[flang] Implement lowering for the PAUSE statement (Fixes #166821) (#167115)

Implements lowering for the Fortran `PAUSE` statement.

- Handles PAUSE with no operand.
- Handles PAUSE with integer argument.
- Handles PAUSE with character literal argument.
- Adds a new lowering test: flang/test/Lower/pause-statement.f90.

Unlike STOP, PAUSE does not unconditionally terminate control flow.
The lowering preserves labels and GOTOs, consistent with legacy Fortran
behavior.

Fixes: #166821

---------

Co-authored-by: aditya nath <adityanath5002@gmail.com>
Co-authored-by: Eugene Epshteyn <eepshteyn@nvidia.com>

Author: 3 people

flang/lib/Lower/Runtime.cpp

@@ -169,12 +169,55 @@ void Fortran::lower::genUnlockStatement(
 
 void Fortran::lower::genPauseStatement(
     Fortran::lower::AbstractConverter &converter,
-    const Fortran::parser::PauseStmt &) {
+    const Fortran::parser::PauseStmt &stmt) {
+
   fir::FirOpBuilder &builder = converter.getFirOpBuilder();
   mlir::Location loc = converter.getCurrentLocation();
-  mlir::func::FuncOp callee =
-      fir::runtime::getRuntimeFunc<mkRTKey(PauseStatement)>(loc, builder);
-  fir::CallOp::create(builder, loc, callee, mlir::ValueRange{});
+  Fortran::lower::StatementContext stmtCtx;
+
+  llvm::SmallVector<mlir::Value> operands;
+  mlir::func::FuncOp callee;
+  mlir::FunctionType calleeType;
+
+  if (stmt.v.has_value()) {
+    const auto &code = stmt.v.value();
+    auto expr =
+        converter.genExprValue(*Fortran::semantics::GetExpr(code), stmtCtx);
+    expr.match(
+        // Character-valued expression -> call PauseStatementText (CHAR, LEN)
+        [&](const fir::CharBoxValue &x) {
+          callee = fir::runtime::getRuntimeFunc<mkRTKey(PauseStatementText)>(
+              loc, builder);
+          calleeType = callee.getFunctionType();
+
+          operands.push_back(
+              builder.createConvert(loc, calleeType.getInput(0), x.getAddr()));
+          operands.push_back(
+              builder.createConvert(loc, calleeType.getInput(1), x.getLen()));
+        },
+        // Unboxed value -> call PauseStatementInt which accepts an integer.
+        [&](fir::UnboxedValue x) {
+          callee = fir::runtime::getRuntimeFunc<mkRTKey(PauseStatementInt)>(
+              loc, builder);
+          calleeType = callee.getFunctionType();
+          assert(calleeType.getNumInputs() >= 1);
+          mlir::Value cast =
+              builder.createConvert(loc, calleeType.getInput(0), x);
+          operands.push_back(cast);
+        },
+        [&](auto) {
+          fir::emitFatalError(loc, "unhandled expression in PAUSE");
+        });
+  } else {
+    callee =
+        fir::runtime::getRuntimeFunc<mkRTKey(PauseStatement)>(loc, builder);
+    calleeType = callee.getFunctionType();
+  }
+
+  fir::CallOp::create(builder, loc, callee, operands);
+
+  // NOTE: PAUSE does not terminate the current block. The program may resume
+  // and continue normal execution, so we do not emit control-flow terminators.
 }
 
 void Fortran::lower::genPointerAssociate(fir::FirOpBuilder &builder,

flang/test/Lower/pause-statement.f90

@@ -2,7 +2,31 @@
 
 ! CHECK-LABEL: pause_test
 subroutine pause_test()
-  ! CHECK: fir.call @_Fortran{{.*}}PauseStatement()
-  ! CHECK-NEXT: return
   pause
+  ! CHECK: fir.call @_FortranA{{.*}}PauseStatement()
+  ! CHECK-NEXT: return
+end subroutine
+
+! CHECK-LABEL: pause_code
+subroutine pause_code()
+  pause 42
+  ! CHECK: %[[c42:.*]] = arith.constant 42 : i32
+  ! CHECK: fir.call @_FortranA{{.*}}PauseStatementInt(%[[c42]])
+  ! CHECK-NEXT: return
 end subroutine
+
+! CHECK-LABEL: pause_msg
+subroutine pause_msg()
+  pause "hello"
+  ! CHECK-DAG: %[[five:.*]] = arith.constant 5 : index
+  ! CHECK-DAG: %[[addr:.*]] = fir.address_of(@_QQ{{.*}}) : !fir.ref<!fir.char<1,5>>
+  ! CHECK-DAG: %[[str:.*]]:2 = hlfir.declare %[[addr]] typeparams %[[five]] {fortran_attrs = #fir.var_attrs<parameter>, uniq_name = "_QQ{{.*}}"} : (!fir.ref<!fir.char<1,5>>, index) -> (!fir.ref<!fir.char<1,5>>, !fir.ref<!fir.char<1,5>>)
+  ! CHECK-DAG: %[[buff:.*]] = fir.convert %[[str]]#0 : (!fir.ref<!fir.char<1,5>>) -> !fir.ref<i8>
+  ! CHECK-DAG: %[[len:.*]] = fir.convert %[[five]] : (index) -> i64
+  ! CHECK: fir.call @_FortranA{{.*}}PauseStatementText(%[[buff]], %[[len]])
+  ! CHECK-NEXT: return
+end subroutine
+
+! CHECK-DAG: func private @_FortranA{{.*}}PauseStatement
+! CHECK-DAG: func private @_FortranA{{.*}}PauseStatementInt
+! CHECK-DAG: func private @_FortranA{{.*}}PauseStatementText