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target: Add VLT_NUM_THREADS to configure threads used by verilator #148

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target: Add VLT_NUM_THREADS to configure threads used by verilator #148

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target: Add `VLT_NUM_THREADS` to threads used by verilator
zero9178 Jun 5, 2024
dab85ad
address review comments
zero9178 Jun 5, 2024
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7 changes: 7 additions & 0 deletions target/common/common.mk
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Expand Up @@ -44,6 +44,7 @@ EVENTVIS_PY ?= $(UTIL_DIR)/trace/eventvis.py
# a two-liner with the OS on the first line, hence the tail -n1
VERILATOR_ROOT ?= $(dir $(shell $(VERILATOR_SEPP) which verilator | tail -n1))..
VLT_ROOT ?= ${VERILATOR_ROOT}
VLT_NUM_THREADS ?= 1

MATCH_END := '/+incdir+/ s/$$/\/*\/*/'
MATCH_BGN := 's/+incdir+//g'
Expand Down Expand Up @@ -86,7 +87,13 @@ VLT_FLAGS += -Wno-UNSIGNED
VLT_FLAGS += -Wno-UNOPTFLAT
VLT_FLAGS += -Wno-fatal
VLT_FLAGS += --unroll-count 1024
ifneq ($(VLT_NUM_THREADS), 1)
VLT_FLAGS += --threads $(VLT_NUM_THREADS)
endif
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I believe we can drop the if statement, as Verilator should by default behave as if --threads 1 is passed.

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I believe this is true for Verilator 5 but not the verilator 4 that the repo currently supports until #76 lands.
That said I think you're right and we can drop this as this is almost certainly a premature optimization and I doubt single-threaded usage will see a measureable difference.

VLT_CFLAGS += -std=c++14 -pthread
ifneq ($(VLT_NUM_THREADS), 1)
VLT_CFLAGS += -DVL_THREADED=1
endif
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Couldn't find much information on this, but perhaps we can also always add this flag, and remove the if statement. I guess it shouldn't break the single-threaded case.

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Information is sadly very sparse on the flag and I only found it referenced through GitHub issues such as verilator/verilator#3668 (comment)
I had to add it as I was getting compilation errors as verilator requires it to be defined when using multiple threads.
I added a comment to clarify.

VLT_CFLAGS +=-I ${VLT_BUILDDIR} -I $(VLT_ROOT)/include -I $(VLT_ROOT)/include/vltstd -I $(VLT_FESVR)/include -I $(TB_DIR) -I ${MKFILE_DIR}/test

ANNOTATE_FLAGS ?= -q --keep-time --addr2line=$(ADDR2LINE)
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3 changes: 3 additions & 0 deletions target/snitch_cluster/Makefile
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Expand Up @@ -103,6 +103,9 @@ VLT_COBJ += $(VLT_BUILDDIR)/tb/tb_bin.o
VLT_COBJ += $(VLT_BUILDDIR)/vlt/verilated.o
VLT_COBJ += $(VLT_BUILDDIR)/vlt/verilated_dpi.o
VLT_COBJ += $(VLT_BUILDDIR)/vlt/verilated_vcd_c.o
ifneq ($(VLT_NUM_THREADS), 1)
VLT_COBJ += $(VLT_BUILDDIR)/vlt/verilated_threads.o
endif
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# Bootdata
VLT_COBJ += $(VLT_BUILDDIR)/generated/bootdata.o

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