[Flang] Avoid crash when a function return is undefined

[ceseo]

Properly terminate the StatementContext cleanup when a function return value is undefined.

Fixes #126452

[llvmbot]

@llvm/pr-subscribers-flang-fir-hlfir

Author: Carlos Seo (ceseo)

Changes

Properly terminate the StatementContext cleanup when a function return value is undefined.

Fixes #126452

---

Full diff: https://github.com/llvm/llvm-project/pull/151577.diff

2 Files Affected:

• (modified) flang/lib/Lower/Bridge.cpp (+7)
• (added) flang/test/Lower/undef-func-result.f90 (+8)

diff --git a/flang/lib/Lower/Bridge.cpp b/flang/lib/Lower/Bridge.cpp
index 1adfb96ab9a98..802bdae9a5f05 100644
--- a/flang/lib/Lower/Bridge.cpp
+++ b/flang/lib/Lower/Bridge.cpp
@@ -1733,6 +1733,13 @@ class FirConverter : public Fortran::lower::AbstractConverter {
     mlir::Location loc = toLocation();
     if (!resultSymBox) {
       mlir::emitError(loc, "internal error when processing function return");
+      // Create a dummy undefined value of the expected return type.
+      // This prevents improper cleanup of StatementContext, which would lead
+      // to a crash due to a block with no terminator. See issue #126452.
+      mlir::FunctionType funcType = builder->getFunction().getFunctionType();
+      mlir::Type resultType = funcType.getResult(0);
+      mlir::Value undefResult = builder->create<fir::UndefOp>(loc, resultType);
+      genExitRoutine(false, undefResult);
       return;
     }
     mlir::Value resultVal = resultSymBox.match(
diff --git a/flang/test/Lower/undef-func-result.f90 b/flang/test/Lower/undef-func-result.f90
new file mode 100644
index 0000000000000..e8f3c8d2cad7d
--- /dev/null
+++ b/flang/test/Lower/undef-func-result.f90
@@ -0,0 +1,8 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1
+! XFAIL: *
+function s(x) result(i)
+!ERROR: internal error when processing function return
+integer::x
+procedure():: i
+end function
+end

[tblah]

Thanks for all of the bug fixes.

[tblah]

Thank you