While debugging the handling of user defined physical types in Vivado (read more), I found a different behavior for type conversions from real to integer.
Here is my example code:
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
--use IEEE.MATH_REAL.all;
entity Top_PhysicalTest_Simple is
port (
Clock : in STD_LOGIC;
Input : in STD_LOGIC;
Output : out STD_LOGIC
);
end;
architecture top of Top_PhysicalTest_Simple is
constant int_1 : INTEGER := natural(0.5);
constant int_2 : INTEGER := integer(-0.5);
-- constant int_2 : INTEGER := natural(-0.5);
begin
assert FALSE report "16 - int_1 (natural(0.5)): " & INTEGER'image(int_1) severity note;
assert FALSE report "17 - int_2 (natural(-0.5)): " & INTEGER'image(int_2) severity note;
Output <= Input when rising_edge(Clock);
end;
The dummy flip flop is used to prevent some tools from complaining about an empty design.
XST 14.7:
Elaborating entity <Top_PhysicalTest_Simple> (architecture <top>) from library <work>.
Note: "16 - int_1 (natural(0.5)): 1"
Note: "17 - int_2 (natural(-0.5)): 0"
XST seems to use the mode round up and it handles the type conversion inclusive range check.
So I must use integer(-0.5)
instead of natural(-0.5)
.
Vivado 2014.4:
[Synth 8-63] RTL assertion: "16 - int_1 (natural(0.5)): 1" ["D:/Temp/PhysicalTest_Vivado2014.4/vhdl/Top_PhysicalTest_Simple.vhdl":80]
[Synth 8-63] RTL assertion: "17 - int_2 (natural(-0.5)): -1" ["D:/Temp/PhysicalTest_Vivado2014.4/vhdl/Top_PhysicalTest_Simple.vhdl":81]
Synth seems to use the mode round to infinity and it handles the type conversion without range check. So maybe natural(..)
is just an alias to integer(..)
.
The commented line: constant int_2 : INTEGER := natural(-0.5);
throws no error.
GHDL 0.29:
GHDL 0.29 does no range check in natural(..)
.
I know it's out dated, but since 0.31 hates me I can't tell if this is already fixed.
GHDL 0.31:
I'll present the results later. GHDL refuses to analyse my code because:
Top_PhysicalTest_Simple.vhdl:29:14: file std_logic_1164.v93 has changed and must be reanalysed
My questions:
From IEEE Std 1076-2002 section 7.3.5 "Type conversions"
The conversion of a floating point value to an integer type rounds to the nearest integer; if the value is halfway between two integers, rounding may be up or down.
If you want something else, maybe functions in IEEE.MATH_REAL
can be of some use (notably CEIL
, FLOOR
and/or TRUNC
).