From 0e73b1821cacb453c307cc1b9f35e989321b8e42 Mon Sep 17 00:00:00 2001
From: Richard-Qin-X <richard.qin.001@gmail.com>
Date: Wed, 12 Aug 2026 02:30:19 +0800
Subject: [CodeGen] Preserve variable-sized stack object offsets

---
 backend/lib/CodeGen/MachineFrameInfo.cpp           | 8 ++------
 backend/unittests/CodeGen/MachineFrameInfoTest.cpp | 6 ++++++
 2 files changed, 8 insertions(+), 6 deletions(-)

(limited to 'backend')

diff --git a/backend/lib/CodeGen/MachineFrameInfo.cpp b/backend/lib/CodeGen/MachineFrameInfo.cpp
index 8c3e6e5..80e1ee0 100644
--- a/backend/lib/CodeGen/MachineFrameInfo.cpp
+++ b/backend/lib/CodeGen/MachineFrameInfo.cpp
@@ -114,9 +114,9 @@ Expected<void> MachineFrameInfo::verifyDescription(const StackObjectDesc &Descri
   const bool IsVariable = Description.Kind == StackObjectKind::VariableSized ||
                           Description.Kind == StackObjectKind::DynamicAlloca;
   if (IsVariable) {
-    if (Description.Size || Description.Offset) {
+    if (Description.Size) {
       return makeUnexpected(ErrorCode::InvalidArgument,
-                            "variable-sized stack objects cannot have a fixed size or offset");
+                            "variable-sized stack objects cannot have a fixed size");
     }
   } else if (!Description.Size) {
     return makeUnexpected(ErrorCode::InvalidArgument,
@@ -254,10 +254,6 @@ Expected<void> MachineFrameInfo::setObjectOffset(FrameIndex Index, std::int64_t
                           "live stack object unexpectedly lacks a description");
   }
   StackObjectDesc Description = *CurrentDescription;
-  if ((*Object)->isVariableSized()) {
-    return makeUnexpected(ErrorCode::InvalidArgument,
-                          "variable-sized stack object cannot have a fixed offset");
-  }
   Description.Offset = Offset;
   (*Object)->Description.emplace<StackObjectDesc>(std::move(Description));
   return {};
diff --git a/backend/unittests/CodeGen/MachineFrameInfoTest.cpp b/backend/unittests/CodeGen/MachineFrameInfoTest.cpp
index 78987c0..b0464a6 100644
--- a/backend/unittests/CodeGen/MachineFrameInfoTest.cpp
+++ b/backend/unittests/CodeGen/MachineFrameInfoTest.cpp
@@ -57,6 +57,8 @@ void testObjectLifecycle(TestContext &Context) {
   rcc::StackObjectDesc VariableDescription;
   VariableDescription.Kind = rcc::StackObjectKind::VariableSized;
   VariableDescription.Align = *rcc::Alignment::create(32);
+  VariableDescription.Offset = 0;
+  VariableDescription.LocalOffset = 0;
   auto Variable = Frame.createObject(VariableDescription);
 
   Context.expect(Local && Fixed && Spill && Variable, "valid stack object kinds must be created");
@@ -72,10 +74,14 @@ void testObjectLifecycle(TestContext &Context) {
 
   Context.expect(Frame.setObjectOffset(*Local, -8).has_value() &&
                      Frame.setObjectOffset(*Spill, -16).has_value() &&
+                     Frame.setObjectOffset(*Variable, -64).has_value() &&
                      Frame.markObjectDead(*Spill).has_value(),
                  "live stack objects must support controlled pre-freeze updates");
   Context.expect(Frame.object(*Spill)->description()->IsDead,
                  "dead stack object state must remain explicit");
+  Context.expect(Frame.object(*Variable)->description()->Offset == -64 &&
+                     Frame.object(*Variable)->description()->LocalOffset == 0,
+                 "variable-sized stack objects must preserve LLVM MIR frame offsets");
 
   Context.expect(Frame.eraseObject(*Fixed).has_value() && Frame.object(*Fixed) == nullptr &&
                      Frame.liveObjectCount() == 3,
-- 
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