[flang-commits] [flang] [flang][OpenMP] Privatize loop IVs in the innermost parallel (PR #227486)

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@llvm/pr-subscribers-flang-fir-hlfir

Author: Leandro Lupori (luporl)

<details>
<summary>Changes</summary>

A sequential loop's iteration variable is predetermined private in the
innermost parallel, teams or task-generating construct that encloses
the loop. When the loop was nested in another construct, such as a
worksharing loop, lowering failed to privatize the variable in the
enclosing parallel region. Instead, it created a new local copy inside
the nested construct, so the parallel region's other references to the
variable used the shared host variable.

Fix this by deciding which construct privatizes a symbol based on the
scope that owns it. This also simplifies DataSharingProcessor: the
OMPConstructSymbolVisitor, which walked the parse tree to track where
symbols were defined, is no longer needed and has been removed.

I have noticed that metadirectives don't always own the symbols that
should be privatized in them, as semantics doesn't create a new scope.
I'm not very familiar with metadirectives, but it seems this causes some
privatization issues with non-explicitly specified DSAs, as
DataSharingProcessor relies on ownership information to determine which
symbols should be privatized. These issues already existed before this
change, and I haven't seen any test regressions after it. I've run the
gfortran and Fujitsu testsuites.

Fixes #<!-- -->207125

Assisted-By: Claude Code Opus 5.5 (tests and documentation only)


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Full diff: https://github.com/llvm/llvm-project/pull/227486.diff


8 Files Affected:

- (modified) flang/lib/Lower/Bridge.cpp (+1-8) 
- (modified) flang/lib/Lower/OpenMP/DataSharingProcessor.cpp (+67-130) 
- (modified) flang/lib/Lower/OpenMP/DataSharingProcessor.h (-73) 
- (modified) flang/test/Lower/OpenMP/lastprivate-simd.f90 (+4-1) 
- (added) flang/test/Lower/OpenMP/predetermined-do-iv.f90 (+208) 
- (modified) flang/test/Lower/OpenMP/shared-loop.f90 (+4-6) 
- (modified) flang/test/Lower/OpenMP/taskgroup02.f90 (+1-2) 
- (modified) flang/test/Lower/OpenMP/unstructured.f90 (+7-8) 


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</details>


https://github.com/llvm/llvm-project/pull/227486