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opt_expr: Optimize div/mod by const 1. 

Turns out the code for div by a power of 2 is already almost capable of
optimizing this to a shift-by-0 or and-with-0, which will be further
folded into nothingness; let's beef it up to handle div by 1 as well.

Fixes #2820.
This commit is contained in:
Marcelina Kościelnicka 2021-06-09 16:14:16 +02:00
parent 55e8f5061a
commit 12b3a9765d
1 changed files with 4 additions and 4 deletions
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8
passes/opt/opt_expr.cc
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@ -1648,7 +1648,7 @@ skip_identity:
goto next_cell;
}
for (int i = 1; i < (b_signed ? sig_b.size()-1 : sig_b.size()); i++)
for (int i = 0; i < (b_signed ? sig_b.size()-1 : sig_b.size()); i++)
if (b_val == (1 << i))
{
if (cell->type.in(ID($div), ID($divfloor)))
@ -1672,7 +1672,7 @@ skip_identity:
// Truncating division is the same as flooring division, except when
// the result is negative and there is a remainder - then trunc = floor + 1
if (is_truncating && a_signed) {
if (is_truncating && a_signed && i != 0) {
Wire *flooring = module->addWire(NEW_ID, sig_y.size());
cell->setPort(ID::Y, flooring);
@ -1698,7 +1698,7 @@ skip_identity:
std::vector<RTLIL::SigBit> new_b = RTLIL::SigSpec(State::S1, i);
if (b_signed)
if (b_signed || i == 0)
new_b.push_back(State::S0);
cell->type = ID($and);
@ -1707,7 +1707,7 @@ skip_identity:
// truncating modulo has the same masked bits as flooring modulo, but
// the sign bits are those of A (except when R=0)
if (is_truncating && a_signed) {
if (is_truncating && a_signed && i != 0) {
Wire *flooring = module->addWire(NEW_ID, sig_y.size());
cell->setPort(ID::Y, flooring);
SigSpec truncating = SigSpec(flooring).extract(0, i);