[flang] handle common block used as BIND(C) module variables (llvm#145669)

Support odd case where a static object is being declared both as a
common block and a BIND(C) module variable name in different modules,
and both modules are used in the same compilation unit.

This is not standard, but happens when using MPI and MPI_F08 in the same
compilation unit, and at least both gfortran and ifx support this.

See added test case for an illustration.

Author: jeanPerier

flang/lib/Lower/ConvertVariable.cpp

@@ -687,8 +687,13 @@ static void instantiateGlobal(Fortran::lower::AbstractConverter &converter,
   }
   auto addrOf = builder.create<fir::AddrOfOp>(loc, global.resultType(),
                                               global.getSymbol());
+  // The type of the global cannot be trusted to be the same as the one
+  // of the variable as some existing programs map common blocks to
+  // BIND(C) module variables (e.g. mpi_argv_null in MPI and MPI_F08).
+  mlir::Type varAddrType = fir::ReferenceType::get(converter.genType(sym));
+  mlir::Value cast = builder.createConvert(loc, varAddrType, addrOf);
   Fortran::lower::StatementContext stmtCtx;
-  mapSymbolAttributes(converter, var, symMap, stmtCtx, addrOf);
+  mapSymbolAttributes(converter, var, symMap, stmtCtx, cast);
 }
 
 //===----------------------------------------------------------------===//

flang/test/Lower/variable-common-viewed-as-module-var.f90

@@ -0,0 +1,37 @@
+! RUN: bbc -emit-hlfir %s -o - | FileCheck %s
+
+! Test non standard definition of a common block as a BIND(C) variable.
+! This happens when MPI and MPI_F08 are used inside the same compilation
+! unit because MPI uses common blocks while MPI_F08 uses BIND(C) variables
+! to refer to the same objects (e.g. mpi_argv_null).
+
+module m_common_var
+ character(1) :: var
+ common /var_storage/var
+end module
+
+module m_bindc_var
+  character(1), bind(c, name="var_storage_") :: var
+end module
+
+subroutine s1()
+  use m_common_var, only : var
+  var = "a"
+end subroutine
+
+subroutine s2()
+  use m_bindc_var, only : var
+  print *, var
+end subroutine
+
+  call s1()
+  call s2()
+end
+
+! CHECK: fir.global common @var_storage_(dense<0> : vector<1xi8>) {alignment = 1 : i64} : !fir.array<1xi8>
+
+! CHECK-LABEL: func.func @_QPs1
+! CHECK: hlfir.declare %{{.*}} typeparams %c1 {uniq_name = "_QMm_common_varEvar"} : (!fir.ref<!fir.char<1>>, index) -> (!fir.ref<!fir.char<1>>, !fir.ref<!fir.char<1>>)
+
+! CHECK-LABEL: func.func @_QPs2
+! CHECK: hlfir.declare %{{.*}} typeparams %c1 {fortran_attrs = #fir.var_attrs<bind_c>, uniq_name = "var_storage_"} : (!fir.ref<!fir.char<1>>, index) -> (!fir.ref<!fir.char<1>>, !fir.ref<!fir.char<1>>)