Fix issue #269, optimize signed compare with 0

add opt_compare pass and add it to opt
for a < 0:
    if a is signed, replace with a[max_bit-1]
for a >= 0:
    if a is signed, replace with ~a[max_bit-1]

diff --git a/passes/opt/Makefile.inc b/passes/opt/Makefile.inc
index a8b1537bb6aa3bdedc93ed910f380c73ac2012ee..a15c4184d048f76226d2784fd9eb8a5fcd659bd5 100644 (file)
--- a/passes/opt/Makefile.inc
+++ b/passes/opt/Makefile.inc
@@ -6,6 +6,7 @@ OBJS += passes/opt/opt_reduce.o
 OBJS += passes/opt/opt_rmdff.o
 OBJS += passes/opt/opt_clean.o
 OBJS += passes/opt/opt_expr.o
+OBJS += passes/opt/opt_compare.o
 
 ifneq ($(SMALL),1)
 OBJS += passes/opt/share.o

diff --git a/passes/opt/opt.cc b/passes/opt/opt.cc
index 021c1a03f0292cf7f9b64aad0bd655b701c2bac7..b689be48056e2d119fc5c6a53e4448c47279ab52 100644 (file)
--- a/passes/opt/opt.cc
+++ b/passes/opt/opt.cc
@@ -128,6 +128,7 @@ struct OptPass : public Pass {
                {
                        while (1) {
                                Pass::call(design, "opt_expr" + opt_expr_args);
+                               Pass::call(design, "opt_compare");
                                Pass::call(design, "opt_merge" + opt_merge_args);
                                design->scratchpad_unset("opt.did_something");
                                Pass::call(design, "opt_rmdff" + opt_rmdff_args);
@@ -141,6 +142,7 @@ struct OptPass : public Pass {
                else
                {
                        Pass::call(design, "opt_expr" + opt_expr_args);
+                       Pass::call(design, "opt_compare");
                        Pass::call(design, "opt_merge -nomux" + opt_merge_args);
                        while (1) {
                                design->scratchpad_unset("opt.did_something");

diff --git a/passes/opt/opt_compare.cc b/passes/opt/opt_compare.cc
new file mode 100644 (file)
index 0000000..15b547e
--- /dev/null
+++ b/passes/opt/opt_compare.cc
@@ -0,0 +1,78 @@
+#include "kernel/yosys.h"
+#include "kernel/sigtools.h"
+#include "kernel/utils.h"
+USING_YOSYS_NAMESPACE
+PRIVATE_NAMESPACE_BEGIN
+void replace_le_cell(Cell* cell, Module* module){
+       RTLIL::SigSpec a = cell->getPort("\\A");
+       RTLIL::SigSpec b = cell->getPort("\\B");
+       RTLIL::SigSpec y(RTLIL::State::S0, cell->parameters["\\Y_WIDTH"].as_int());
+       if(b.is_fully_const() && b.is_fully_zero() ){ 
+               if(cell->parameters["\\A_SIGNED"].as_bool()){
+                       // a < 0, can be replaced with a[MAX_BIT]
+                       log("Found x < 0 (signed), replacing with the last bit\n");
+                       int a_width = cell->parameters["\\A_WIDTH"].as_int();
+                       if(a_width > 0){
+                               y[0] = a[a_width-1];
+                               module->connect(cell->getPort("\\Y"), y);
+                               module->remove(cell);
+                       }
+               }
+       } 
+}
+void replace_ge_cell(Cell* cell, Module* module){
+       RTLIL::SigSpec a = cell->getPort("\\A");
+       RTLIL::SigSpec b = cell->getPort("\\B");
+       RTLIL::SigSpec y = cell->getPort("\\Y");
+       if(b.is_fully_const() && b.is_fully_zero()){
+               if(cell->parameters["\\A_SIGNED"].as_bool()){
+                       log("Found x >= 0 (signed), optimizing\n");
+                       RTLIL::SigSpec a_prime(RTLIL::State::S0, cell->parameters["\\Y_WIDTH"].as_int());
+                       int a_width = cell->parameters["\\A_WIDTH"].as_int();
+                       if(a_width > 0){
+                               a_prime[0] = a[a_width-1];
+                               module->remove(cell);
+                               module->addNot("$not", a_prime, y,false);
+                       }
+               }
+       }
+}
+void optimize_compares(Design* design, Module* module){
+       log_header(design, "Executing OPT_COMPARE pass.\n");
+       log_push();
+       TopoSort<RTLIL::Cell*, RTLIL::IdString::compare_ptr_by_name<RTLIL::Cell>> cells;
+       for(auto cell: module->cells())
+               if(design->selected(module,cell) && cell->type[0] == '$'){
+                       cells.node(cell);
+               }
+       cells.sort();
+       for (auto cell: cells.sorted){
+               if (cell->type == "$lt"){
+                       replace_le_cell(cell,module);
+               }
+               else if(cell->type == "$ge"){
+                       replace_ge_cell(cell,module);
+               }
+       }
+
+}
+struct OptCompare : public Pass {
+       OptCompare() : Pass("opt_compare") {}
+       virtual void execute(vector<string>, Design* design){
+               for(auto module: design->selected_modules())
+                       optimize_compares(design,module);
+       }
+       virtual void help() {
+               log("\n");
+               log("opt_compare\n");
+               log("\n");
+               log("This pass optimizes some signed compares with 0.\n");
+               log("In particular, it replaces a < 0 with the msb of a,\n");
+               log("and a >= 0 with the inverted msb of a.\n");
+               log("\n");
+
+       }
+} OptCompare;
+
+
+PRIVATE_NAMESPACE_END