CODEWITHSUNDEEP
type-conversionvhdlxilinxxilinx-isevivado
Paebbels
Paebbels

Reputation: 16221

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

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:

Upvotes: 10

Views: 25563

Answers (2)

Brian Davis
Brian Davis

Reputation: 23

( 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\

Upvotes: 0

Philippe Aubertin
Philippe Aubertin

Reputation: 1063

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).

Upvotes: 11

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