I am wondering why ballotARB(false) returns always 0 and not return the bitfield where the corresponding bits are set for all active invocations in the sub-group.
Because of that, I do not see how can I use ballotARB fonction in my program.
Is my understanding of this function false?
What should ballotARB(false) return, if not 0?
It does return the count of the true votes in the local subgroup. Since all your invocations vote false no matter what, the result must be 0. This is even explicitely stated in the ARB_shader_ballot extension spec:
The following trivial assumptions can be made:
ballotARB(true) returns bitfield where the corresponding bits are set for all active invocations in the sub-group.
ballotARB(false) returns zero.
So to make use of the votes, you should use it on an experession which is actually not dynamically uniform.
Related
Updates:
The reason I was getting true is because anything that is other than 0 would be considered true which obviously makes sense to how unlikely it would have been for me to get false when uninitialized.
I have read a post similar to my question on StackOverflow, it talked about that it is good practise to initialize all member variables, which I agree with.
Post: Boolean variables aren't always false by default?
This post is a bit old (9 years ago) so I just think maybe somethings might have been changed in the new C++ versions, I am currently using C++17. I also have one slight different question from the ones talked about in the post.
I am aware that if a variable is uninitialized it may contain some "garbage data" or as one of the answers in the post said (which I think that is what they meant but I'm not 100% sure), "if not explicitly initialized -- will contain an arbitrary value.".
I have tried testing that, and the results showed that when I didn't initialize my variables, they contained random numbers (for int, double). I also tested std::string but they are set to "" by default from what I saw.
Anyways, now when I tried for the built in type bool I would always get true (after class is constructed, but again that boolean value never initialized, I would go into debug and see that the value would be true), what I am confused is that no matter how many times I tried to test if that was just a random value out of true and false, and if sometimes it would be false, it was always set to true. If uninitialized shouldn't the value be "random" kind of? Why did it always set to true (again when a member variable of my class which wasn't initialized on construction).
Solutions I tried:
Obviously one is just to initialize on construction, but I thought of another one...
What if on construction I wanted it to be true and not false but when it hasn't been constructed to be set to false then that way when I have a vector of pointers to my object I can check that I am not reading uninitialized objects when following the pointer by checking whether that boolean is set to true (if initialized) or false otherwise. I wouldn't be able to use method 1 which was to initialize on construction to false, also because if we are reversing the behaviour I can't rely on what that uninitialized boolean member variable would be as I mentioned in the above paragraphs I am unsure what behaviour that has due to the results I had been getting. I did the following and it worked...
class Testing{
private:
bool condition{false} // Initalize it here which kind of makes me confused but it works
public:
Testing() : condition{true} {} // Constructor setting the condition value to true
};
Could someone explain if it is wrong to do this, personally I have never seen someone do this but I tried it and no errors were given.
While bool conceptually contains only one bit of information, the requirements of the C++ standard mean that a bool object must take up at least eight bits. There's three main ways that a compiler might represent bool at a bitwise level:
All bits zeroed for false; all bits one'd for true (0x00 versus 0xFF)
All bits zeroed for false; the lowest bit one'd for true (0x00 versus 0x01)
All bits zeroed for false; at least one bit one'd for true(0x00 versus anything else)
(Note that this choice of representation is not ordinarily visible in the effects of a program. Regardless of how the bits are represented, a bool becomes a 0 or 1 when casted to a wider integer type. It's only relevant to the machine code being generated.)
In practice, modern x86/x64 compilers go with option 2. There are instructions which make this straightforward, it makes casting bool to int trivial, and comparing bools works without additional effort.
A side effect is that if the bits making up a bool end up set to, say, 0x37, weird stuff can happen, because the executable code isn't expecting that. For instance, both branches of an if-statement might be taken. A good debugger should loudly yell at you when it sees a bool with an unexpected bit pattern, but in practice they tend to show the value as true.
The common theme of all those options is that most random bit patterns are not the bit pattern for false. So if the allocator really did set it to a "random" value, it almost certainly would be shown as true in the debugger.
I am using Arena simulation software. I need to define "if" in the expression section of the Process Module. I could not find any instruction to define "ifelse" function.
I never found IF statement in Arena simulation.
But don't worry. There is workaround with using expressions.
According to official Documentation:
Logical Expression Evaluation
ASSIGN: InventoryLevel = 50 *
(SystemStatus==Early) +
30 * (SystemStatus==Late);
Assign InventoryLevel a value of 50 if the variable SystemStatus is
equal to Early (logical expressions evaluate to 1 for TRUE, 0 for
FALSE). If SystemStatus equals Late, assign InventoryLevel to 30. If
neither is true, assign SystemStatus equal to 0. This single ASSIGN
block may have been used to replace a BRANCH block (to check the value
of SystemStatus) and two ASSIGN blocks (to assign the correct value).
Is there a way to determine the minimum and maximum value I can pass to a compiler option. E.g.:
-fconstexpr-depth=n
or
-falign-jumps[=n]
What are the min and max values for n? Or even better would be to know what the whole value range is with all intermediate values.
I do know that this can and will depend on the code I want to compile. But I guess for a few compiler options the maximum and minimum input values can be determined independent of the code to compile.
Lets assume you are asking about GCC (it follows from tags).
fconstexpr-depth option defined in gcc/c-family/c.opt this way:
fconstexpr-depth=
C++ ObjC++ Joined RejectNegative UInteger Var(max_constexpr_depth) Init(512)
-fconstexpr-depth=<number> Specify maximum constexpr recursion depth.
What you can see immediately: GCC option description has no explicit value limits. Just default value (512) and max_constexpr_depth variable to which this option value tied up. Lets look it up in source code...
static bool
push_cx_call_context (tree call)
{
..... some code .....
if (call_stack.length () > (unsigned) max_constexpr_depth)
return false;
return true;
}
As you see, this variable is being used without any limits checking. So correct answer: there are NO LIMITS at all. You may pass 5000 or 5000000 nobody cares, everybody assumes that you know what you are doing.
And of course, having no limits, compiler have no way to report you them.
Many times people will combine a boolean check by just re-using an int variable they already have and checking for -1 if something exists or not.
However, what if someone wants to use unsigned integers but still wants to use this method and also where 0 actually has a different meaning besides existance.
Is there a way to have a data range be -1 to 4,294,967,294?
The obvious choice here is to just use a bool that detects what you are after but it is my understanding that a bool is a byte, and can really add to the storage size if you have an array of structs. This is why I wondered if there was a way to get the most useful numbers you can (postivies) all while leaving just one number to act as a flag.
Infact, if it is possible to do something like shifting the data range of a data type, it would seem like shifting it to something like -10 to 4,294,967,285 would allow you to have 10 boolean flags at no additional cost (bits).
The obvious hacky method here is just to add whatever number to what your storing and remember to account for it later on, but I wanted to keep it a bit more readable (I guess if thats the case I shouldnt even be using -1, but meh).
If you simply want to pick a value which can not exist in your interpretation of the variable and to use it to indicate an exception or error value, why not to simply do it? You can take such a value, define it as a macro and use it. For example if you are sure that your variable never reaches the max limit, put:
#define MY_FUN_ERROR_VALUE (UINT_MAX)
then you can use it as:
unsigned r = my_function_maybe_returning_error();
if (r == MY_FUN_ERROR_VALUE) {handle error}
you shall also ensure that my_function_maybe_returning_error does not return MY_FUN_ERROR_VALUE in normal conditions when actually no error happens. For this you may use an assert:
unsigned my_function_maybe_returning_error() {
...
// branch going to return normal (not error) value r
assert(r != MY_FUN_ERROR_VALUE);
return(r);
}
I do not see anything wrong on this.
You just asked how to use a value that can be 0 or something greater than 0 to hold the three states: whatever 0 means, something greater than 0, and does not exist. So no, (by the pigeonhole principle I guess) it's not possible.
Nor should it be. Overloading a variable is bad practice unless you're down to your last 3 bytes left of RAM, which you almost certainly aren't. So yes, please use another variable with a correct name and clear purpose.
My program requires several floats to be set to a default number when the program launches. As the program runs these integers will be set to their true values. These true values however can be any real number. My program will be consistently be checking these numbers to see if their value has been changed from the default.
For example lets say I have integers A,B,C. All these integers will be set to a default value at the start (lets say -1). Then as the program progresses, lets say A and B are set to 3 and 2 respectfully. Since C is still at the default value, the program can conclude than C hasn't been assigned a non-default value yet.
The problem arises when trying to find a unique default value. Since the values of the numbers can be set to anything, if the value its set to is identical to the default value, my program won't know if a float still has the default value or its true value is just identical to the default value.
I considered NULL as a default value, but NULL is equal to 0 in C++, leading to the same problem!
I could create a whole object consisting of an bool and a float as members, where the bool indicates whether the float has been assigned its own value yet or not. This however seems like an overkill. Is there a default value I can set my floats to such that the value isn't identical to any other value? (Examples include infinity or i)
I am asking for C/C++ solutions.
I could create a whole object consisting of an bool and a integer as
members, where the bool indicates whether the number has been assigned
its own value yet or not. This however seems like an overkill.
What you described is called a "nullable type" in .NET. A C++ implementation is boost::optional:
boost::optional<int> A;
if (A)
do_something(*A);
On a two's complement machine there's an integer value that is less useful than the others: INT_MIN. You can't make a valid positive value by negating it. Since it's the least useful value in the integer range, it makes a good choice for a marker value. It also has an easily recognizable hex value, 0x80000000.
There is no bit pattern you can assign to an int that isn't an actual int. You need to keep separate flags if you really have no integer values that are out of bounds.
If the domain of valid int values is unlimited, the only choice is a management bit indicating whether it is assigned or not.
But, are you sure MAX_INT is a desired choice?
There is no way to guarantee that a value you assign an int to is not going to be equal to another random int. The only way to assure that what you want to happen occurs, is to create a separate bool to account for changes.
No, you will have to create your own data type which contains the information about whether it has been assigned or not.
If as you say, no integer value is off limits, then you cannot assign a default "uninitialised" value. Just use a struct with an int and a bool as you suggest in your question.
I could create a whole object consisting of an bool and a integer as
members, where the bool indicates whether the number has been assigned
its own value yet or not. This however seems like an overkill.
My first guess would be to effectively use a flag and mark each variable. But this is not your only choice of course.
You can use pointers (which can be NULL) and assign dynamically the memory. Not very convenient.
You can pick a custom value which is almost never used. You can then define this value to be the default value. Ofc, some time, you will need to assign this value to your floats, but this case won't happen often and you just need to keep track of this variables. Given the occurrence of such case, a simple linked list should do.