optimizing fabric bitsteream memory footprint

openfpga/src/fpga_bitstream/build_fabric_bitstream.cpp

@@ -495,6 +495,10 @@ FabricBitstream build_fabric_dependent_bitstream(const BitstreamManager& bitstre
   VTR_ASSERT(1 == top_block.size());
   VTR_ASSERT(0 == top_module_name.compare(bitstream_manager.block_name(top_block[0])));
 
+  /* Reserve bits before build-up */
+  fabric_bitstream.reserve(bitstream_manager.bits().size());
+
+  /* Start build-up formally */
   build_module_fabric_dependent_bitstream(config_protocol,
                                           bitstream_manager, top_block[0],
                                           module_manager, top_module, 

openfpga/src/fpga_bitstream/fabric_bitstream.cpp

@@ -55,6 +55,13 @@ bool FabricBitstream::bit_din(const FabricBitId& bit_id) const {
 /******************************************************************************
  * Public Mutators
  ******************************************************************************/
+void FabricBitstream::reserve(const size_t& num_bits) {
+  bit_ids_.reserve(num_bits);
+  config_bit_ids_.reserve(num_bits);
+  bit_addresses_.reserve(num_bits);
+  bit_dins_.reserve(num_bits);
+}
+
 FabricBitId FabricBitstream::add_bit(const ConfigBitId& config_bit_id) {
   FabricBitId bit = FabricBitId(bit_ids_.size());
   /* Add a new bit, and allocate associated data structures */

openfpga/src/fpga_bitstream/fabric_bitstream.h

@@ -60,6 +60,9 @@ class FabricBitstream {
     bool bit_din(const FabricBitId& bit_id) const;
 
   public:  /* Public Mutators */
+    /* Reserve config bits */
+    void reserve(const size_t& num_bits);
+
     /* Add a new configuration bit to the bitstream manager */
     FabricBitId add_bit(const ConfigBitId& config_bit_id);