[PTO][Lowering] subset lowering forces contiguous child strides and may break address semantics

[WenboCodes]

Problem

pto.subset is described as a strided view inheriting source strides, but lowering rewrites result strides to contiguous [child_width, 1].

Evidence

• ODS description says inherited strides: include/PTO/IR/PTOOps.td:168
• Lowering hardcodes child strides:
  • lib/PTO/Transforms/PTOViewToMemref.cpp:527
  • lib/PTO/Transforms/PTOViewToMemref.cpp:533
  • lib/PTO/Transforms/PTOViewToMemref.cpp:542

Repro

build/tools/ptoas/ptoas test/samples/_out/Subset/subset-pto-ir.pto

Observed lowering output includes:

• parent: memref<32x64xf32, strided<[64, 1], ...>
• child subset: to memref<32x32xf32, strided<[32, 1], ...>

Why this is risky

For parent 32x64 and child 32x32, row step changes from 64 to 32, so address formula changes for rows > 0.

Expected behavior

Either:

1. Preserve view semantics by keeping source-compatible strides, or
2. Explicitly materialize contiguous memory (alloc+copy) if contiguous child layout is intended.

Acceptance criteria

• Decide and document canonical subset semantics.
• Align ODS docs + lowering implementation.
• Add regression test proving row-stride correctness for 32x64 -> subset 32x32.