[flang] Support CHARACTER(4) pointer targets

[jeanPerier]

Summary

fir.rebox is emitting an llvm.sdiv to compute the character length
given the byte size from the input descriptor.
Inside a fir.global, this is not needed given the target length must
be accessible via the type, and it caused MLIR to fail LLVM IR
code generation (and crash).

Use the input type length when available instead.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Optimizer/CodeGen/CodeGen.cpp

if (isInGlobalOp(rewriter))
       if (auto unrealizedCast =
               loweredBox.getDefiningOp<mlir::UnrealizedConversionCastOp>())
         loweredBox = unrealizedCast.getInputs()[0];
 
     // Create new descriptor and fill its non-shape related data.
     llvm::SmallVector<mlir::Value, 2> lenParams;
     mlir::Type inputEleTy = getInputEleTy(rebox);
     if (auto charTy = inputEleTy.dyn_cast<fir::CharacterType>()) {
+      if (charTy.hasConstantLen()) {
+        mlir::Value len =
+            genConstantIndex(loc, idxTy, rewriter, charTy.getLen());
+        lenParams.emplace_back(len);
+      } else {
         mlir::Value len = getElementSizeFromBox(
             loc, idxTy, rebox.getBox().getType(), loweredBox, rewriter);
         if (charTy.getFKind() != 1) {
+          assert(!isInGlobalOp(rewriter) &&
+                 "character target in global op must have constant length");
           mlir::Value width =
               genConstantIndex(loc, idxTy, rewriter, charTy.getFKind());
           len = rewriter.create<mlir::LLVM::SDivOp>(loc, idxTy, len, width);
         }
         lenParams.emplace_back(len);
+      }
     } else if (auto recTy = inputEleTy.dyn_cast<fir::RecordType>()) {
       if (recTy.getNumLenParams() != 0)
         TODO(loc, "reboxing descriptor of derived type with length parameters");
     }
 
     // Rebox on polymorphic entities needs to carry over the dynamic type.
     mlir::Value typeDescAddr;
     if (rebox.getBox().getType().isa<fir::ClassType>() &&

flang/test/Fir/rebox-global.fir

 // RUN: tco %s | FileCheck %s
 // RUN: %flang_fc1 -emit-llvm  %s -o - | FileCheck %s
 
 fir.global @x target : i32
 
 fir.global @p : !fir.box<!fir.ptr<i32>> {
   %0 = fir.address_of(@x) : !fir.ref<i32>
   %1 = fir.embox %0 : (!fir.ref<i32>) -> !fir.box<i32>
   %2 = fir.rebox %1 : (!fir.box<i32>) -> !fir.box<!fir.ptr<i32>>
   fir.has_value %2 : !fir.box<!fir.ptr<i32>>
 }
 // CHECK: @p = global { ptr, i64, i32, i8, i8, i8, i8 } { ptr @x, {{.*}}, i32 {{.*}}, i8 0, i8 9, i8 1, i8 0 }
+
+// Test that sdiv is not generated inside fir.global.
+fir.global @char4 target : !fir.char<4,10>
+fir.global @pointer_char4_init : !fir.box<!fir.ptr<!fir.char<4,10>>> {
+  %0 = fir.address_of(@char4) : !fir.ref<!fir.char<4,10>>
+  %1 = fir.embox %0 : (!fir.ref<!fir.char<4,10>>) -> !fir.box<!fir.char<4,10>>
+  %2 = fircg.ext_rebox %1 : (!fir.box<!fir.char<4,10>>) -> !fir.box<!fir.ptr<!fir.char<4,10>>>
+  fir.has_value %2 : !fir.box<!fir.ptr<!fir.char<4,10>>>
+}
+// CHECK-LABEL: @pointer_char4_init
+// CHECK-SAME: { ptr @char4, i64 ptrtoint (ptr getelementptr ([10 x i32], ptr null, i32 1) to i64), i32 20180515, i8 0, i8 44, i8 1, i8 0 }