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6.7.3. Registers and Compiler Optimizations

[] Advice to users.

All the material in this section is an advice to users. ( End of advice to users.)
A coherence problem exists between variables kept in registers and the memory value of these variables. An RMA call may access a variable in memory (or cache), while the up-to-date value of this variable is in register. A get will not return the latest variable value, and a put may be overwritten when the register is stored back in memory.

The problem is illustrated by the following code:

In this example, variable buff is allocated in the register reg_A and therefore ccc will have the old value of buff and not the new value 777.

This problem, which also afflicts in some cases send/receive communication, is discussed more at length in Section A Problem with Register Optimization .

MPI implementations will avoid this problem for standard conforming C programs. Many Fortran compilers will avoid this problem, without disabling compiler optimizations. However, in order to avoid register coherence problems in a completely portable manner, users should restrict their use of RMA windows to variables stored in COMMON blocks, or to variables that were declared VOLATILE (while VOLATILE is not a standard Fortran declaration, it is supported by many Fortran compilers). Details and an additional solution are discussed in Section A Problem with Register Optimization , ``A Problem with Register Optimization,'' . See also, ``Problems Due to Data Copying and Sequence Association,'', for additional Fortran problems.

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