Reduce Yosys ECP5 cell usage by 30% with -abc9 -nowidelut

We've been investigating why the barrel rotator uses an enormous
number of cells on the yosys ECP5 target. Eventually it was narrowed
down to the -abc9 -nowidelut options, which see the cell count go from
4985 cells to 841 cells.

Using the same options on an Orange Crab build reduces the cell count
from 50864 to 36085. The main differences:

     LUT4                        31040 -> 25270
     PFUMX                        6956 ->     0
     L6MUX21                      1746 ->     0
     CCU2C                        2066 ->  1759

Signed-off-by: Anton Blanchard <anton@linux.ibm.com>

diff --git a/Makefile b/Makefile
index f57d33379a05c6e8f8230a3cf2cee597161376ad..e486a29caac5e77760928fd25569d01c8fe50f87 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -196,7 +196,7 @@ fpga_files = fpga/soc_reset.vhdl \
 synth_files = $(core_files) $(soc_files) $(fpga_files) $(clkgen) $(toplevel) $(dmi_dtm)
 
 microwatt.json: $(synth_files) $(RAM_INIT_FILE)
-       $(YOSYS) -m $(GHDLSYNTH) -p "ghdl --std=08 --no-formal $(GHDL_IMAGE_GENERICS) $(synth_files) -e toplevel; synth_ecp5 -json $@  $(SYNTH_ECP5_FLAGS)" $(uart_files)
+       $(YOSYS) -m $(GHDLSYNTH) -p "ghdl --std=08 --no-formal $(GHDL_IMAGE_GENERICS) $(synth_files) -e toplevel; synth_ecp5 -abc9 -nowidelut -json $@  $(SYNTH_ECP5_FLAGS)" $(uart_files)
 
 microwatt.v: $(synth_files) $(RAM_INIT_FILE)
        $(YOSYS) -m $(GHDLSYNTH) -p "ghdl --std=08 --no-formal $(GHDL_IMAGE_GENERICS) $(synth_files) -e toplevel; write_verilog $@"