[flang] Handle absent optional in LOC (llvm#73530)

LOC is a GNU extension, so gfortran is the reference for it, and it
accepts absent OPTIONAL and returns zero. Support this use case in flang
too.

Update the LOC test to use HLFIR while touching it.

Fixes llvm#72823.

Author: jeanPerier

flang/lib/Optimizer/Builder/IntrinsicCall.cpp

@@ -4138,17 +4138,46 @@ IntrinsicLibrary::genCharacterCompare(mlir::Type resultType,
       fir::getBase(args[1]), fir::getLen(args[1]));
 }
 
+static bool isOptional(mlir::Value value) {
+  auto varIface = mlir::dyn_cast_or_null<fir::FortranVariableOpInterface>(
+      value.getDefiningOp());
+  return varIface && varIface.isOptional();
+}
+
 // LOC
 fir::ExtendedValue
 IntrinsicLibrary::genLoc(mlir::Type resultType,
                          llvm::ArrayRef<fir::ExtendedValue> args) {
   assert(args.size() == 1);
-  mlir::Value argValue = fir::getBase(args[0]);
-  assert(fir::isa_box_type(argValue.getType()) &&
+  mlir::Value box = fir::getBase(args[0]);
+  assert(fir::isa_box_type(box.getType()) &&
          "argument must have been lowered to box type");
-  bool isFunc = argValue.getType().isa<fir::BoxProcType>();
-  mlir::Value argAddr = getAddrFromBox(builder, loc, args[0], isFunc);
-  return builder.createConvert(loc, fir::unwrapRefType(resultType), argAddr);
+  bool isFunc = box.getType().isa<fir::BoxProcType>();
+  if (!isOptional(box)) {
+    mlir::Value argAddr = getAddrFromBox(builder, loc, args[0], isFunc);
+    return builder.createConvert(loc, resultType, argAddr);
+  }
+  // Optional assumed shape case.  Although this is not specified in this GNU
+  // intrinsic extension, LOC accepts absent optional and returns zero in that
+  // case.
+  // Note that the other OPTIONAL cases do not fall here since `box` was
+  // created when preparing the argument cases, but the box can be safely be
+  // used for all those cases and the address will be null if absent.
+  mlir::Value isPresent =
+      builder.create<fir::IsPresentOp>(loc, builder.getI1Type(), box);
+  return builder
+      .genIfOp(loc, {resultType}, isPresent,
+               /*withElseRegion=*/true)
+      .genThen([&]() {
+        mlir::Value argAddr = getAddrFromBox(builder, loc, args[0], isFunc);
+        mlir::Value cast = builder.createConvert(loc, resultType, argAddr);
+        builder.create<fir::ResultOp>(loc, cast);
+      })
+      .genElse([&]() {
+        mlir::Value zero = builder.createIntegerConstant(loc, resultType, 0);
+        builder.create<fir::ResultOp>(loc, zero);
+      })
+      .getResults()[0];
 }
 
 // MASKL, MASKR