[flang] Support CHARACTER(4) pointer targets

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.

Reviewed By: PeteSteinfeld, vzakhari

Differential Revision: https://reviews.llvm.org/D154072

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -1894,14 +1894,22 @@ struct XReboxOpConversion : public EmboxCommonConversion<fir::cg::XReboxOp> {
     llvm::SmallVector<mlir::Value, 2> lenParams;
     mlir::Type inputEleTy = getInputEleTy(rebox);
     if (auto charTy = inputEleTy.dyn_cast<fir::CharacterType>()) {
-      mlir::Value len = getElementSizeFromBox(
-          loc, idxTy, rebox.getBox().getType(), loweredBox, rewriter);
-      if (charTy.getFKind() != 1) {
-        mlir::Value width =
-            genConstantIndex(loc, idxTy, rewriter, charTy.getFKind());
-        len = rewriter.create<mlir::LLVM::SDivOp>(loc, idxTy, len, width);
+      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);
       }
-      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");

flang/test/Fir/rebox-global.fir

@@ -10,3 +10,14 @@ fir.global @p : !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 }