[vhdl-200x] Performance Proposals

Hi,

While we are waiting for more details on Perf 1 to 7, I would like to make
some comments on them:

Perf 1: Removal of simulation deltas

If this means removal of simulation deltas from existing code, I would be
against it on the grounds that it could change the simulation results. If
this means a new kind of process to execute during the update phase, I would
support a copy only to an unresolved signal.  This would allow an ungated
clock tree to have no delta cycles. If a general expression is allowed,
multiple changes can cause multiple executions yielding different results.

Perf 2: Expressions in the sensitivity list

The question here is: when is the expression evaluated? If it is evaluated
at the update phase, then some of the signals may not yet have been updated.
If it is at the evaluation phase, then it is just like running a process.
The only worthwhile case is a posedge or negedge on a scalar signal, which
can be correctly evaluated at the update phase.

Perf 3: Define 2 and 4 state semantics

Since the language allows 2 state and 4 state signals to be defined, it
seems unnecessary to change the language.  Of course a tool can attempt to
optimise std_logic into fewer states, at the risk of getting the wrong
results!

Perf 4: Light-weight signals

Signals without resolution functions can only have a single driver.  I do
not understand the proposal.

Perf 5: Signal atomicity

I do not understand the effect of this enhancement, and its purpose.

Perf 6: Removal of constructs indicated in VHDL 2002

I would want to keep postponed processes.

Perf 7: Zero-delay ordering of signals

It is unclear what algorithm is required here.  It must cope with multiple
clock domains and asynchronous circuits. 

Regards,

Peter Flake


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