Also, this worked in Visual Studio 2008... I will assume it works in 2005 as well but anyone who can testify to this is encouraged to let me know.
Anyway, I have googled and searched gamedev occasionally to find a way to get cmov instructions consistently, but have not found a single answer that works every time. Closest is "use ? :". After doing so, I've still had problems getting it to EVER happen. Anyway, I took another stab and here's my results for anyone interested.
As far as project configuration options go, SSE has to be enabled, and any form of the overall Optimizations need to be turned on (/O1 /O2 /Ox). Whether you favor size, speed, or neight (/Ot, /Os, none) is irrelavent. Also, this is for code generated for x86, and is only dependable with 32 bit integers, although sometimes works with __int64's for some reason.
Essentially, it will generate a cmov with a single conditional and 2 variables, "X = Cond ? X : Y" or "X = Cond ? Y : X". Expressions and constants of any kind will NOT work in place of the variables, but the conditional can be anything, as long is it is only 1 conditional.
Another stipulation I found, is that the compiler needs to think the variable ALREADY has something assigned to it, even if that assignment was undefined.
For an example:
const int ConstVar; // int ConstVar = 5 would be equivalent if it doesn't change.
int Condition;
int Var;
int InitVal;
int Result = InitVal; // If this was just 'int Result;' than it wouldn't always work
Result = Condition ? Result : Var; // Works every time
if (Condition) Result = Var; // Also works every time
Result = Condition ? InitVal : Var; // Only sometimes (suprisingly)
Result = Condition ? Result : ConstVar; // Doesn't! Usually turns into a jump!
Result = (Condition && Condition2) ? Result : Var; // Doesn't! Only 1 conditional seems to work.
Result = Condition ? Result : 10; // Just like a const var of course
Result = Condition ? Var : ConstVar; // Nope!
Result = Condition ? Result : Var / 5; // Nope!
So a conditional statement, containing only 1 comparison (an expression like 'Var1 || Var2' is 2 comparisons of course where 'Var1 | Var2 / 10 + (Var3 - 6) > Var2 * Var3' isn't), followed by assigning a single variable (not an expression!) to a single result variable, and the original variable to itself otherwise, results in a cmov every time.
Anything else might work, but I have found it to be either inconsistent, or not produce it at all.
Also, arrays are a bit trickier.
// Works consistently
for (int Index = 0; Index < 10; Index++) {
long In1 = rand();
long SomeResult = rand();
long Out = OutBuffer[Index]; // OutBuffer doesn't have to be volatile
Out = In1 < InBuffer[Index] ? Out : SomeResult;
OutBuffer[Index] = Out; // Notice we did not use an array directly in the resulting assignment
}
You CANNOT use an array in the assignment at the end, directly. If SomeResult, in this case, was assigned an array entry called InBufferTwo[Index], InBufferTwo would have to be declared as volatile to work!!!! Otherwise the compiler will optimize it into the conditional, skipping using 'SomeResult', and not recognize it as a cmovable operation, sadly to say.
It also doesn't like pointer dereferencing in place of a straight array lookup (in the conditional). So either assign the dereferenced pointer to another var, or use a direct array lookup [] style, or just assign all your array values to an integer variable!
Very interesting the confinements to get cmov ops to generate. If anyone has anything to add, it'd be great... it seems like I'm not the only one that has tried to find a dependable solution to this problem. And of course, always check the assembly output to make sure it worked!
Cheers
-Scott
[Edited by - popsoftheyear on July 14, 2008 3:48:20 PM]
