Merge pull request #401 from The-OpenROAD-Project-staging/sta_fix_const_net_ptr

dcalc: no slew merge for constant disabled arcs

dcalc/GraphDelayCalc.cc

@@ -47,6 +47,7 @@
 #include "Sdc.hh"
 #include "Scene.hh"
 #include "SearchPred.hh"
+#include "search/Sim.hh"
 #include "Stats.hh"
 #include "TimingArc.hh"
 #include "TimingRole.hh"
@@ -88,8 +89,10 @@ DcalcPred::searchFrom(const Vertex *from_vertex,
   const Pin *from_pin = from_vertex->pin();
   const Sdc *sdc = mode->sdc();
   const Network *network = sta_->network();
+  const Sim *sim = mode->sim();
   Net *net = network->net(from_pin);
   return !(sdc->isDisabledConstraint(from_pin)
+           || sim->isConstant(from_vertex)
 	   || (net && (network->isPower(net)
                        || network->isGround(net))));
 }

dcalc/test/CMakeLists.txt

@@ -1,5 +1,6 @@
 sta_module_tests("dcalc"
   TESTS
+    case_analysis_leak
     prima_report
 )
 

dcalc/test/dcalc_case_analysis_leak.ok

@@ -0,0 +1,10 @@
+
+load on constant net cn (ff)    arrival at z (ps)
+0                               158.79
+50                              158.79
+200                             158.79
+500                             158.79
+1000                            158.79
+
+arrival spread across the sweep: 0.00 ps (expected 0.00)
+

dcalc/test/dcalc_case_analysis_leak.tcl

@@ -0,0 +1,61 @@
+# Delay calculation must not merge slew from set_case_analysis constants.
+#
+# Regression guard for "dcalc no slew merge for constant disabled arcs".
+#
+# GraphDelayCalc uses DcalcPred (dcalc/GraphDelayCalc.cc).  Its searchFrom
+# used to stop only at power/ground nets:
+#
+#   return !(sdc->isDisabledConstraint(from_pin)
+#            || (net && (network->isPower(net) || network->isGround(net))));
+#
+# SearchPred0::searchFrom (search/SearchPred.cc) stops at
+# sim->isConstant(from_vertex); DcalcPred did not, so in
+# findDriverEdgeDelays an arc whose source pin is an SDC constant still
+# passed searchFrom/searchThru and had its delay and slew merged into the
+# driver vertex -- including when that driver is a live, non-constant pin.
+#
+# The fix adds sim->isConstant(from_vertex) to DcalcPred::searchFrom, and
+# makes Sta::delayCalcPreamble propagate constants before delay calculation
+# so the predicate sees them.
+#
+# Circuit (dcalc_case_analysis_leak.v):
+#
+#   c --BUF u0--> cn ---A2\
+#                          AND2 u1 --> n1 --BUF u2--> z
+#   a -------------------A1/
+#
+# set_case_analysis 1 c makes net cn constant.  1 is non-controlling for an
+# AND, so n1 and z stay live and the A2->ZN arc is dead.  The reported path is
+# a -> u1 -> u2 -> z, which never traverses cn.
+#
+# Sweeping the wire load on cn must not change anything on that path.  Before
+# the fix it did: cn's slew rode the dead A2->ZN arc into n1, and n1's slew
+# set u2's delay, so the arrival spread was 46.38 ps.  The arrival column
+# below must now be flat.
+
+read_liberty ../../examples/nangate45_slow.lib.gz
+read_verilog dcalc_case_analysis_leak.v
+link_design top
+
+create_clock -name clk -period 10
+set_input_delay 0 -clock clk [get_ports {a c}]
+set_output_delay 0 -clock clk [get_ports z]
+set_input_transition 0.010 [get_ports {a c}]
+
+set_case_analysis 1 [get_ports c]
+
+puts ""
+puts "load on constant net cn (ff)    arrival at z (ps)"
+set arrivals {}
+foreach load {0 50 200 500 1000} {
+  set_load $load [get_nets cn]
+  set path [lindex [find_timing_paths -path_delay max] 0]
+  set arrival [format %.2f [expr [$path data_arrival_time] * 1e12]]
+  lappend arrivals $arrival
+  puts [format "%-32s%s" $load $arrival]
+}
+
+set delta [expr [lindex $arrivals end] - [lindex $arrivals 0]]
+puts ""
+puts "arrival spread across the sweep: [format %.2f $delta] ps (expected 0.00)"
+puts ""

dcalc/test/dcalc_case_analysis_leak.v

@@ -0,0 +1,12 @@
+module top (a, c, z);
+  input a, c;
+  output z;
+  wire cn, n1;
+
+  // set_case_analysis 1 on port c makes net cn constant.
+  BUF_X1  u0 (.A(c),  .Z(cn));
+  // 1 is non-controlling for AND2, so n1 is NOT constant.
+  AND2_X1 u1 (.A1(a), .A2(cn), .ZN(n1));
+  // Consumes n1's slew, so slew corruption becomes a delay change.
+  BUF_X1  u2 (.A(n1), .Z(z));
+endmodule

graph/test/graph_modify.ok

@@ -2415,7 +2415,7 @@ Corner: slow
    0.14    1.14 v buf1/Z (BUF_X1)
    0.09    1.23 v and1/ZN (AND2_X1)
    0.09    1.32 ^ nand1/ZN (NAND2_X1)
-   0.07    1.38 ^ buf4/Z (BUF_X4)
+   0.06    1.38 ^ buf4/Z (BUF_X4)
    0.00    1.38 ^ q3 (out)
            1.38   data arrival time
 

sdc/test/sdc_port_delay_advanced.ok

@@ -111,7 +111,7 @@ max slew
 
 Pin                                    Limit    Slew   Slack
 ------------------------------------------------------------
-nor1/ZN                                 0.20    0.01    0.18 (MET)
+reg1/QN                                 0.20    0.01    0.19 (MET)
 
 max fanout
 

search/Sta.cc

@@ -191,6 +191,7 @@ StaSimObserver::StaSimObserver(StaState *sta) :
 void
 StaSimObserver::valueChangeAfter(const Pin *pin)
 {
+  graph_delay_calc_->delayInvalid(pin);
   Vertex *vertex = graph_->pinDrvrVertex(pin);
   if (vertex) {
     search_->arrivalInvalid(vertex);
@@ -3655,8 +3656,10 @@ void
 Sta::delayCalcPreamble()
 {
   ensureLevelized();
-  for (Mode *mode : modes_)
+  for (Mode *mode : modes_) {
+    mode->sim()->ensureConstantsPropagated();
     mode->clkNetwork()->ensureClkNetwork();
+  }
 }
 
 void