[flang][hlfir] work towards handling char_convert in hlfir

This patch aims to address the TODO for handling character conversion in HLFIR found [here](https://github.com/llvm/llvm-project/blob/1defa781243f9d0bc66719465e4de33e9fb7a243/flang/lib/Lower/ConvertExprToHLFIR.cpp#L1388) using [this similar operation but for FIR as inspiration](https://github.com/llvm/llvm-project/blob/3ea673a97b0583affc22345b9d62e863ba36b3d8/flang/lib/Lower/ConvertExpr.cpp#L1212-L1271).

Reviewed By: vzakhari, tblah

Differential Revision: https://reviews.llvm.org/D155650

Author: cabreraam

flang/lib/Lower/ConvertExprToHLFIR.cpp

@@ -1385,7 +1385,34 @@ struct UnaryOp<
                                          hlfir::Entity lhs) {
     if constexpr (TC1 == Fortran::common::TypeCategory::Character &&
                   TC2 == TC1) {
-      TODO(loc, "character conversion in HLFIR");
+      // TODO(loc, "character conversion in HLFIR");
+      auto kindMap = builder.getKindMap();
+      mlir::Type fromTy = lhs.getFortranElementType();
+      mlir::Value origBufferSize = genCharLength(loc, builder, lhs);
+      mlir::Value bufferSize{origBufferSize};
+      auto fromBits = kindMap.getCharacterBitsize(
+          fir::unwrapRefType(fromTy).cast<fir::CharacterType>().getFKind());
+      mlir::Type toTy = Fortran::lower::getFIRType(
+          builder.getContext(), TC1, KIND, /*params=*/std::nullopt);
+      auto toBits = kindMap.getCharacterBitsize(
+          toTy.cast<fir::CharacterType>().getFKind());
+      if (toBits < fromBits) {
+        // Scale by relative ratio to give a buffer of the same length.
+        auto ratio = builder.createIntegerConstant(loc, bufferSize.getType(),
+                                                   fromBits / toBits);
+        bufferSize =
+            builder.create<mlir::arith::MulIOp>(loc, bufferSize, ratio);
+      }
+      // allocate space on the stack for toBuffer
+      auto dest = builder.create<fir::AllocaOp>(loc, toTy,
+                                                mlir::ValueRange{bufferSize});
+      builder.create<fir::CharConvertOp>(loc, lhs.getFirBase(), origBufferSize,
+                                         dest);
+
+      return hlfir::EntityWithAttributes{builder.create<hlfir::DeclareOp>(
+          loc, dest, "ctor.temp", /*shape=*/nullptr,
+          /*typeparams=*/mlir::ValueRange{origBufferSize},
+          fir::FortranVariableFlagsAttr{})};
     }
     mlir::Type type = Fortran::lower::getFIRType(builder.getContext(), TC1,
                                                  KIND, /*params=*/std::nullopt);

flang/test/Lower/charconvert.f90

@@ -0,0 +1,39 @@
+! RUN: bbc -emit-hlfir %s -o - | FileCheck %s
+
+subroutine test_c1_to_c4(c4, c1)
+  character(len=*, kind=4) :: c4                           
+  character(len=*, kind=1) :: c1            
+  c4 = c1                                         
+end subroutine                                           
+
+subroutine test_c4_to_c1(c4, c1)
+  character(len=*, kind=4) :: c4                           
+  character(len=*, kind=1) :: c1            
+  c1 = c4                                         
+end subroutine                                           
+                                                
+! CHECK: func.func @_QPtest_c1_to_c4(%[[ARG0:.*]]: !fir.boxchar<4> {fir.bindc_name = "c4"}, %[[ARG1:.*]]: !fir.boxchar<1> {fir.bindc_name = "c1"}) {
+! CHECK:   %[[VAL_0:.*]]:2 = fir.unboxchar %[[ARG1]] : (!fir.boxchar<1>) -> (!fir.ref<!fir.char<1,?>>, index)
+! CHECK:   %[[VAL_1:.*]]:2 = hlfir.declare %[[VAL_0]]#0 typeparams %[[VAL_0]]#1 {uniq_name = "_QFtest_c1_to_c4Ec1"} : (!fir.ref<!fir.char<1,?>>, index) -> (!fir.boxchar<1>, !fir.ref<!fir.char<1,?>>)
+! CHECK:   %[[VAL_2:.*]]:2 = fir.unboxchar %[[ARG0]] : (!fir.boxchar<4>) -> (!fir.ref<!fir.char<4,?>>, index)
+! CHECK:   %[[VAL_3:.*]]:2 = hlfir.declare %[[VAL_2]]#0 typeparams %[[VAL_2]]#1 {uniq_name = "_QFtest_c1_to_c4Ec4"} : (!fir.ref<!fir.char<4,?>>, index) -> (!fir.boxchar<4>, !fir.ref<!fir.char<4,?>>)
+! CHECK:   %[[VAL_4:.*]] = fir.alloca !fir.char<4,?>(%[[VAL_0]]#1 : index)
+! CHECK:   fir.char_convert %[[VAL_1]]#1 for %[[VAL_0]]#1 to %[[VAL_4:.*]] : !fir.ref<!fir.char<1,?>>, index, !fir.ref<!fir.char<4,?>>
+! CHECK:   %[[VAL_5:.*]]:2 = hlfir.declare %[[VAL_4]] typeparams %[[VAL_0]]#1 {uniq_name = "ctor.temp"} : (!fir.ref<!fir.char<4,?>>, index) -> (!fir.boxchar<4>, !fir.ref<!fir.char<4,?>>)
+! CHECK:   hlfir.assign %[[VAL_5]]#0 to %[[VAL_3]]#0 : !fir.boxchar<4>, !fir.boxchar<4>
+! CHECK:   return
+! CHECK: }
+
+! CHECK: func.func @_QPtest_c4_to_c1(%[[ARG0:.*]]: !fir.boxchar<4> {fir.bindc_name = "c4"}, %[[ARG1:.*]]: !fir.boxchar<1> {fir.bindc_name = "c1"}) {
+! CHECK:   %[[VAL_0:.*]]:2 = fir.unboxchar %[[ARG1]] : (!fir.boxchar<1>) -> (!fir.ref<!fir.char<1,?>>, index)
+! CHECK:   %[[VAL_1:.*]]:2 = hlfir.declare %[[VAL_0]]#0 typeparams %[[VAL_0]]#1 {uniq_name = "_QFtest_c4_to_c1Ec1"} : (!fir.ref<!fir.char<1,?>>, index) -> (!fir.boxchar<1>, !fir.ref<!fir.char<1,?>>)
+! CHECK:   %[[VAL_2:.*]]:2 = fir.unboxchar %[[ARG0]] : (!fir.boxchar<4>) -> (!fir.ref<!fir.char<4,?>>, index)
+! CHECK:   %[[VAL_3:.*]]:2 = hlfir.declare %[[VAL_2]]#0 typeparams %[[VAL_2]]#1 {uniq_name = "_QFtest_c4_to_c1Ec4"} : (!fir.ref<!fir.char<4,?>>, index) -> (!fir.boxchar<4>, !fir.ref<!fir.char<4,?>>)
+! CHECK:   %[[C4:.*]] = arith.constant 4 : index
+! CHECK:   %[[VAL_4:.*]] = arith.muli %[[VAL_2]]#1, %[[C4]] : index
+! CHECK:   %[[VAL_5:.*]] = fir.alloca !fir.char<1,?>(%[[VAL_4]] : index)
+! CHECK:   fir.char_convert %[[VAL_3]]#1 for %[[VAL_2]]#1 to %[[VAL_5:.*]] : !fir.ref<!fir.char<4,?>>, index, !fir.ref<!fir.char<1,?>>
+! CHECK:   %[[VAL_6:.*]]:2 = hlfir.declare %[[VAL_5]] typeparams %[[VAL_2]]#1 {uniq_name = "ctor.temp"} : (!fir.ref<!fir.char<1,?>>, index) -> (!fir.boxchar<1>, !fir.ref<!fir.char<1,?>>)
+! CHECK:   hlfir.assign %[[VAL_6]]#0 to %[[VAL_1]]#0 : !fir.boxchar<1>, !fir.boxchar<1>
+! CHECK:   return
+! CHECK: }