The $div and $mod cells use truncating division semantics (rounding
towards 0), as defined by e.g. Verilog. Another rounding mode, flooring
(rounding towards negative infinity), can be used in e.g. VHDL. The
new $divfloor cell provides this flooring division.
This commit also fixes the handling of $div in opt_expr, which was
previously optimized as if it was $divfloor.
The $div and $mod cells use truncating division semantics (rounding
towards 0), as defined by e.g. Verilog. Another rounding mode, flooring
(rounding towards negative infinity), can be used in e.g. VHDL. The
new $modfloor cell provides this flooring modulo (also known as "remainder"
in several languages, but this name is ambiguous).
This commit also fixes the handling of $mod in opt_expr, which was
previously optimized as if it was $modfloor.
Before this commit, memory_map (which is always a part of a synth
script) would always pick up any $mem cell that was not processed
by a preceding pass and lower it down to $dff/$mux cells.
This is undesirable for two reasons:
* If there is an explicit inference attribute set on a $mem cell,
e.g. (* ram_block *), then it is arguably incorrect to map such
a memory to $dff/$mux cells.
* If memory_map tries to lower a memory that was intended to
be mapped to a large BRAM, it often takes extraordinarily long
time to finish, produces an extremely large log file, and outputs
an unusable design.
After this commit, properly invoked memory_map will not map any
memory that has an explicit inference attribute specified, solving
the first issue, and alleviating the second. The default behavior
is not changed.
The semantics of an RTLIL constant that has less bits than its
declared bit width is zero padding. Therefore, if the output of
memory_collect will be used for simulation, truncating 'x from
the end of \INIT will produce incorrect simulation results.