`--Mdir` does not play well with user c/cpp files

[FanShupei]

• Issue explained

It's best explained by running the following example. The command and error message is given below. The key point is that user c file test.c and a "complicated" Mdir option co-exists. --Mdir obj_dir or --Mdir obj_dir_other is OK, but --Mdir build/obj_dir is not OK.

$ verilator --cc --build test.sv dpi.c -Mdir build/obj_dir
make: Entering directory '/home/fsp/my/test/build/obj_dir'
/usr/sbin/python3 /usr/share/verilator/bin/verilator_includer -DVL_INCLUDE_OPT=include Vtest.cpp Vtest___024root__DepSet_h9b2d61ff__0.cpp Vtest___024root__DepSet_hc07518e4__0.cpp Vtest___024root__Slow.cpp Vtest___024root__DepSet_hc07518e4__0__Slow.cpp Vtest__Syms.cpp > Vtest__ALL.cpp
g++ -Os  -I.  -MMD -I/usr/share/verilator/include -I/usr/share/verilator/include/vltstd -DVM_COVERAGE=0 -DVM_SC=0 -DVM_TRACE=0 -DVM_TRACE_FST=0 -DVM_TRACE_VCD=0 -faligned-new -fcf-protection=none -Wno-bool-operation -Wno-overloaded-virtual -Wno-shadow -Wno-sign-compare -Wno-uninitialized -Wno-unused-but-set-parameter -Wno-unused-but-set-variable -Wno-unused-parameter -Wno-unused-variable      -c -o Vtest__ALL.o Vtest__ALL.cpp
make: *** No rule to make target 'dpi.c', needed by 'dpi.o'.  Stop.
make: Leaving directory '/home/fsp/my/test/build/obj_dir'
%Error: make -C build/obj_dir -f Vtest.mk -j 1 exited with 2
%Error: Command Failed ulimit -s unlimited 2>/dev/null; exec /usr/bin/verilator_bin --cc --build test.sv dpi.c -Mdir build/obj_dir

This is a Makefile issue, the contents of the two files are irrelevant. A minimal test.sv like below and an empty dpi.c is sufficient

module top;
    initial $display("hello");
endmodule

• Investigation

It seems Verilator assumes --Mdir must specify a direct child of current directory. VPATH=.. is hardcoded in verilated.mk. When it is not the case, e.g. given "--Mdir build/obj_dir", Makefile does not know where to find user c/cpp files.

https://github.com/verilator/verilator/blob/dace2c0a9a8f9d0559c4e4cbdb56ebdcae93b097/include/verilated.mk.in#L94

• Verilator version

Verilator 5.026 2024-06-15 rev UNKNOWN.REV on ArchLinux, reproduced on master branch.

• Proposed fix

We may remove the line VPATH=.. in verilated.mk. Instead, we can record the absolute path of working directory of verilator in Vprefix.mk. As an example,

# contents of Vtest.mk
...
# Include list of all generated classes
include Vtest_classes.mk
# Include global rules
include $(VERILATOR_ROOT)/include/verilated.mk
VPATH += /home/fsp/my/test # absolute path of where verilator runs
...

Any comments on this strategy?