Template:Memory qualifiers

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coherent​
Normally, the compiler is free to assume that this shader invocation is the only invocation that modifies values read through this variable. It also can freely assume that other shader invocations may not see values written through this variable.
Using this qualifier is required to allow dependent shader invocations to communicate with one another, as it enforces the coherency of memory accesses. Using this requires the appropriate memory barriers to be executed, so that visibility can be achieved.
When communicating between shader invocations for different rendering commands, glMemoryBarrier should be used instead of this qualifier.
volatile​
The compiler normally is free to assume that values accessed through variables will only change after memory barriers or other synchronization. With this qualifier, the compiler assumes that the contents of the storage represented by the variable could be changed at any time.
restrict​
Normally, the compiler must assume that you could access the same image/buffer object separate variables in the same shader. Therefore, if you write to one variable, and read from a second, the compiler assumes that it is possible that you could be reading the value you just wrote. With this qualifier, you are telling the compiler that this particular variable is the only variable that can modify the memory visible through that variable within this shader invocation (other shader stages don't count here). This allows the compiler to optimize reads/writes better.
You should use this wherever possible.
readonly​
Normally, the compiler allows you to read and write from variables as you wish. If you use this, the variable can only be used for reading operations.
writeonly​
Normally, the compiler allows you to read and write from variables as you wish. If you use this, the variable can only be used for writing operations (atomic writes are forbidden because they also count as reads).