytest tests/ut -n 8`: 10599 passed, 3 skipped, 3 xfailed - `pytest tests/st/codegen -n 8`: 60 passed - `pytest tests/st/runtime/control_flow`: 30 passed on device, 2 skipped - The new test was confirmed to fail without the restore, with the exact substitution: `while: post-loop use was rewritten to i + 1`. - lint: `check_headers`, `check_english_only`, `check_docs_en_zh_parity`, `check_docs_nav`, `clang-format --dry-run --Werror`, `ruff check` / `ruff format --check`: exit 0 each.
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Summary
Simplifysplit awhileloop'sIterArgin two: the loop header kept the original node while every reference in the condition and body pointed at an undefined clone of it.UseAfterDefreported one error per reference.An
IterArgis not a plainVar— a use is the same node as its declaration, and it carriesinitValue_. SoIRMutator::VisitExpr_(IterArgPtr)(src/ir/transforms/mutator.cpp:264) re-visitsinitValue_at every occurrence and mints a freshIterArgwhenever it changed:Simplify's own constant propagation is what makes the init change. A top-level
i: pl.Scalar[pl.INDEX] = 0has a constant RHS atscope_depth_ == 0, soVisitStmt_(AssignStmtPtr)full-binds it; the first use of the loop variable — thewhilecondition — then foldsi__ssa_v0 -> 0and trips the rebuild.ForStmtis immune because it rebuildsiter_args_before its body, seedingvar_remap_so the header and every body reference resolve to one node. Its comment says so explicitly:The
WhileStmthandler had no such call: it simplified the condition and body, thenMutableCopy'd the originaliter_args_back in.The user-visible shape
This is
tests/st/runtime/control_flow/test_ctrl_flow.py::TestCtrlFlowOperations::test_while_loop_add. AfterSimplifyits body reads (the printer disambiguates the orphan for you):Node identities confirm it: header
IterArgwithinit = i__ssa_v0, uses withinit = 0.Nothing miscompiled — codegen resolves by name and both nodes are named
i__iter_v1, which is why the ST case passed on device. But the SSA edge was dangling, and it madeUseAfterDefunverifiable at every point after pass 5.The trigger needs all three of: a
while(aforis immune), the induction variable carried as anIterArg, and a constant initializer. Change the seed to a scalar parameter and no substitution happens, so the node never splits — which is why this shape went unnoticed.Changes
src/ir/transforms/simplify_pass.cpp:VisitStmt_(WhileStmtPtr)rebuildsiter_args_before the condition and body. This is the fix.It also rebuilds
return_vars_after the body, matchingForStmt. That was the same omission one field over — latent rather than active, since it only bites when a return var's type simplifies andMaybeRebuildVaris a no-op otherwise. Included for symmetry rather than left as a second copy of the gap.tests/ut/ir/transforms/test_simplify_pass.py: regression test inTestControlFlow. It authors thepl.while_form directly so it exercisesSimplifyalone, and asserts noUseAfterDeferrors, that the init actually folded to0(so it cannot pass vacuously), and that the condition operandsame_asthe header'sIterArg. Reading the return var after the loop keeps the carry live and covers thereturn_vars_rebuild too.docs/{en,zh}/dev/passes/05-simplify.md:WhileStmtwas documented as merely "visit the body with scoped scalar unbinding", sharing a bullet withSpmdScopeStmt. Split into its own bullet documenting the rebuild order and why it is required rather than cosmetic.Verification
cmake --build build --parallel 20: exit 0pytest tests/ut -n 8: 10598 passed, 3 skipped, 3 xfailedpytest tests/st/codegen -n 8: 60 passedpytest tests/st/runtime/control_flow: 9 passed on device, includingtest_while_loop_add[a2a3]— the case this was reported againstctest --parallel 8: 1/1 passedUseAfterDefonkernel_while_addnow verifies clean after every pass of the Default pipeline (52 passes); previously it broke at pass 5 and stayed broken.check_headers,check_english_only,check_docs_en_zh_parity,check_docs_nav,check_no_broad_raises,check_op_name_literals,clang-format --dry-run --Werror,ruff check/ruff format --check,pyright: exit 0 each.Reviewer notes
UseAfterDefverifiable pipeline-wide on its own. A separate known dangling reference (OutlineIncoreScopeshanding a loop-body-local tensor version to a post-loop submit, which needs amanual_dep=Truetensor) still blocks adding it toGetVerifiedProperties().kernel_while_adddoes not have that shape, which is why it now verifies clean end to end.