[flang] Lower length on character storage

[clementval]

Summary

This patch adds lowering for SetLength used to set
different length on character storage around calls where
the dummy and actual length differ.

This patch is part of the upstreaming effort from fir-dev branch.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Lower/ConvertExpr.cpp

if (charType.hasConstantLen()) {
       charAddr = builder.createConvert(loc, newType, charAddr);
       return fir::CharBoxValue{charAddr, newLenValue};
     }
     return fir::CharBoxValue{charAddr, newLenValue};
   }
 
   template <int KIND>
   ExtValue genval(const Fortran::evaluate::SetLength<KIND> &x) {
-    TODO(getLoc(), "genval SetLength<KIND>");
+    mlir::Value newLenValue = genunbox(x.right());
+    fir::ExtendedValue lhs = gen(x.left());
+    return replaceScalarCharacterLength(lhs, newLenValue);
   }
 
   template <int KIND>
   ExtValue genval(const Fortran::evaluate::Relational<Fortran::evaluate::Type<
                       Fortran::common::TypeCategory::Integer, KIND>> &op) {
     return createCompareOp<mlir::arith::CmpIOp>(op,
                                                 translateRelational(op.opr));
   }

flang/test/Lower/set-length.f90

+! Test evaluate::SetLength lowering (used to set a different length on a
+! character storage around calls where the dummy and actual length differ).
+! RUN: bbc -emit-fir -o - %s | FileCheck %s
+
+
+subroutine takes_length_4(c)
+  character c(3)*4
+  !do i = 1,3
+  print *, c(i)
+  !enddo
+end
+  
+! CHECK-LABEL: func @_QPfoo(
+subroutine foo(c)
+  character c(4)*3
+  ! evaluate::Expr is: CALL s(%SET_LENGTH(c(1_8),4_8)) after semantics.
+  call takes_length_4(c(1))
+! CHECK:         %[[VAL_2:.*]] = arith.constant 4 : i64
+! CHECK:         %[[VAL_6:.*]] = fir.coordinate_of %{{.*}}, %{{.*}} : (!fir.ref<!fir.array<4x!fir.char<1,3>>>, i64) -> !fir.ref<!fir.char<1,3>>
+! CHECK:         %[[VAL_7:.*]] = fir.convert %[[VAL_6]] : (!fir.ref<!fir.char<1,3>>) -> !fir.ref<!fir.char<1,?>>
+! CHECK:         %[[VAL_8:.*]] = fir.convert %[[VAL_2]] : (i64) -> index
+! CHECK:         %[[VAL_9:.*]] = fir.emboxchar %[[VAL_7]], %[[VAL_8]] : (!fir.ref<!fir.char<1,?>>, index) -> !fir.boxchar<1>                                                                   
+! CHECK:         fir.call @_QPtakes_length_4(%[[VAL_9]]) : (!fir.boxchar<1>) -> ()
+end subroutine
+
+  character(3) :: c(4) = ["abc", "def", "ghi", "klm"] 
+  call foo(c)
+end