[AutoDiff] Disable EliminateRecomputableAdStackPushes pending mutated-SNode chain-leaf fix

Author: duburcqa

quadrants/transforms/auto_diff/auto_diff.cpp

@@ -37,14 +37,13 @@ void auto_diff(IRNode *root, const CompileConfig &config, AutodiffMode autodiff_
         replace_local_var_with_stacks(ib, config.ad_stack_size);
         type_check(root, config);
 
-        // Drop AdStackAllocas whose pushed value is recomputable from already-stack-backed allocas + args + const +
-        // loop-index. Trades forward-pass adstack memory traffic (one push + one load per iter per intermediate spill)
-        // for cloned arithmetic in the reverse scope, which `BackupSSA::generic_visit` generates on demand via the same
-        // RecomputableChainCloner path. Must run after `replace_local_var_with_stacks` (so the analyzer sees the
-        // AdStackAlloca shape, not the alloca-with-store shape promote_ssa_to_local_var emits) and before
-        // `make_adjoint` (so the reverse pass is generated against the cleaned forward IR with no spurious push/load
-        // scaffolding).
-        eliminate_recomputable_ad_stack_pushes(ib);
+        // Disabled: `RecomputableChainAnalyzer` admits `GlobalLoadStmt` as a recomputable interior node without
+        // verifying SNode read-only-ness. When the chain reaches a `GlobalLoadStmt` of an SNode written elsewhere in
+        // the same kernel execution, `BackupSSA`'s reverse-side re-clone reads post-write state instead of the iter-k
+        // value the original `AdStackLoadTopStmt` would have returned, silently corrupting gradients on MPM-style
+        // kernels that mix per-particle reads with grid writes. Re-enable once the chain analyzer rejects
+        // mutated-SNode loads (or the cost model gates on read-only-verified leaves).
+        // eliminate_recomputable_ad_stack_pushes(ib);
         type_check(root, config);
 
         make_adjoint(ib);