[llvm] [Flang-rt] Implement same beahvior as -O3 for zero-length arrays (PR #171480)

[Michael Klemm via llvm-commits]

https://github.com/mjklemm created https://github.com/llvm/llvm-project/pull/171480

This PR addresses an issue of incorrect code where an unallocated array is passed to a routine as a zero-length array.  When compiling such a code with -O3, the assignment is ignored, but -O0 chokes in flang-rt due to the above condition in the original code. Adding the extra condition makes the behavior consistent with -O3.

>From b1fd7cfbbe960c552dee77d896ee05a211c48fad Mon Sep 17 00:00:00 2001
From: Michael Klemm <michael.klemm at amd.com>
Date: Tue, 9 Dec 2025 18:43:53 +0100
Subject: [PATCH] [Flang-rt] Implement same beahvior as -O3 for zero-length
 arrays

This PR addresses an issue of incorrect code where an unallocated array
is passed to a routine as a zero-length array.  When compiling such a
code with -O3, the assignment is ignored, but -O0 chokes in flang-rt due
to the above condition in the original code. Adding the extra condition
makes the behavior consistent with -O3.
---
 flang-rt/lib/runtime/assign.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/flang-rt/lib/runtime/assign.cpp b/flang-rt/lib/runtime/assign.cpp
index b70182ccb3178..dd5d4b945881e 100644
--- a/flang-rt/lib/runtime/assign.cpp
+++ b/flang-rt/lib/runtime/assign.cpp
@@ -366,7 +366,7 @@ RT_API_ATTRS int AssignTicket::Begin(WorkQueue &workQueue) {
                                    "assignment to unallocated allocatable",
           to_.rank(), from_->rank());
     }
-  } else if (!to_.IsAllocated()) {
+  } else if (!to_.IsAllocated() && to_.Elements()) {
     workQueue.terminator().Crash(
         "Assign: left-hand side variable is neither allocated nor allocatable");
   }