Scope-of-defines
How VL and other tools handle the scope of `defines.
What is the scope of a `define?
Until the end of 2009, we treated `defines as having file-scope, and
processed every file using only the initial defines to begin with. But
now we are treating `defines as cumulative, allowing them to spill over
from one file into the next. This is convenient in that it allows us to see
what defines have been encountered, and hence we can programmatically extract
the values associated with particular `define symbols.
This is scary. The order of file loading is not especially predictable when
vl-flush-out-descriptions is used, so different things might happen
depending on which files happen to get loaded first!
Well, our behavior appears to be at least similar to what other Verilog
tools do. We found that, on both Verilog-XL and NCVerilog,
- `defines that occur before an `include do seem to carry over into
the included file.
- When we give the tool multiple files, e.g., verilog foo.v bar.v or
ncverilog foo.v bar.v, the includes from the earlier files do carry over
to the later files. Hence, switching the argument order can affect simulation
results.
- `defines that occur in the main files (those that are given as
explicit command line arguments) are indeed carried over into library
files
- `defines from earlier-loaded library files are carried over into
later-loaded library files.
Our behavior is approximately the same. However, it seems very
likely that our particular way of loading missing descriptions with vl-flush-out-descriptions will be at least somewhat different from how other tools
look for missing descriptions. Roughly, after parsing the main files, we figure out
what descriptions are missing and try to load them in alphabetical order. Other
tools probably use different orders and may produce very different
behaviors.
BOZO we could probably defend against this by tracking which ifdef
tests have ever been considered and what their values are. If we find that two
files have ever done an ifdef and gotten different results, we could add a
warning that maybe something is amiss with file loading.
