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peepopt: handle offset too large in `shiftadd` 

If the offset is larger than the signal itself,
meaning the signal is completely shifted out,
it tried to extract a negative amount of bits from the old signal.

This RTL pattern is suspicious since it is a complicated way of
arriving at a constant value, so we warn the user.
This commit is contained in:
Philippe Sauter 2024-01-26 11:44:44 +01:00
parent 80511ced71
commit 68a9aa7c29
1 changed files with 11 additions and 1 deletions
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12
passes/pmgen/peepopt_shiftadd.pmg
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@ -103,8 +103,18 @@ code
new_a.append(old_a);
} else {
// data >> (...+c) transformed to data[MAX:c] >> (...)
new_a.append(old_a.extract_end(offset));
if(offset < GetSize(old_a)) // some signal bits left?
new_a.append(old_a.extract_end(offset));
else {
if(shift->type.in($shiftx))
log_warning("at %s: result of indexed part-selection is always constant (selecting from '%s' with index '%s + %d')\n", \
shift->get_src_attribute().c_str(), log_signal(old_a), log_signal(var_signal), offset);
else
log_warning("at %s: result of shift operation is always constant (shifting '%s' by '%s + %d'-bits)\n", \
shift->get_src_attribute().c_str(), log_signal(old_a), log_signal(var_signal), offset);
}
// is it fine to leave new_a empty? (size 0)
}
SigSpec new_b = {var_signal, SigSpec(State::S0, log2scale)};