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celledges: Clean up shift rules
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@povik
povik committed Jan 19, 2024
1 parent a96c257 commit 134eb15
Showing 1 changed file with 89 additions and 65 deletions.
154 changes: 89 additions & 65 deletions kernel/celledges.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -179,107 +179,131 @@ void shift_op(AbstractCellEdgesDatabase *db, RTLIL::Cell *cell)
int a_width = GetSize(cell->getPort(ID::A));
int b_width = GetSize(cell->getPort(ID::B));
int y_width = GetSize(cell->getPort(ID::Y));
int effective_a_width = a_width;

if (cell->type.in(ID($shift), ID($shiftx)) && is_signed) {
effective_a_width = std::max(y_width, a_width);
//is_signed = false;
// Behavior of the different shift cells:
//
// $shl, $sshl -- shifts left by the amount on B port, B always unsigned
// $shr, $sshr -- ditto right
// $shift, $shiftx -- shifts right by the amount on B port, B optionally signed
//
// Sign extension (if A signed):
//
// $shl, $shr, $shift -- only sign-extends up to size of Y, then shifts in zeroes
// $sshl, $sshr -- fully sign-extends
// $shiftx -- no sign extension
//
// Because $shl, $sshl only shift left, and $shl sign-extens up to size of Y, they
// are effectively the same.

// the cap below makes sure we don't overflow in the arithmetic further down, though
// it makes the edge data invalid once a_width approaches the order of 2**30
// (that ever happening is considered improbable)
int b_width_capped = min(b_width, 30);

int b_high, b_low;
if (!is_b_signed) {
b_high = (1 << b_width_capped) - 1;
b_low = 0;
} else {
b_high = (1 << (b_width_capped - 1)) - 1;
b_low = -(1 << (b_width_capped - 1));
}

// how far the maximum value of B is able to shift
int b_range = (1<<b_width) - 1;
// highest position of Y that can change with the value of B
int b_range_upper;
// 1 + highest position of A that can be moved to Y[i]
int a_range_upper;
// lowest position of A that can be moved to Y[i]
int a_range_lower;

for (int i = 0; i < y_width; i++){
// highest position of Y that can change with the value of B
int b_range_upper = 0;
// 1 + highest position of A that can be moved to Y[i]
int a_range_upper;
// lowest position of A that can be moved to Y[i]
int a_range_lower;

if (cell->type.in(ID($shl), ID($sshl))) {
// << and <<<
b_range_upper = a_width + b_range;
b_range_upper = a_width + b_high;
if (is_signed) b_range_upper -= 1;
a_range_lower = max(0, i - b_range);
a_range_lower = max(0, i - b_high);
a_range_upper = min(i+1, a_width);
} else if (cell->type.in(ID($shr), ID($sshr)) || (cell->type.in(ID($shift), ID($shiftx)) && !is_b_signed)){
// >> and >>>
} else if (cell->type.in(ID($shr), ID($sshr)) || (cell->type.in(ID($shift), ID($shiftx)) && !is_b_signed)) {
b_range_upper = a_width;
a_range_lower = min(i, a_width - 1); // technically the min is unneccessary as b_range_upper check already skips any i >= a_width, but let's leave the logic in since this is hard enough
a_range_upper = min(i+1 + b_range, a_width);
a_range_lower = min(i, a_width - 1);
a_range_upper = min(i+1 + b_high, a_width);
} else if (cell->type.in(ID($shift), ID($shiftx)) && is_b_signed) {
// can go both ways depending on sign of B
// 2's complement range is different depending on direction
int b_range_left = (1<<(b_width - 1));
int b_range_right = (1<<(b_width - 1)) - 1;
b_range_upper = effective_a_width + b_range_left;
a_range_lower = max(0, i - b_range_left);
b_range_upper = a_width - b_low;
a_range_lower = max(0, i + b_low);
if (is_signed)
a_range_lower = min(a_range_lower, a_width - 1);
a_range_upper = min(i+1 + b_range_right, a_width);
a_range_upper = min(i+1 + b_high, a_width);
} else {
log_assert(false && "unreachable");
}

if (i < b_range_upper) {
for (int k = a_range_lower; k < a_range_upper; k++)
db->add_edge(cell, ID::A, k, ID::Y, i, -1);
} else {
// the only possible influence value is sign extension
// only influence is through sign extension
if (is_signed)
db->add_edge(cell, ID::A, a_width - 1, ID::Y, i, -1);
}

for (int k = 0; k < b_width; k++) {
if (cell->type.in(ID($shl), ID($sshl)) && a_width == 1 && is_signed) {
int skip = (1<<(k+1));
int base = skip -1;
if (i % skip != base && i - a_width + 2 < 1 << b_width)
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
} else if (true && cell->type.in(ID($shift), ID($shiftx)) && a_width == 1 && is_signed && is_b_signed) {
if (k != b_width - 1) {
// can we jump into the zero-padding by toggling B[k]?
bool zpad_jump = (((y_width - i) & ((1 << (k + 1)) - 1)) != 0 \
&& (((y_width - i) & ~(1 << k)) < (1 << (b_width - 1))));
if (((~(i - 1) & ((1 << (k + 1)) - 1)) != 0 && i < 1 << (b_width - 1)) || zpad_jump)
// left shifts
if (cell->type.in(ID($shl), ID($sshl))) {
if (a_width == 1 && is_signed) {
int skip = 1 << (k + 1);
int base = skip -1;
if (i % skip != base && i - a_width + 2 < 1 << b_width)
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
} else if (is_signed) {
if (i - a_width + 2 < 1 << b_width)
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
} else {
if ((y_width - 1 - i < (1 << (b_width - 1)) - 1) || (i < (1 << (b_width - 1))))
if (i - a_width + 1 < 1 << b_width)
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
}
} else if ((cell->type.in(ID($shr), ID($sshr)) || (cell->type.in(ID($shift), ID($shiftx)) && !is_b_signed)) && is_signed) {
bool shift_in_bulk = i < a_width - 1;
// can we jump into the zero-padding by toggling B[k]?
bool zpad_jump = (((y_width - i) & ((1 << (k + 1)) - 1)) != 0 \
&& (((y_width - i) & ~(1 << k)) < (1 << b_width)));

if (shift_in_bulk || (cell->type.in(ID($shr), ID($shift), ID($shiftx)) && zpad_jump))
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
} else if (cell->type.in(ID($sshl), ID($shl)) && is_signed) {
if (i - a_width + 2 < 1 << b_width)
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
} else if (cell->type.in(ID($shl), ID($sshl))) {
if (i - a_width + 1 < 1 << b_width)
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
} else if (cell->type.in(ID($shift), ID($shiftx)) && is_b_signed && !is_signed) {
if (i - a_width < (1 << (b_width - 1)))
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
} else if (cell->type.in(ID($shift), ID($shiftx)) && is_b_signed && is_signed) {
if (k != b_width - 1) {
bool r_shift_in_bulk = i < a_width - 1;
// right shifts
} else if (cell->type.in(ID($shr), ID($sshr)) || (cell->type.in(ID($shift), ID($shiftx)) && !is_b_signed)) {
if (is_signed) {
bool shift_in_bulk = i < a_width - 1;
// can we jump into the zero-padding by toggling B[k]?
bool r_zpad_jump = (((y_width - i) & ((1 << (k + 1)) - 1)) != 0 \
&& (((y_width - i) & ~(1 << k)) < (1 << (b_width - 1))));
if (r_shift_in_bulk || r_zpad_jump || i - a_width + 2 <= 1 << (b_width - 1))
bool zpad_jump = (((y_width - i) & ((1 << (k + 1)) - 1)) != 0 \
&& (((y_width - i) & ~(1 << k)) < (1 << b_width)));

if (shift_in_bulk || (cell->type.in(ID($shr), ID($shift), ID($shiftx)) && zpad_jump))
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
} else {
if ((i - a_width + 2) <= (1 << (b_width - 1)) || (y_width - i) < (1 << (b_width - 1)))
if (i < a_width)
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
}
// bidirectional shifts (positive B shifts right, negative left)
} else if (cell->type.in(ID($shift), ID($shiftx)) && is_b_signed) {
if (is_signed) {
if (k != b_width - 1) {
bool r_shift_in_bulk = i < a_width - 1;
// assuming B is positive, can we jump into the upper zero-padding by toggling B[k]?
bool r_zpad_jump = (((y_width - i) & ((1 << (k + 1)) - 1)) != 0 \
&& (((y_width - i) & ~(1 << k)) <= b_high));
// assuming B is negative, can we influence Y[i] by toggling B[k]?
bool l = a_width - 2 - i >= b_low;
if (a_width == 1) {
// in case of a_width==1 we go into more detailed reasoning
l = l && (~(i - a_width) & ((1 << (k + 1)) - 1)) != 0;
}
if (r_shift_in_bulk || r_zpad_jump || l)
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
} else {
if (y_width - i <= b_high || a_width - 2 - i >= b_low)
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
}
} else {
if (a_width - 1 - i >= b_low)
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
}
} else {
if (i < effective_a_width)
db->add_edge(cell, ID::B, k, ID::Y, i, -1);
log_assert(false && "unreachable");
}
}

}
}

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