[flang] Add a placeholder argument for a format descriptor on IO calls

[klausler]

LGTM, thanks!

[vdonaldson]

https://reviews.llvm.org/D132259

[PeteSteinfeld]

Note that, although this revision has been approved, the associated changes have not yet been pushed into the repository.

[klausler]

Merged as commit d0e03bf06b2a7c05d8c478845282a68cba7cdda2.

flang/lib/Lower/IO.cpp

if (isInternal) {
                                  stmtCtx));
     }
   } else { // PRINT - maybe explicit format; default unit
     maybeGetFormatArgs();
     ioArgs.push_back(builder.create<mlir::arith::ConstantOp>(
         loc, builder.getIntegerAttr(ioFuncTy.getInput(ioArgs.size()),
                                     Fortran::runtime::io::DefaultUnit)));
   }
-  // File name and line number are always the last two arguments.
+  // File name and line number.
   ioArgs.push_back(
       locToFilename(converter, loc, ioFuncTy.getInput(ioArgs.size())));
   ioArgs.push_back(
       locToLineNo(converter, loc, ioFuncTy.getInput(ioArgs.size())));
+  // Placeholder for format passed as a descriptor.
+  if (isFormatted && !isListOrNml)
+    ioArgs.push_back(
+        builder.createNullConstant(loc, ioFuncTy.getInput(ioArgs.size())));
 }
 
 template <bool isInput, bool hasIOCtrl = true, typename A>
 static mlir::Value
 genDataTransferStmt(Fortran::lower::AbstractConverter &converter,
                     const A &stmt) {
   fir::FirOpBuilder &builder = converter.getFirOpBuilder();
   Fortran::lower::StatementContext stmtCtx;