Shifting toward C99

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Revision as of 18:36, 26 October 2018 by Zhouh (talk | contribs) (Motivating reasons)

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Motivating reasons

  • Third party components are shifting to C99.
hwloc 
hwloc is required in MPICH and it requires C99.
ucx 
ucx is optional in MPICH and it requires C99.
ofi. 
ofi is optional in MPICH and it requires C99.
ch4 
CH4 device requires either ucx or ofi.
  • C99 requirement simplifies portability burden.
    • Boolean type in <stdbool.h>
    • long long
    • Restricted pointer
    • Mixed declaration
  • C99 allows declaration of local variables close to where it is being used. It can make the code much more readable.

Potential impact

The main concern of shifting to C99 is that certain platforms with absent or less-complete C99 compiler support will not be able to build MPICH.

Reference compiler quirks page for the status of compiler support.

MPICH specific C99 features

Even as we shifting to C99 requirement, we are not opening the flood gate to allow all C99 features into the codebase. The full support of C99 features may put unnecessary restrictions on compilers. By limiting allowed C99 features, we intend to support as many compilers as we can. In addition, even when certain feature may not be supported by a particular compiler, the limited feature allowance may still provide feasible path for back porting.

The following are a list of feature we are considering to adopt:

We are holding off the adoption of the features that are not listed above. Some of them are listed below: