Original Redmine Comment
Author Name: Wei Song
Original Date: 2015-09-08T11:38:08Z

Typo:

I did NOT define any "pure" key word when declaring DPI-C.

Original Redmine Comment
Author Name: Jie Xu (@jiexu)
Original Date: 2015-09-09T07:09:29Z

This is not a scheduling problem. Instead the issue is introduced during optimization. Verilator will try to simplify

IF(valid) ready = dpi_function; else ready = 1'b0

into

ready = valid ? dpi_function : 1'b0

And then in your code since @Valid == 1'b0@ Verilator further simplify the code into

ready = valid & dpi_function

The second optimization is the issue here. In C++, the @&@ operator is actually eager operator [[ https://en.wikipedia.org/wiki/Short-circuit_evaluation]] which means the @dpi_function@ will be always called.

Original Redmine Comment
Author Name: Wei Song
Original Date: 2015-09-09T08:02:35Z

Thanks for the rewording. Surely I am not fully aware of the process of synthesis.
I do understand the reason to do this optimization when possible.
However, it is difficult to see whether a DPI function is pure or not.

I would suggest Verilator treats all DPI functions as impure unless a "pure" key word is defined. The SystemVerilog does define this key word to indicate the DPI function is pure, isn't it?

Hopefully it is not a big trouble.

Original Redmine Comment
Author Name: Wei Song
Original Date: 2015-09-09T08:07:06Z

Actually, if

ready = valid & dpi_function

really follows the short cut rule, when valid is 0, dpi_function() does not need to be evaluated?

Original Redmine Comment
Author Name: Jie Xu (@jiexu)
Original Date: 2015-09-09T08:14:10Z

@&@ is an eager operator instead of short-circuit operator in C++. @&&@ is a short-circuit operator.

Original Redmine Comment
Author Name: Wilson Snyder (@wsnyder)
Original Date: 2015-09-18T23:08:34Z

Fixed in git towards 3.877.

Note that the short-circuit operator handing e.g. &&, still does not generally work, so do not rely on this. However if to conditional was intended to be safe.

Original Redmine Comment
Author Name: Wilson Snyder (@wsnyder)
Original Date: 2015-11-01T13:23:11Z

In 3.878.