Basic flow

The Place and Route process in VPR consists of several steps:

  • Packing (combinines primitives into complex blocks)

  • Placement (places complex blocks within the FPGA grid)

  • Routing (determines interconnections between blocks)

  • Analysis (analyzes the implementation)

Each of these steps provides additional configuration options that can be used to customize the whole process.

Packing

The packing algorithm tries to combine primitive netlist blocks (e.g. LUTs, FFs) into groups, called Complex Blocks (as specified in the FPGA architecture file). The results from the packing process are written into a .net file. It contains a description of complex blocks with their inputs, outputs, used clocks and relations to other signals. It can be useful in analyzing how VPR packs primitives together.

A detailed description of the .net file format can be found in the Packed Netlist Format (.net) section.

Placement

This step assigns a location to the Complex Blocks (produced by packing) with the FPGA grid, while optimizing for wirelength and timing. The output from this step is written to the .place file, which contains the physical location of the blocks from the .net file.

The file has the following format:

block_name    x        y   subblock_number

where x and y are positions in the VPR grid and block_name comes from the .net file.

Example of a placing file:

Netlist_File: top.net Netlist_ID: SHA256:ce5217d251e04301759ee5a8f55f67c642de435b6c573148b67c19c5e054c1f9
Array size: 149 x 158 logic blocks

#block name x       y       subblk  block number
#---------- --      --      ------  ------------
$auto$alumacc.cc:474:replace_alu$24.slice[1].carry4_full    53      32      0       #0
$auto$alumacc.cc:474:replace_alu$24.slice[2].carry4_full    53      31      0       #1
$auto$alumacc.cc:474:replace_alu$24.slice[3].carry4_full    53      30      0       #2
$auto$alumacc.cc:474:replace_alu$24.slice[4].carry4_full    53      29      0       #3
$auto$alumacc.cc:474:replace_alu$24.slice[5].carry4_full    53      28      0       #4
$auto$alumacc.cc:474:replace_alu$24.slice[6].carry4_part    53      27      0       #5
$auto$alumacc.cc:474:replace_alu$24.slice[0].carry4_1st_full        53      33      0       #6
out:LD7             9       5       0       #7
clk         42      26      0       #8
$false              35      26      0       #9

A detailed description of the .place file format can be found in the Placement File Format (.place) section.

Routing

This step determines how to connect the placed Complex Blocks together, according to the netlist connectivity and the routing resources of the FPGA chip. The router uses a Routing Resource (RR) Graph [BRM99] to represent the FPGA’s available routing resources. The RR graph can be created in two ways:

  1. Automatically generated by VPR from the FPGA architecture description [BR00], or

  2. Loaded from an external RR graph file.

The output of routing is written into a .route file. The file describes each connection from input to its output through different routing resources of the FPGA. Each net starts with a SOURCE node and ends in a SINK node, potentially passing through complex block input/output pins (IPIN/OPIN nodes) and horizontal/vertical routing wires (CHANX/CHANY nodes). The pair of numbers in round brackets provides information on the (x, y) resource location on the VPR grid. The additional field provides information about the specific node.

An example routing file could look as follows:

 Placement_File: top.place Placement_ID: SHA256:88d45f0bf7999e3f9331cdfd3497d0028be58ffa324a019254c2ae7b4f5bfa7a
 Array size: 149 x 158 logic blocks.

 Routing:

 Net 0 (counter[4])

 Node:       203972  SOURCE (53,32)  Class: 40  Switch: 0
 Node:       204095    OPIN (53,32)  Pin: 40   BLK-TL-SLICEL.CQ[0] Switch: 189
 Node:       1027363  CHANY (52,32)  Track: 165  Switch: 7
 Node:       601704   CHANY (52,32)  Track: 240  Switch: 161
 Node:       955959   CHANY (52,32) to (52,33)  Track: 90  Switch: 130
 Node:       955968   CHANY (52,32)  Track: 238  Switch: 128
 Node:       955976   CHANY (52,32)  Track: 230  Switch: 131
 Node:       601648   CHANY (52,32)  Track: 268  Switch: 7
 Node:       1027319  CHANY (52,32)  Track: 191  Switch: 183
 Node:       203982    IPIN (53,32)  Pin: 1   BLK-TL-SLICEL.A2[0] Switch: 0
 Node:       203933    SINK (53,32)  Class: 1  Switch: -1

Net 1 ($auto$alumacc.cc:474:replace_alu$24.O[6])
...

A detailed description of the .route file format can be found in the Routing File Format (.route) section.

Analysis

This step analyzes the resulting implementation, producing information about:

  • Resource usage (e.g. block types, wiring)

  • Timing (e.g. critical path delays and timing paths)

  • Power (e.g. total power used, power broken down by blocks)

Note that VPR’s analysis can be used independently of VPR’s optimization stages, so long as the appropriate .net/.place/.route files are available.