opt_expr: improve simplification of comparisons with large constants.

The idea behind this simplification is that a N-bit signal X being
compared with an M-bit constant where M>N and the constant has Nth
or higher bit set, it either always succeeds or always fails.

However, the existing implementation only worked with one-hot signals
for some reason. It also printed incorrect messages.

This commit adjusts the simplification to have as much power as
possible, and fixes other bugs.

diff --git a/passes/opt/opt_expr.cc b/passes/opt/opt_expr.cc
index 9abbdc6ac13c40b3a9ca711966c822da7d2dd2e2..76bcfd44f605e07f237353909805793a062e1a7e 100644 (file)
--- a/passes/opt/opt_expr.cc
+++ b/passes/opt/opt_expr.cc
@@ -259,6 +259,22 @@ bool is_one_or_minus_one(const Const &value, bool is_signed, bool &is_negative)
        return last_bit_one;
 }
 
+int get_highest_hot_index(RTLIL::SigSpec signal)
+{
+       for (int i = GetSize(signal) - 1; i >= 0; i--)
+       {
+               if (signal[i] == RTLIL::State::S0)
+                       continue;
+
+               if (signal[i] == RTLIL::State::S1)
+                       return i;
+
+               break;
+       }
+
+       return -1;
+}
+
 // if the signal has only one bit set, return the index of that bit.
 // otherwise return -1
 int get_onehot_bit_index(RTLIL::SigSpec signal)
@@ -1345,9 +1361,6 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool cons
                }
 
                // simplify comparisons
-               // currently, only replaces comparisons with an extreme of the signal range with a constant
-               // TODO: since, unlike the following optimization, this one does not assume that both inputs to the cell are constant,
-               // it is more robust; the following optimization should be folded into this one at some point
                if (do_fine && (cell->type == "$lt" || cell->type == "$ge" || cell->type == "$gt" || cell->type == "$le"))
                {
                        IdString cmp_type = cell->type;
@@ -1405,29 +1418,53 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool cons
                                                replace = true;
                                        }
 
-                                       int const_bit_set = get_onehot_bit_index(const_sig);
-                                       if (const_bit_set >= 0 && const_bit_set < var_width)
+                                       int const_bit_hot = get_onehot_bit_index(const_sig);
+                                       if (const_bit_hot >= 0 && const_bit_hot < var_width)
                                        {
-                                               RTLIL::SigSpec var_high_sig(RTLIL::State::S0, var_width - const_bit_set);
-                                               for (int i = const_bit_set; i < var_width; i++) {
-                                                       var_high_sig[i - const_bit_set] = var_sig[i];
+                                               RTLIL::SigSpec var_high_sig(RTLIL::State::S0, var_width - const_bit_hot);
+                                               for (int i = const_bit_hot; i < var_width; i++) {
+                                                       var_high_sig[i - const_bit_hot] = var_sig[i];
                                                }
 
                                                if (cmp_type == "$lt")
                                                {
                                                        condition   = stringf("unsigned X<%s", log_signal(const_sig));
-                                                       replacement = stringf("!X[%d:%d]", var_width - 1, const_bit_set);
+                                                       replacement = stringf("!X[%d:%d]", var_width - 1, const_bit_hot);
                                                        module->addLogicNot(NEW_ID, var_high_sig, cell->getPort("\\Y"));
                                                        remove = true;
                                                }
                                                if (cmp_type == "$ge")
                                                {
                                                        condition   = stringf("unsigned X>=%s", log_signal(const_sig));
-                                                       replacement = stringf("|X[%d:%d]", var_width - 1, const_bit_set);
+                                                       replacement = stringf("|X[%d:%d]", var_width - 1, const_bit_hot);
                                                        module->addReduceOr(NEW_ID, var_high_sig, cell->getPort("\\Y"));
                                                        remove = true;
                                                }
                                        }
+
+                                       int const_bit_set = get_highest_hot_index(const_sig);
+                                       if(const_bit_set >= var_width)
+                                       {
+                                               string cmp_name;
+                                               if (cmp_type == "$lt" || cmp_type == "$le")
+                                               {
+                                                       if (cmp_type == "$lt") cmp_name = "<";
+                                                       if (cmp_type == "$le") cmp_name = "<=";
+                                                       condition   = stringf("unsigned X[%d:0]%s%s", var_width - 1, cmp_name.c_str(), log_signal(const_sig));
+                                                       replacement = "constant 1";
+                                                       replace_sig[0] = State::S1;
+                                                       replace = true;
+                                               }
+                                               if (cmp_type == "$gt" || cmp_type == "$ge")
+                                               {
+                                                       if (cmp_type == "$gt") cmp_name = ">";
+                                                       if (cmp_type == "$ge") cmp_name = ">=";
+                                                       condition   = stringf("unsigned X[%d:0]%s%s", var_width - 1, cmp_name.c_str(), log_signal(const_sig));
+                                                       replacement = "constant 0";
+                                                       replace_sig[0] = State::S0;
+                                                       replace = true;
+                                               }
+                                       }
                                }
                                else
                                { /* signed */
@@ -1460,66 +1497,6 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool cons
                        }
                }
 
-               // replace a<0 or a>=0 with the top bit of a
-               if (do_fine && (cell->type == "$lt" || cell->type == "$ge" || cell->type == "$gt" || cell->type == "$le"))
-               {
-                       //used to decide whether the signal needs to be negated
-                       bool is_lt = false;
-
-                       //references the variable signal in the comparison
-                       RTLIL::SigSpec sigVar;
-
-                       //references the constant signal in the comparison
-                       RTLIL::SigSpec sigConst;
-
-                       // note that this signal must be constant for the optimization
-                       // to take place, but it is not checked beforehand.
-                       // If new passes are added, this signal must be checked for const-ness
-
-                       //width of the variable port
-                       int width;
-                       int const_width;
-
-                       bool var_signed;
-
-                       if (cell->type == "$lt" || cell->type == "$ge") {
-                               is_lt = cell->type == "$lt" ? 1 : 0;
-                               sigVar = cell->getPort("\\A");
-                               sigConst = cell->getPort("\\B");
-                               width = cell->parameters["\\A_WIDTH"].as_int();
-                               const_width = cell->parameters["\\B_WIDTH"].as_int();
-                               var_signed = cell->parameters["\\A_SIGNED"].as_bool();
-                       } else
-                       if (cell->type == "$gt" || cell->type == "$le") {
-                               is_lt = cell->type == "$gt" ? 1 : 0;
-                               sigVar = cell->getPort("\\B");
-                               sigConst = cell->getPort("\\A");
-                               width = cell->parameters["\\B_WIDTH"].as_int();
-                               const_width = cell->parameters["\\A_WIDTH"].as_int();
-                               var_signed = cell->parameters["\\B_SIGNED"].as_bool();
-                       } else
-                               log_abort();
-
-                       if (sigConst.is_fully_const() && sigConst.is_fully_def() && var_signed == false)
-                       {
-                               int const_bit_set = get_onehot_bit_index(sigConst);
-                               if(const_bit_set >= width && const_bit_set >= 0){
-                                       RTLIL::SigSpec a_prime(RTLIL::State::S0, 1);
-                                       if(is_lt){
-                                               a_prime[0] = RTLIL::State::S1;
-                                               log("Replacing %s cell `%s' (implementing unsigned X[%d:0] < %s[%d:0]) with constant 0.\n", log_id(cell->type), log_id(cell), width-1, log_signal(sigConst),const_width-1);
-                                       }
-                                       else{
-                                               log("Replacing %s cell `%s' (implementing unsigned X[%d:0]>= %s[%d:0]) with constant 1.\n", log_id(cell->type), log_id(cell), width-1, log_signal(sigConst),const_width-1);
-                                       }
-                                       module->connect(cell->getPort("\\Y"), a_prime);
-                                       module->remove(cell);
-                                       did_something = true;
-                                       goto next_cell;
-                               }
-                       }
-               }
-
        next_cell:;
 #undef ACTION_DO
 #undef ACTION_DO_Y

diff --git a/tests/opt/opt_expr_cmp.v b/tests/opt/opt_expr_cmp.v
index 500b15f1b05e8a4ccf209d28bd8afdb5c80d92f7..5aff4b80f14500d1cf9e9d4201f8afa39ad226c1 100644 (file)
--- a/tests/opt/opt_expr_cmp.v
+++ b/tests/opt/opt_expr_cmp.v
@@ -19,4 +19,22 @@ module top(...);
   output o3_2 = 4'b0100 <= a;
   output o3_3 = a <  4'b0100;
   output o3_4 = a >= 4'b0100;
+
+  output o4_1 = 5'b10000 >  a;
+  output o4_2 = 5'b10000 >= a;
+  output o4_3 = 5'b10000 <  a;
+  output o4_4 = 5'b10000 <= a;
+  output o4_5 = a <  5'b10000;
+  output o4_6 = a <= 5'b10000;
+  output o4_7 = a >  5'b10000;
+  output o4_8 = a >= 5'b10000;
+
+  output o5_1 = 5'b10100 >  a;
+  output o5_2 = 5'b10100 >= a;
+  output o5_3 = 5'b10100 <  a;
+  output o5_4 = 5'b10100 <= a;
+  output o5_5 = a <  5'b10100;
+  output o5_6 = a <= 5'b10100;
+  output o5_7 = a >  5'b10100;
+  output o5_8 = a >= 5'b10100;
 endmodule