peepopt: Fix padding for the peepopt_shiftmul_right pattern

The previous version could easily generate a large amount of padding
when the constant factor was significantly larger than the width of the
shift data input. This could lead to huge amounts of logic being
generated before then being optimized away at a huge performance and
memory cost.

Additionally and more critically, when the input width was not a
multiple of the constant factor, the input data was padded with 'x bits
to such a multiple before interspersing the 'x padding needed to align
the selectable windows to power-of-two offsets.

Such a final padding would not be correct for shifts besides $shiftx,
and the previous version did attempt to remove that final padding at the
end so that the native zero/sign/x-extension behavior of the shift cell
would be used, but since the last selectable window also got
power-of-two padding appended after the padding the code is trying to
remove got added, it did not actually fully remove it in some cases.

I changed the code to only add 'x padding between selectable windows,
leaving the last selectable window unpadded. This omits the need to add
final padding to a multiple of the constant factor in the first place.
In turn, that means the only 'x bits added are actually impossible to
select. As a side effect no padding is added when the constant factor is
equal to or larger than the width of the shift data input, also solving
the reported performance bug.

This fixes #4056

passes/pmgen/peepopt_shiftmul_right.pmg

@@ -82,22 +82,17 @@ code
 	int new_const_factor = 1 << factor_bits;
 	SigSpec padding(State::Sx, new_const_factor-const_factor);
 	SigSpec old_a = port(shift, \A), new_a;
-	int trunc = 0;
-
-	if (GetSize(old_a) % const_factor != 0) {
-		trunc = const_factor - GetSize(old_a) % const_factor;
-		old_a.append(SigSpec(State::Sx, trunc));
-	}
 
 	for (int i = 0; i*const_factor < GetSize(old_a); i++) {
-		SigSpec slice = old_a.extract(i*const_factor, const_factor);
-		new_a.append(slice);
-		new_a.append(padding);
+		if ((i+1)*const_factor < GetSize(old_a)) {
+			SigSpec slice = old_a.extract(i*const_factor, const_factor);
+			new_a.append(slice);
+			new_a.append(padding);
+		} else {
+			new_a.append(old_a.extract_end(i*const_factor));
+		}
 	}
 
-	if (trunc > 0)
-		new_a.remove(GetSize(new_a)-trunc, trunc);
-
 	SigSpec new_b = {mul_din, SigSpec(State::S0, factor_bits)};
 	if (param(shift, \B_SIGNED).as_bool())
 		new_b.append(State::S0);

tests/various/peepopt.ys

@@ -46,7 +46,31 @@ design -import gold -as gold peepopt_shiftmul_2
 design -import gate -as gate peepopt_shiftmul_2
 
 miter -equiv -make_assert -make_outputs -ignore_gold_x -flatten gold gate miter
-sat -show-public -enable_undef -prove-asserts miter
+sat -verify -show-public -enable_undef -prove-asserts miter
+cd gate
+select -assert-count 1 t:$shr
+select -assert-count 1 t:$mul
+select -assert-count 0 t:$shr t:$mul %% t:* %D
+
+####################
+
+design -reset
+read_verilog <<EOT
+module peepopt_shiftmul_3 (input [7:0] D, input [0:0] S, output [3:0] Y);
+	assign Y = D >> (S*5);
+endmodule
+EOT
+
+prep
+design -save gold
+peepopt
+design -stash gate
+
+design -import gold -as gold peepopt_shiftmul_3
+design -import gate -as gate peepopt_shiftmul_3
+
+miter -equiv -make_assert -make_outputs -ignore_gold_x -flatten gold gate miter
+sat -verify -show-public -enable_undef -prove-asserts miter
 cd gate
 select -assert-count 1 t:$shr
 select -assert-count 1 t:$mul