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[flang][Lower] Always generate namelist group locally #109303

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[flang][Lower] Always generate namelist group locally #109303

wants to merge 1 commit into from

Commits on Sep 19, 2024

  1. [flang][Lower] Always generate namelist group locally 

    Namelists are allowed inside of OpenMP reduction regions. Those regions
map the symbol for the reduction variable to a thread-local reduction
variable, but this mapping is escaped if the namelist is generated as a
global variable because the global variable will refer the original
symbol mapping. See llvm#101907 for a code example.

We can't do this only when inside of an OpenMP region because the IO
function might be inside of a subroutine/function which is called from
inside the OpenMP region.

There is also no restriction against namelist variables for the `reduce`
clause of `do concurrent` in F23. We currently don't seem to implement
initial values for reduction variables (F23 table 11.1), but when we do
I expect this would see the same issue as OpenMP.

A disadvantage of doing this is that the namelist has to be constructed
in place every time. If for some reason this is not hoisted out of hot
loops (e.g. it is inside of another subroutine called in the loop body)
this might have some performance impact (although I suspect this won't
be huge compared to the context switches required for I/O).

Fixes llvm#101907
    @tblah
    tblah committed Sep 19, 2024
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