Previous we generated a "wait for 0 ns" statement after
every blocking assignment that wasn't the last statement
in the process. While this implements the Verilog semantics,
it generates excessive waits, and cannot usually be synthesised.
This patch only generates "wait for 0 ns" statements when it
cannot be avoid (e.g. when the target of a blocking assignment
is read in the same process).
An example:
begin
x = 5;
if (x == 2)
y = 7;
end
Becomes:
x <= 5;
wait for 0 ns; -- Required to implement assignment semantics
if x = 2 then
y <= 7; -- No need for wait here, not read
-- wait for 0 ns (previously)
end if;
Conflicts:
tgt-vhdl/process.cc
tgt-vhdl/stmt.cc
tgt-vhdl/vhdl_target.h
Previous we generated a "wait for 0 ns" statement after
every blocking assignment that wasn't the last statement
in the process. While this implements the Verilog semantics,
it generates excessive waits, and cannot usually be synthesised.
This patch only generates "wait for 0 ns" statements when it
cannot be avoid (e.g. when the target of a blocking assignment
is read in the same process).
An example:
begin
x = 5;
if (x == 2)
y = 7;
end
Becomes:
x <= 5;
wait for 0 ns; -- Required to implement assignment semantics
if x = 2 then
y <= 7; -- No need for wait here, not read
-- wait for 0 ns (previously)
end if;
This patch optimises away straight line sequences like:
wait for 0 ns;
wait for X ns;
to:
wait for X ns;
This tidies up the output a bit.
It also has the effect of removing all code from initial
processes where the assignments have been extracted as
VHDL signal intialisers. (c.f. pr2391337)
Initial processes set a magic flag in the code generator
which allows it to push constant assignments into the
VHDL signal initialisation and omit the assignment.
However, it should only do this if the signal has not
already been read (otherwise the previous read would
not get the undefined value as expected)
Cary R
Nick Gasson
Stephen Williams