routley@tle.enet.dec.com (10/30/90)
I have submitted the following TC++ bug report to Borland.
Kevin Routley - Digital Equipment Corporation
routley@tle.enet.dec.com
Product: Turbo C++
Version: 1.00
OS: MS-DOS V3.2
1. Bitfield Operation Performance
Bitfield operations are not optimal. See Example 1
for an example program.
Testing the first bit of a multi-bit bitfield struc-
ture is done in 20 (8086) cycles with the TEST
instruction:
TEST WORD PTR [BP-06], 0001
The third bit in a multi-bit bitfield structure is
testing by performing a pair of shift instructions,
requiring 25 cycles:
MOV AX, WORD PTR [BP-06]
SHR AX, 1
SHR AX, 1
TEST AX, 0001
Larger bitfield structures use the multiple shift
instruction to test higher bits. For example, the
eighth bit requires seven shifts and 61 cycles to
test:
MOV AX, WORD PTR [BP-06]
MOV CL, 7
SHR AX, CL
TEST AX, 0001
This poor optimization places severe penalties on
those programs that might choose to use bitfields
to store flags instead of char objects. As far as
space is concerned, it is more efficient to use a
bitfield structure for three or more boolean flags
rather than chars.
The bitwise & operator illustrates that all three
tests above could have been performed with individ-
ual TEST instructions (a total of 63 cycles compared
to the 106 cycles required by the above tests):
TEST WORD PTR [BP-06], 0001
TEST WORD PTR [BP-06], 0004
TEST WORD PTR [BP-06], 0080
In fact, a series of similar (& or |) operations on
bitfield structure elements could be optimized to a
single TEST instruction.
Example 1: Example Bitfield Program
_______________________________________________________
#include <stdio.h>
main()
{
struct foo
{
unsigned flag1 : 1;
unsigned flag2 : 1;
unsigned flag3 : 1;
unsigned flag4 : 1;
unsigned flag5 : 1;
unsigned flag6 : 1;
unsigned flag7 : 1;
unsigned flag8 : 1;
} flags;
char lflag1 = 0;
char lflag2 = 1;
char lflag3 = 0;
char lflag4 = 1;
flags.flag1 = flags.flag8 = 1;
if ( flags.flag1 &&
flags.flag8 &&
lflag2 &&
lflag4 )
printf("True\n");
else
printf("False\n");
____}__________________________________________________
2. Macro Expansion Limit
The limitation of 4096 characters for a macro ex-
pansion is too small for non-toy programs. This can
prevent Turbo C++ from being used as a development
environment for large programs. Ideally a macro
expansion should only be limited by the compiling
machine's memory (if that, using memory swapping).
As an example, multiple data structures can be
easily and conveniently initialized from a single
instance of the entered information (illustrated
in Example 2. If table_data contains more than a
few lines with more than a couple of items, and the
author uses spaces to insure readibility, you can
easily exceed the 4096 character limit.
Example 2: Struct Init. from a Single Set of Data
_______________________________________________________
/* setup the data to init all the structures with
*/
#define table_data \
X( "text1", 0, CONST1, 1, 1, 0, struct1, 1 )\
X( "text1234", 0, CONST1234, 1, 1, 0, struct2, 1 )\
. . .
X( "text99", 0, CONST99, 0, 0, 0, structure99, 0 )
/* setup the macro to init table1. Only some of the fields
are used.
*/
#define X (string, foo, bar, fum, foobar, dataptr, flag )\
{ foo, bar, fum, flag }
/* table1 is an array of structs, each with four int fields
*/
table1[MAX_TABLE] = { table_data };
#undef X
/* setup a different macro to init table2. Different fields
are used this time.
*/
#define X (string, foo, bar, fum, foobar, dataptr, flag )\
{ flag, string }
table2[MAX_TABLE] = { table_data };
#undef_X_______________________________________________
3. TC.EXE Display Customization
I am unable to customize the colors of the inte-
grated environment so that they display reasonably
on my monochrome VGA monitor.
TCINST has an option "Mode for Display". No matter
which option I select (e.g. Black and White, or
Monochrome) to customize TC.EXE to have, I always
get the same default color display mode when I
invoke TC.EXE. The default mode is very difficult to
read on my monochrome display.Ralf.Brown@B.GP.CS.CMU.EDU (10/30/90)
In article <16797@shlump.nac.dec.com>, routley@tle.enet.dec.com wrote: } 1. Bitfield Operation Performance } } The third bit in a multi-bit bitfield structure is } testing by performing a pair of shift instructions, } requiring 25 cycles: } } MOV AX, WORD PTR [BP-06] } SHR AX, 1 } SHR AX, 1 } TEST AX, 0001 I just checked, and TC 2.00 does the same thing. -- UUCP: {ucbvax,harvard}!cs.cmu.edu!ralf -=- 412-268-3053 (school) -=- FAX: ask ARPA: ralf@cs.cmu.edu BIT: ralf%cs.cmu.edu@CMUCCVMA FIDO: 1:129/3.1 Disclaimer? | I was gratified to be able to answer promptly, and I did. What's that? | I said I didn't know. --Mark Twain