gemini-code-assist[bot] commented on code in PR #19496:
URL: https://github.com/apache/tvm/pull/19496#discussion_r3177152662
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src/tirx/transform/common_subexpr_elim.cc:
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@@ -275,6 +282,13 @@ class CSEPlanner : public StmtExprVisitor {
if (IsForbiddenNode(expr)) return false;
if (expr.as<RampNode>() || expr.as<BroadcastNode>()) return false;
if (CheckContains::ExprContains(expr, IsForbiddenNode)) return false;
+ // Reject wholly-constant expressions (no Var anywhere in the tree).
+ // BufferLoad is already filtered above by IsForbiddenNode, so
+ // "contains no Var" is sufficient to declare the expression a
+ // compile-time constant. Hoisting it adds noise; the constant
+ // folder will collapse it.
+ auto contains_var = [](const PrimExpr& e) { return e.as<VarNode>() !=
nullptr; };
+ if (!CheckContains::ExprContains(expr, contains_var)) return false;
Review Comment:
The `IsEligible` predicate now performs two separate full traversals of the
expression tree (lines 284 and 291) for every node visited during the CSE
planning phase. This leads to $O(N^2)$ complexity relative to the expression
depth, which can be significant for large generated TIR programs.
Furthermore, the check at line 282 is redundant because
`CheckContains::ExprContains` at line 284 already checks the root node.
Additionally, since `IsEligible` is only invoked from `RecordExpr` within
specific `VisitExpr_` overrides (which exclude `Call` and `BufferLoad`), the
root-level check at line 282 will always be false.
Consider combining the 'forbidden node' and 'contains var' checks into a
single pass or, ideally, caching these properties during the bottom-up
traversal of `CSEPlanner` to maintain linear complexity.
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