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opt_expr: saturate shift amount instead of overflowing for large shifts

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This commit is contained in:
George Rennie 2025-05-07 14:41:13 +02:00
parent 7cbe6ed048
commit 0dcd94b6ad
1 changed files with 6 additions and 1 deletions
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7
passes/opt/opt_expr.cc
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@ -1307,7 +1307,12 @@ skip_fine_alu:
if (cell->type.in(ID($shl), ID($shr), ID($sshl), ID($sshr), ID($shift), ID($shiftx)) && (keepdc ? assign_map(cell->getPort(ID::B)).is_fully_def() : assign_map(cell->getPort(ID::B)).is_fully_const()))
{
bool sign_ext = cell->type == ID($sshr) && cell->getParam(ID::A_SIGNED).as_bool();
int shift_bits = assign_map(cell->getPort(ID::B)).as_int(cell->type.in(ID($shift), ID($shiftx)) && cell->getParam(ID::B_SIGNED).as_bool());
RTLIL::SigSpec sig_b = assign_map(cell->getPort(ID::B));
const bool b_sign_ext = cell->type.in(ID($shift), ID($shiftx)) && cell->getParam(ID::B_SIGNED).as_bool();
// We saturate the value to prevent overflow, but note that this could
// cause incorrect opimization in the impractical case that A is 2^32 bits
// wide
int shift_bits = sig_b.as_int_saturating(b_sign_ext);
if (cell->type.in(ID($shl), ID($sshl)))
shift_bits *= -1;