PR #181277: Preserve symbolic relationships in after-aot repro scripts

• Repository: pytorch/pytorch
• Author: eellison
• State: open | merged: False
• Link: https://github.com/pytorch/pytorch/pull/181277

Description (problem / solution / changelog)

Stack from ghstack (oldest at bottom):

• -> #181277
• #181276
• #181275
• #179091
• #179090
• #176927

When an after-aot repro script serializes symint inputs, it previously stored only the concrete hint value (e.g., reader.symint(160)), discarding the symbolic expression. When run_repro retraces with tracing_mode='symbolic', each symint becomes an independent symbol, losing algebraic relationships like sym_size_int_65 = (s0 + s3 + ... + s97) // 160 + 866. This causes make_fx to produce a graph with many independent symbols instead of a few related ones, which prevents reproducing compile-time pathologies caused by wide symbolic expressions (e.g., 50+ min compiles from 64-variable polynomial GCD).

Changes:

• InputWriter.symint now emits the expression alongside the hint: reader.symint(160, expr='s0') for free symbols, and reader.symint(930, expr='((...sum...)//160) + 866') for derived.
• InputReader.symint accepts the optional expr kwarg and stores the index-to-expression mapping in symint_exprs.
• repro_common, when tracing_mode='symbolic' and expressions are available, calls _build_symbolic_wrapper() to build a wrapper module that takes carrier tensors for free symbols, extracts SymInts via .size(0), computes derived expressions via Python arithmetic, and delegates to the inner Repro. make_fx then traces the wrapper, preserving the algebraic structure.
• Backward-compatible: old repro scripts without expr= still work (fall back to independent-symbol behavior).

Authored with Claude.

cc @voznesenskym @penguinwu @EikanWang @jgong5 @Guobing-Chen @XiaobingSuper @zhuhaozhe @blzheng @wenzhe-nrv @jiayisunx @kadeng @chauhang @amjames @Lucaskabela @jataylo @azahed98

Changed files

• torch/_dynamo/debug_utils.py (modified, +10/-3)
• torch/_dynamo/repro/after_aot.py (modified, +94/-0)