Merge pull request #2692 from The-OpenROAD-Project-staging/rsz-hold-area

rsz: select hold buffer by delay/area rather than just max delay

Author: maliberty

src/rsz/src/RepairHold.cc

@@ -169,21 +169,53 @@ RepairHold::repairHold(Pin *end_pin,
   resizer_->incrementalParasiticsEnd();
 }
 
-// Find the buffer with the most delay in the fastest corner.
+// Find a good hold buffer using delay/area as the metric.
 LibertyCell *
 RepairHold::findHoldBuffer()
 {
-  LibertyCell *max_buffer = nullptr;
-  float max_delay = 0.0;
+  // Build a vector of buffers sorted by the metric
+  struct MetricBuffer {
+    float metric;
+    LibertyCell* cell;
+  };
+  std::vector<MetricBuffer> buffers;
   for (LibertyCell *buffer : resizer_->buffer_cells_) {
-    float buffer_min_delay = bufferHoldDelay(buffer);
-    if (max_buffer == nullptr
-        || buffer_min_delay > max_delay) {
-      max_buffer = buffer;
-      max_delay = buffer_min_delay;
+    const float buffer_area = buffer->area();
+    if (buffer_area != 0.0) {
+      float buffer_cost = bufferHoldDelay(buffer) / buffer_area;
+      buffers.push_back({buffer_cost, buffer});
     }
   }
-  return max_buffer;
+
+  std::sort(buffers.begin(),
+            buffers.end(),
+            [this](const MetricBuffer& lhs, const MetricBuffer& rhs) {
+              return lhs.metric < rhs.metric;
+            });
+
+  if (buffers.empty()) {
+    return nullptr;
+  }
+
+  // Select the highest metric
+  MetricBuffer& best_buffer = *buffers.rbegin();
+  const MetricBuffer& highest_metric = best_buffer;
+
+  // See if there is a smaller choice with nearly as good a metric.
+  const float margin = 0.95;
+  for (auto itr = buffers.rbegin() + 1; itr != buffers.rend(); itr++) {
+    if (itr->metric >= margin * highest_metric.metric) {
+      // buffer within margin, so check if area is smaller
+      const float best_buffer_area = best_buffer.cell->area();
+      const float buffer_area = itr->cell->area();
+
+      if (buffer_area < best_buffer_area) {
+        best_buffer = *itr;
+      }
+    }
+  }
+
+  return best_buffer.cell;
 }
 
 float

src/rsz/test/gcd_resize.ok

@@ -14,9 +14,9 @@ worst slack 1.34
 [INFO RSZ-0058] Using max wire length 720um.
 [INFO RSZ-0039] Resized 50 instances.
 [INFO RSZ-0046] Found 35 endpoints with hold violations.
-[INFO RSZ-0032] Inserted 98 hold buffers.
+[INFO RSZ-0032] Inserted 99 hold buffers.
 worst slack 0.20
-worst slack 1.23
+worst slack 1.24
 max slew
 
 Pin                                    Limit    Slew   Slack
@@ -47,4 +47,4 @@ _847_/Z manhtn 60.3 steiner 60.3 0.00
 _851_/Z manhtn 58.3 steiner 58.3 0.00
 _771_/Z manhtn 58.1 steiner 58.1 0.00
 _725_/Z manhtn 56.1 steiner 56.1 0.00
-Design area 836 u^2 13% utilization.
+Design area 784 u^2 12% utilization.

src/rsz/test/repair_hold12.ok

@@ -10,4 +10,4 @@
  (clk ^) r -0.04:INF f -0.04:INF
 [INFO RSZ-0046] Found 2 endpoints with hold violations.
 [INFO RSZ-0032] Inserted 4 hold buffers.
- (clk ^) r 0.01:INF f 0.00:INF
+ (clk ^) r 0.02:INF f 0.01:INF

src/rsz/test/repair_hold2.ok

@@ -90,12 +90,12 @@ Path Type: min
 
 [INFO RSZ-0046] Found 4 endpoints with hold violations.
 [INFO RSZ-0032] Inserted 10 hold buffers.
-worst slack 0.00
+worst slack 0.01
 worst slack 1.88
 Net in2
- Pin capacitance: 1.25-1.42
+ Pin capacitance: 0.70-0.78
  Wire capacitance: 5.22
- Total capacitance: 6.47-6.64
+ Total capacitance: 5.92-6.00
  Number of drivers: 1
  Number of loads: 1
  Number of pins: 2
@@ -104,18 +104,18 @@ Driver pins
  in2 input port (20, 0)
 
 Load pins
- hold10/A input (CLKBUF_X3) 1.25-1.42 (40, 41)
+ hold10/A input (CLKBUF_X1) 0.70-0.78 (40, 41)
 
 Net out
  Pin capacitance: 0.00
- Wire capacitance: 6.96
- Total capacitance: 6.96
+ Wire capacitance: 6.95
+ Total capacitance: 6.95
  Number of drivers: 1
  Number of loads: 1
  Number of pins: 2
 
 Driver pins
- hold2/Z output (CLKBUF_X3) (41, 41)
+ hold2/Z output (CLKBUF_X1) (41, 41)
 
 Load pins
  out output port (0, 0)

src/rsz/test/repair_hold3.ok

@@ -12,4 +12,4 @@ worst slack 1.95
 [INFO RSZ-0046] Found 2 endpoints with hold violations.
 [INFO RSZ-0032] Inserted 2 hold buffers.
 worst slack 0.01
-worst slack 1.92
+worst slack 1.93

src/rsz/test/repair_hold7.ok

@@ -13,9 +13,9 @@ worst slack 0.15
 worst slack 1.34
 [INFO RSZ-0046] Found 35 endpoints with hold violations.
 [WARNING RSZ-0064] Unable to repair all hold checks within margin.
-[INFO RSZ-0032] Inserted 163 hold buffers.
+[INFO RSZ-0032] Inserted 411 hold buffers.
 [ERROR RSZ-0050] Max utilization reached.
 RSZ-0050
-Design area 887 u^2 14% utilization.
-worst slack 0.22
-worst slack 1.22
+Design area 998 u^2 16% utilization.
+worst slack 0.37
+worst slack 1.03

src/rsz/test/repair_hold8.ok

@@ -10,6 +10,6 @@
 worst slack -0.09
 worst slack 1.84
 [INFO RSZ-0046] Found 4 endpoints with hold violations.
-[INFO RSZ-0032] Inserted 37 hold buffers.
-worst slack 0.20
-worst slack 1.66
+[INFO RSZ-0032] Inserted 38 hold buffers.
+worst slack 0.21
+worst slack 1.64

src/rsz/test/repair_hold9.ok

@@ -14,4 +14,4 @@ Design area 670 u^2 10% utilization.
 [INFO RSZ-0032] Inserted 163 hold buffers.
 [ERROR RSZ-0060] Max buffer count reached.
 RSZ-0060
-Design area 887 u^2 14% utilization.
+Design area 800 u^2 13% utilization.