Type conversion in VHDL: real to integer - Is the rounding mode specified? - type-conversion

Question

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:
Does VHDL define a rounding mode? And if so which one?
How should I handle rounding if no mode is defined?

Answer 1

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).

Answer 2

( posting this as an answer because I can't post a comment inline... )
Here are the results using the prebuilt ghdl-0.31-mcode-win32 :
C:\brian\jobs\ghdl_test\paebbels>md work.ghd
C:\brian\jobs\ghdl_test\paebbels>ghdl -a --workdir=work.ghd Top_PhysicalTest_Simple.vhd
C:\brian\jobs\ghdl_test\paebbels>ghdl -r --workdir=work.ghd Top_PhysicalTest_Simple
Top_PhysicalTest_Simple.vhd:18:3:#0ms:(assertion note): 16 - int_1 (natural(0.5)): 1
Top_PhysicalTest_Simple.vhd:19:3:#0ms:(assertion note): 17 - int_2 (natural(-0.5)): -1
"0.31 is my Windows machine (mcode version)"
"GHDL refuses to analyse my code "
If you're having trouble with libraries on the Windows mcode build of 0.31, try uninstalling any 0.29 or earlier NSIS-installer versions of GHDL on that machine.
Also make sure you ran the through the whole setup process as described in the 0.31 Windows INSTALL, particularly reanalyze_libraries.bat
Here's the version used for the above test:
C:\brian\jobs\ghdl_test\paebbels>ghdl -v
GHDL 0.31 (20140108) [Dunoon edition] + ghdl-0.31-mcode-win32.patch
Compiled with GNAT Version: GPL 2013 (20130314)
mcode code generator
Written by Tristan Gingold.
Copyright (C) 2003 - 2014 Tristan Gingold.
GHDL is free software, covered by the GNU General Public License. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
And the library path info :
C:\brian\jobs\ghdl_test\paebbels>ghdl --dispconfig
command line prefix (--PREFIX): (not set)
environment prefix (GHDL_PREFIX): C:\Ghdl\ghdl-0.31-mcode-win32\lib
default prefix: C:\Ghdl\ghdl-0.31-mcode-win32\lib
actual prefix: C:\Ghdl\ghdl-0.31-mcode-win32\lib
command_name: C:\Ghdl\ghdl-0.31-mcode-win32\bin\ghdl.exe
default library pathes:
C:\Ghdl\ghdl-0.31-mcode-win32\lib\v93\std\
C:\Ghdl\ghdl-0.31-mcode-win32\lib\v93\ieee\