473,705 Members | 3,580 Online
Bytes | Software Development & Data Engineering Community
+ Post

Home Posts Topics Members FAQ

Way to tell if parameter is variable vs constant?

Is there a way to determine if a parameter to a function is a constant
(e.g. 2.0f) versus a variable? Is there some way to determine if this
is the case? (Say some metaprogramming tip or type trait?)

I have a function with an if statement, that I would like to optimize
away somehow. I was hoping the compiler would do it for me, but it
doesn't seem to.

Jay

May 1 '06 #1
7 3168
icosahedron wrote:
Is there a way to determine if a parameter to a function is a constant
(e.g. 2.0f) versus a variable? Is there some way to determine if this
is the case? (Say some metaprogramming tip or type trait?)

I have a function with an if statement, that I would like to optimize
away somehow. I was hoping the compiler would do it for me, but it
doesn't seem to.


Please post the code you want to change. So far your question does not
make sense. If your function is declared so that you may pass a constant
to it, then the argument is passed by value or by a reference to const.
Even if you could determine where the argument originated, how would it
help you? What kind of optimization are you trying to accomplish?

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
May 1 '06 #2

icosahedron wrote:
Is there a way to determine if a parameter to a function is a constant
(e.g. 2.0f) versus a variable? Is there some way to determine if this
is the case? (Say some metaprogramming tip or type trait?)

I have a function with an if statement, that I would like to optimize
away somehow. I was hoping the compiler would do it for me, but it
doesn't seem to.


A compiler does it for you provided the definition of the function is
available in the same translation unit as the call site. I.e. it will
propagate a compile time constant through the call eliminating
conditions and dead code branches.

May 1 '06 #3
icosahedron wrote:
Is there a way to determine if a parameter to a function is a constant
(e.g. 2.0f) versus a variable? Is there some way to determine if this
is the case? (Say some metaprogramming tip or type trait?) I have a function with an if statement, that I would like to optimize
away somehow. I was hoping the compiler would do it for me, but it
doesn't seem to.
Please post the code you want to change. So far your
question does not make sense. If your function is declared
so that you may pass a constant to it, then the argument is
passed by value or by a reference to const. Even if you could determine where the argument originated,
how would it help you? What kind of optimization are you
trying to accomplish?


template <typename Element, int N>
float scale( Element (&array)[N], Element scale, int index )
{
if( index == 0 ) {
return array[ 0 ] * scale;
}
else {
return array[ index ];
}
}
int main( void )
{
float test_array[ 5 ] = { 3.0f, 2.0f, 1.0f, 0.0f, -1.0f };
int i = 0;
cout << scale( test_array, 3.0f, 0 ) << endl;
cout << scale( test_array, 3.0f, i ) << endl;

return 0;
}

Okay, so I would like to optimize the case when a 0 is passed into
scale, so that the if will automatically recognize it's a constant and
return array[index] * scale. When a variable is passed, I can
understand the compiler keeping the if. If a constant is passed, I
would have thought that the if would be optimized away, but that
doesn't seem to be the case.

So, what I would like to do is something similar to using
boost::mpl::if_ c would do with a type trait or something that could
tell if index were a constant passed or a variable.

I know this is probably not doable. I was just asking in the event
there were something I was unaware of.

Thanks,

Jay

May 1 '06 #4
icosahedron wrote:
[...]
template <typename Element, int N>
float scale( Element (&array)[N], Element scale, int index )
{
if( index == 0 ) {
return array[ 0 ] * scale;
}
else {
return array[ index ];
}
}
int main( void )
{
float test_array[ 5 ] = { 3.0f, 2.0f, 1.0f, 0.0f, -1.0f };
int i = 0;
cout << scale( test_array, 3.0f, 0 ) << endl;
cout << scale( test_array, 3.0f, i ) << endl;

return 0;
}

Okay, so I would like to optimize the case when a 0 is passed into
scale, so that the if will automatically recognize it's a constant and
return array[index] * scale. When a variable is passed, I can
understand the compiler keeping the if. If a constant is passed, I
would have thought that the if would be optimized away, but that
doesn't seem to be the case.

So, what I would like to do is something similar to using
boost::mpl::if_ c would do with a type trait or something that could
tell if index were a constant passed or a variable.

I know this is probably not doable. I was just asking in the event
there were something I was unaware of.


How about overloading:

template <typename Element, int N>
float scale( Element (&array)[N], Element scale, int& index)
{
if ( index == 0 ) {
return array[ 0 ] * scale;
}
else {
return array[ index ];
}
}

template <typename Element, int N>
float scale( Element (&array)[N], Element scale, int*)
{
return array[ 0 ] * scale;
}

If you pass explicit 0 (a compile-time constant), then the compiler
should be unable to bind it to a reference to non-const, but should
be able to convert it into a pointer to int. Now, if you somehow
would want to also pass 1 or 2 or 666, it won't work.
V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
May 1 '06 #5

icosahedron wrote:

[]
template <typename Element, int N>
float scale( Element (&array)[N], Element scale, int index )
{
if( index == 0 ) {
return array[ 0 ] * scale;
}
else {
return array[ index ];
}
}
int main( void )
{
float test_array[ 5 ] = { 3.0f, 2.0f, 1.0f, 0.0f, -1.0f };
int i = 0;
cout << scale( test_array, 3.0f, 0 ) << endl;
cout << scale( test_array, 3.0f, i ) << endl;

return 0;
}

Okay, so I would like to optimize the case when a 0 is passed into
scale, so that the if will automatically recognize it's a constant and
return array[index] * scale. When a variable is passed, I can
understand the compiler keeping the if. If a constant is passed, I
would have thought that the if would be optimized away, but that
doesn't seem to be the case.


It is the case. Compile the code with a high optimization level and
check the assembly language output.

May 2 '06 #6
>> Okay, so I would like to optimize the case when a 0 is passed into
scale, so that the if will automatically recognize it's a constant and
return array[index] * scale. When a variable is passed, I can
understand the compiler keeping the if. If a constant is passed, I
would have thought that the if would be optimized away, but that
doesn't seem to be the case.
It is the case. Compile the code with a high optimization level and
check the assembly language output.


Yes, it does indeed appear that way. Thanks for your reply.

May 22 '06 #7

icosahedron wrote:
template <typename Element, int N>
float scale( Element (&array)[N], Element scale, int index )
{
if( index == 0 ) {
return array[ 0 ] * scale;
}
else {
return array[ index ];
}
}


Shouldnt you add inline to function signature, to get earlier
optimisation?

regards
Andy Little

May 23 '06 #8

This thread has been closed and replies have been disabled. Please start a new discussion.

Similar topics

1
2224
by: Michael Kochendoerfer | last post by:
Hi, I'm new to PHP and have a small problem. Assume you define a function that is called with a constant as parameter. Within that function, based on some condition it calls itself recursively. Example: <?php function Test($firstcall) { echo "(enter) firstcall=$firstcall<br>\n";
4
3121
by: ulysses | last post by:
Hi, I use PyQt 3.8 non-commercial version in win32. I get a big question. I Can't show PY variable in QT filedialog as initially parameter. Code sample is following: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ fileName="test.avi" def saveSomething(aString):
7
1443
by: Richard Cavell | last post by:
Hi, The point of using const on a parameter to a function should be to let your compiler know that the parameter shouldn't be modified during your program. This allows you to keep your code safe and bug-free. Now, it also occurs to me that a const something-or-other could be passed as a reference (since it's guaranteed not to change) , or that the address of the object could be passed rather than the whole thing in the case of a...
8
3151
by: Tony Johansson | last post by:
Hello Experts! What does this mean actually. If you have a template with a type and non-type template argument, say, like this template<typename T, int a> class Array {. . .}; then A<int, 1> and A<int, 2> are different types. Now, if the A template
2
4625
by: systemutvecklare | last post by:
Hi! I have an application that generates an html-form from an xml-file using an xsl-file. My problem is that I want the xsl to use some "unknown" parameters that I pass to the xslt processor before processing the xml. The parameters are not totally unknown but they are not static, they are built by an attribute in the xml and a constant name. Is it possible to define a "runtime" parameter and use the value passed
4
4937
by: moondaddy | last post by:
I'm passing a parameter in the url when I open up a particular page. When the page loads and finds this parameter, I know that I need to clear a variable out of the session cache and reset it with a new value. On the following postbacks I want this variable to remain constant which means I need to clear the parameter out of the URL before any of the postbacks occur. Otherwise if this parameter is still in the URL when the page loads...
7
1142
by: dwg1011 | last post by:
I have a global class GClass with 2 contants. Both are declared public in the form: PUBLIC Const Const1 = "xxx". When I try to use the constant as a default value (see code excerpt below) the program bombs, but when I use a hardcoded value it works fine. Any ideas? Failing Version: <SelectParameters>
16
3160
by: hzmonte | last post by:
Correct me if I am wrong, declaring formal parameters of functions as const, if they should not be/is not changed, has 2 benefits; 1. It tells the program that calls this function that the parameter will not be changed - so don't worry. 2. It tells the implementor and the maintainer of this function that the parameter should not be changed inside the function. And it is for this reason that some people advocate that it is a good idea to...
5
3622
by: Dennis | last post by:
Hi I'm trying to alter my stored procedure to take a parameter for the Database Name, but as usual the syntax is killing me. Thanks for any help Dennis '--------------------------------------------------------------------------­-------------------------------- Before - This Works without a paramater '--------------------------------------------------------------------------­--------------------------------
0
9273
Oralloy
by: Oralloy | last post by:
Hello folks, I am unable to find appropriate documentation on the type promotion of bit-fields when using the generalised comparison operator "<=>". The problem is that using the GNU compilers, it seems that the internal comparison operator "<=>" tries to promote arguments from unsigned to signed. This is as boiled down as I can make it. Here is my compilation command: g++-12 -std=c++20 -Wnarrowing bit_field.cpp Here is the code in...
0
9136
jinu1996
by: jinu1996 | last post by:
In today's digital age, having a compelling online presence is paramount for businesses aiming to thrive in a competitive landscape. At the heart of this digital strategy lies an intricately woven tapestry of website design and digital marketing. It's not merely about having a website; it's about crafting an immersive digital experience that captivates audiences and drives business growth. The Art of Business Website Design Your website is...
0
8979
tracyyun
by: tracyyun | last post by:
Dear forum friends, With the development of smart home technology, a variety of wireless communication protocols have appeared on the market, such as Zigbee, Z-Wave, Wi-Fi, Bluetooth, etc. Each protocol has its own unique characteristics and advantages, but as a user who is planning to build a smart home system, I am a bit confused by the choice of these technologies. I'm particularly interested in Zigbee because I've heard it does some...
1
6605
isladogs
by: isladogs | last post by:
The next Access Europe User Group meeting will be on Wednesday 1 May 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM). In this session, we are pleased to welcome a new presenter, Adolph Dupré who will be discussing some powerful techniques for using class modules. He will explain when you may want to use classes instead of User Defined Types (UDT). For example, to manage the data in unbound forms. Adolph will...
0
4439
by: TSSRALBI | last post by:
Hello I'm a network technician in training and I need your help. I am currently learning how to create and manage the different types of VPNs and I have a question about LAN-to-LAN VPNs. The last exercise I practiced was to create a LAN-to-LAN VPN between two Pfsense firewalls, by using IPSEC protocols. I succeeded, with both firewalls in the same network. But I'm wondering if it's possible to do the same thing, with 2 Pfsense firewalls...
0
4703
by: adsilva | last post by:
A Windows Forms form does not have the event Unload, like VB6. What one acts like?
1
3137
by: 6302768590 | last post by:
Hai team i want code for transfer the data from one system to another through IP address by using C# our system has to for every 5mins then we have to update the data what the data is updated we have to send another system
2
2486
muto222
by: muto222 | last post by:
How can i add a mobile payment intergratation into php mysql website.
3
2081
bsmnconsultancy
by: bsmnconsultancy | last post by:
In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence can significantly impact your brand's success. BSMN Consultancy, a leader in Website Development in Toronto offers valuable insights into creating effective websites that not only look great but also perform exceptionally well. In this comprehensive...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.