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Fix truncation issue in opt_balance_tree pass

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Only allow rebalancing of cells with "natural" output widths (no truncation).
This prevents equivalence failures when moving operands between adders
with different intermediate truncation points.

For each operation type, the natural width is:
- Addition: max(A_WIDTH, B_WIDTH) + 1 (for carry bit)
- Multiplication: A_WIDTH + B_WIDTH
- Logic ops: max(A_WIDTH, B_WIDTH)

Fixes widlarizer's counterexample in YosysHQ/yosys#5605 where an 8-bit
intermediate wire was intentionally truncating adder results, and
rebalancing would change where that truncation occurred.
This commit is contained in:
Natalia 2026-01-14 13:12:55 -08:00
parent 6aef8ea8ab
commit 60ac3670cb
1 changed files with 24 additions and 3 deletions
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27
passes/opt/opt_balance_tree.cc
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@ -36,10 +36,31 @@ struct OptBalanceTreeWorker {
dict<IdString, int> cell_count;
// Check if cell is of the right type and has matching input/output widths
// Only allow cells with "natural" output widths (no truncation) to prevent
// equivalence issues when rebalancing (see YosysHQ/yosys#5605)
bool is_right_type(Cell* cell, IdString cell_type) {
return cell->type == cell_type &&
cell->getParam(ID::Y_WIDTH).as_int() >= cell->getParam(ID::A_WIDTH).as_int() &&
cell->getParam(ID::Y_WIDTH).as_int() >= cell->getParam(ID::B_WIDTH).as_int();
if (cell->type != cell_type)
return false;
int y_width = cell->getParam(ID::Y_WIDTH).as_int();
int a_width = cell->getParam(ID::A_WIDTH).as_int();
int b_width = cell->getParam(ID::B_WIDTH).as_int();
// Calculate the "natural" output width for this operation
int natural_width;
if (cell_type == ID($add)) {
// Addition produces max(A_WIDTH, B_WIDTH) + 1 (for carry bit)
natural_width = std::max(a_width, b_width) + 1;
} else if (cell_type == ID($mul)) {
// Multiplication produces A_WIDTH + B_WIDTH
natural_width = a_width + b_width;
} else {
// Logic operations ($and/$or/$xor) produce max(A_WIDTH, B_WIDTH)
natural_width = std::max(a_width, b_width);
}
// Only allow cells where Y_WIDTH equals the natural width (no truncation)
return y_width == natural_width;
}
// Create a balanced binary tree from a vector of source signals