rsz: Caching drive resistance value.

Calculating drive resistance takes about 2.8 hours of the currently
30 hour runtime of our block. It's very expensive to calculate so we
should cache it.

Signed-off-by: Ethan Mahintorabi <[email protected]>

Author: QuantamHD

src/rsz/src/Rebuffer.cc

@@ -386,7 +386,17 @@ bool Rebuffer::bufferSizeCanDriveLoad(const BufferSize& size,
 
   const float extra_cap = resizer_->dbuToMeters(extra_wire_length) * wire_cap
                           + outp->capacitance();
-  const float r_drvr = outp->driveResistance();
+
+  // Cache the value of drive resistance since it's expensive to calculate.
+  float r_drvr;
+  auto it_drive_resistance = drive_resistance_cache_.find(outp);
+  if (it_drive_resistance != drive_resistance_cache_.end()) {
+    r_drvr = it_drive_resistance->second;
+  } else {
+    r_drvr = outp->driveResistance();
+    drive_resistance_cache_[outp] = r_drvr;
+  }
+
   const float load_slew
       = (r_drvr
          + resizer_->dbuToMeters(bnet->maxLoadWireLength() + extra_wire_length)

src/rsz/src/Rebuffer.hh

@@ -138,6 +138,7 @@ class Rebuffer : public sta::dbStaState
 
   std::vector<BufferSize> buffer_sizes_;
   std::map<LibertyCell*, BufferSize*> buffer_sizes_index_;
+  std::unordered_map<LibertyPort*, float> drive_resistance_cache_;
 
   Pin* pin_;
   float fanout_limit_;