[flang] Allow lowering of sub-expressions to be overridden (#69944)

OpenACC/OpenMP atomic lowering needs a finer control over expression
lowering. This patch allows mapping evaluate::Expr<T> to mlir::Value so
that any subsequent expression lowering will use these values when an
operand is a mapped Expr<T>.

This is an alternative to
https://github.com/llvm/llvm-project/pull/69866 From which I took the
test and some of the logic to extract the non-atomic sub-expression.

---------

Co-authored-by: Nimish Mishra <neelam.nimish@gmail.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/flang/lib/Lower/Bridge.cpp b/flang/lib/Lower/Bridge.cpp
index ff31625..761cedb 100644
--- a/flang/lib/Lower/Bridge.cpp
+++ b/flang/lib/Lower/Bridge.cpp
@@ -513,6 +513,15 @@ public:
     addSymbol(sym, exval, /*forced=*/true);
   }
 
+  void
+  overrideExprValues(const Fortran::lower::ExprToValueMap *map) override final {
+    exprValueOverrides = map;
+  }
+
+  const Fortran::lower::ExprToValueMap *getExprOverrides() override final {
+    return exprValueOverrides;
+  }
+
   bool lookupLabelSet(Fortran::lower::SymbolRef sym,
                       Fortran::lower::pft::LabelSet &labelSet) override final {
     Fortran::lower::pft::FunctionLikeUnit &owningProc =
@@ -4903,6 +4912,8 @@ private:
   /// Whether an OpenMP target region or declare target function/subroutine
   /// intended for device offloading has been detected
   bool ompDeviceCodeFound = false;
+
+  const Fortran::lower::ExprToValueMap *exprValueOverrides{nullptr};
 };
 
 } // namespace