[Clang][CodeGen] Do not set inbounds flag for struct GEP with null base pointers #130734

dtcxzyw · 2025-03-11T08:37:10Z

In the LLVM middle-end we want to fold gep inbounds null, idx -> null: https://alive2.llvm.org/ce/z/5ZkPx-
This pattern is common in real-world programs (dtcxzyw/llvm-opt-benchmark#55 (comment)). Generally, it exists in some (actually) unreachable blocks, which is introduced by JumpThreading.

However, some old-style offsetof macros are still widely used in real-world C/C++ code (e.g., hwloc/slurm/luajit). To avoid breaking existing code and inconvenience to downstream users, this patch removes the inbounds flag from the struct gep if the base pointer is null.

llvmbot · 2025-03-11T08:37:44Z

@llvm/pr-subscribers-clang-codegen

Author: Yingwei Zheng (dtcxzyw)

Changes

In the LLVM middle-end we want to fold gep inbounds null, idx -> null: https://alive2.llvm.org/ce/z/5ZkPx-
This pattern is common in real-world programs. Generally, it exists in some (actually) unreachable blocks, which is introduced by JumpThreading.

However, some old-style offsetof macros are still widely used in real-world C/C++ code (e.g., hwloc/slurm/luajit). To avoid breaking existing code and inconvenience to downstream users, this patch removes the inbounds flag from the struct gep if the base pointer is null.

Full diff: https://github.com/llvm/llvm-project/pull/130734.diff

3 Files Affected:

(modified) clang/lib/CodeGen/CGBuilder.h (+10-5)
(modified) clang/test/CodeGen/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.c (+1-1)
(modified) clang/test/CodeGenCXX/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.cpp (+1-1)

diff --git a/clang/lib/CodeGen/CGBuilder.h b/clang/lib/CodeGen/CGBuilder.h
index b8036cf6e6a30..11e8818b33397 100644
--- a/clang/lib/CodeGen/CGBuilder.h
+++ b/clang/lib/CodeGen/CGBuilder.h
@@ -223,11 +223,16 @@ class CGBuilderTy : public CGBuilderBaseTy {
     const llvm::StructLayout *Layout = DL.getStructLayout(ElTy);
     auto Offset = CharUnits::fromQuantity(Layout->getElementOffset(Index));
 
-    return Address(CreateStructGEP(Addr.getElementType(), Addr.getBasePointer(),
-                                   Index, Name),
-                   ElTy->getElementType(Index),
-                   Addr.getAlignment().alignmentAtOffset(Offset),
-                   Addr.isKnownNonNull());
+    // Specially, we don't add inbounds flags if the base pointer is null.
+    // This is a workaround for old-style offsetof macros.
+    llvm::GEPNoWrapFlags NWFlags = llvm::GEPNoWrapFlags::noUnsignedWrap();
+    if (!isa<llvm::ConstantPointerNull>(Addr.getBasePointer()))
+      NWFlags |= llvm::GEPNoWrapFlags::inBounds();
+    return Address(
+        CreateConstGEP2_32(Addr.getElementType(), Addr.getBasePointer(), 0,
+                           Index, Name, NWFlags),
+        ElTy->getElementType(Index),
+        Addr.getAlignment().alignmentAtOffset(Offset), Addr.isKnownNonNull());
   }
 
   /// Given
diff --git a/clang/test/CodeGen/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.c b/clang/test/CodeGen/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.c
index 68c0ee3a3a885..a7cfd77766712 100644
--- a/clang/test/CodeGen/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.c
+++ b/clang/test/CodeGen/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.c
@@ -17,7 +17,7 @@ struct S {
 
 // CHECK-LABEL: @get_offset_of_y_naively(
 // CHECK-NEXT:  entry:
-// CHECK-NEXT:    ret i64 ptrtoint (ptr getelementptr inbounds nuw ([[STRUCT_S:%.*]], ptr null, i32 0, i32 1) to i64)
+// CHECK-NEXT:    ret i64 ptrtoint (ptr getelementptr nuw ([[STRUCT_S:%.*]], ptr null, i32 0, i32 1) to i64)
 //
 uintptr_t get_offset_of_y_naively(void) {
   return ((uintptr_t)(&(((struct S *)0)->y)));
diff --git a/clang/test/CodeGenCXX/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.cpp b/clang/test/CodeGenCXX/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.cpp
index 34d4f4c9e34eb..f00a2c486574c 100644
--- a/clang/test/CodeGenCXX/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.cpp
+++ b/clang/test/CodeGenCXX/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.cpp
@@ -10,7 +10,7 @@ struct S {
 
 // CHECK-LABEL: @_Z23get_offset_of_y_naivelyv(
 // CHECK-NEXT:  entry:
-// CHECK-NEXT:    ret i64 ptrtoint (ptr getelementptr inbounds nuw ([[STRUCT_S:%.*]], ptr null, i32 0, i32 1) to i64)
+// CHECK-NEXT:    ret i64 ptrtoint (ptr getelementptr nuw ([[STRUCT_S:%.*]], ptr null, i32 0, i32 1) to i64)
 //
 uintptr_t get_offset_of_y_naively() {
   return ((uintptr_t)(&(((S *)nullptr)->y)));

llvmbot · 2025-03-11T08:37:44Z

@llvm/pr-subscribers-clang

Author: Yingwei Zheng (dtcxzyw)

Changes

In the LLVM middle-end we want to fold gep inbounds null, idx -> null: https://alive2.llvm.org/ce/z/5ZkPx-
This pattern is common in real-world programs. Generally, it exists in some (actually) unreachable blocks, which is introduced by JumpThreading.

However, some old-style offsetof macros are still widely used in real-world C/C++ code (e.g., hwloc/slurm/luajit). To avoid breaking existing code and inconvenience to downstream users, this patch removes the inbounds flag from the struct gep if the base pointer is null.

Full diff: https://github.com/llvm/llvm-project/pull/130734.diff

3 Files Affected:

(modified) clang/lib/CodeGen/CGBuilder.h (+10-5)
(modified) clang/test/CodeGen/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.c (+1-1)
(modified) clang/test/CodeGenCXX/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.cpp (+1-1)

diff --git a/clang/lib/CodeGen/CGBuilder.h b/clang/lib/CodeGen/CGBuilder.h
index b8036cf6e6a30..11e8818b33397 100644
--- a/clang/lib/CodeGen/CGBuilder.h
+++ b/clang/lib/CodeGen/CGBuilder.h
@@ -223,11 +223,16 @@ class CGBuilderTy : public CGBuilderBaseTy {
     const llvm::StructLayout *Layout = DL.getStructLayout(ElTy);
     auto Offset = CharUnits::fromQuantity(Layout->getElementOffset(Index));
 
-    return Address(CreateStructGEP(Addr.getElementType(), Addr.getBasePointer(),
-                                   Index, Name),
-                   ElTy->getElementType(Index),
-                   Addr.getAlignment().alignmentAtOffset(Offset),
-                   Addr.isKnownNonNull());
+    // Specially, we don't add inbounds flags if the base pointer is null.
+    // This is a workaround for old-style offsetof macros.
+    llvm::GEPNoWrapFlags NWFlags = llvm::GEPNoWrapFlags::noUnsignedWrap();
+    if (!isa<llvm::ConstantPointerNull>(Addr.getBasePointer()))
+      NWFlags |= llvm::GEPNoWrapFlags::inBounds();
+    return Address(
+        CreateConstGEP2_32(Addr.getElementType(), Addr.getBasePointer(), 0,
+                           Index, Name, NWFlags),
+        ElTy->getElementType(Index),
+        Addr.getAlignment().alignmentAtOffset(Offset), Addr.isKnownNonNull());
   }
 
   /// Given
diff --git a/clang/test/CodeGen/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.c b/clang/test/CodeGen/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.c
index 68c0ee3a3a885..a7cfd77766712 100644
--- a/clang/test/CodeGen/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.c
+++ b/clang/test/CodeGen/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.c
@@ -17,7 +17,7 @@ struct S {
 
 // CHECK-LABEL: @get_offset_of_y_naively(
 // CHECK-NEXT:  entry:
-// CHECK-NEXT:    ret i64 ptrtoint (ptr getelementptr inbounds nuw ([[STRUCT_S:%.*]], ptr null, i32 0, i32 1) to i64)
+// CHECK-NEXT:    ret i64 ptrtoint (ptr getelementptr nuw ([[STRUCT_S:%.*]], ptr null, i32 0, i32 1) to i64)
 //
 uintptr_t get_offset_of_y_naively(void) {
   return ((uintptr_t)(&(((struct S *)0)->y)));
diff --git a/clang/test/CodeGenCXX/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.cpp b/clang/test/CodeGenCXX/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.cpp
index 34d4f4c9e34eb..f00a2c486574c 100644
--- a/clang/test/CodeGenCXX/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.cpp
+++ b/clang/test/CodeGenCXX/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.cpp
@@ -10,7 +10,7 @@ struct S {
 
 // CHECK-LABEL: @_Z23get_offset_of_y_naivelyv(
 // CHECK-NEXT:  entry:
-// CHECK-NEXT:    ret i64 ptrtoint (ptr getelementptr inbounds nuw ([[STRUCT_S:%.*]], ptr null, i32 0, i32 1) to i64)
+// CHECK-NEXT:    ret i64 ptrtoint (ptr getelementptr nuw ([[STRUCT_S:%.*]], ptr null, i32 0, i32 1) to i64)
 //
 uintptr_t get_offset_of_y_naively() {
   return ((uintptr_t)(&(((S *)nullptr)->y)));

nikic

I think this makes sense, but please wait for a second opinion...

efriedma-quic

I somehow thought we had already done something about this, but I can't find any reference to it.

Anyway, if we're going to support this as an extension, I don't want to bury it in CGBuilder.h. We should decide exactly when we want it to trigger, and make it trigger for those language constructs. Not any construct that happens to go through CreateStructGEP, and not any pointer that happens to get folded to ConstantPointerNull. (Note on some targets, some null pointers are not ConstantPointerNull.)

dtcxzyw · 2025-03-12T05:33:30Z

Not any construct that happens to go through CreateStructGEP

Agree. We hit other cases where the optimization will break the stage2 build. See also #130742 (comment)

not any pointer that happens to get folded to ConstantPointerNull. (Note on some targets, some null pointers are not ConstantPointerNull.)

We only perform this optimization in addrspace(0). If some patterns cannot be constant folded into null in the clang codegen, we just treat them as UB.

efriedma-quic · 2025-03-12T20:18:06Z

We only perform this optimization in addrspace(0).

I'd still prefer to get this right for all address-spaces, so we don't need to revisit later.

But really, I'm more concerned about the "happens to get folded" part: there isn't a stable set of values that get constant-folded, at the LLVM IR level, it's just whether IRBuilder happens to do today. The only way to get a stable answer is to query the AST (Expr::isNullPointerConstant).

dtcxzyw · 2025-03-13T02:08:46Z

I'd still prefer to get this right for all address-spaces, so we don't need to revisit later.

Sentinel pointer value support for non-0 address spaces is still work in progress: #83109
I am ok to add a helper like Address::isConstantNull if needed.

But really, I'm more concerned about the "happens to get folded" part: there isn't a stable set of values that get constant-folded, at the LLVM IR level, it's just whether IRBuilder happens to do today.

I don't expect it to work on more complex patterns like (int*)(x-x).

The only way to get a stable answer is to query the AST (Expr::isNullPointerConstant).

Expr::isNullPointerConstant is unavailable in clang CodeGen unless we store the result into Address.

efriedma-quic · 2025-03-13T05:36:04Z

I don't expect it to work on more complex patterns like (int*)(x-x).

Sure, that particular pattern probably doesn't need to work, but at the very least, you're depending on certain casts getting constant-folded. And it's not clear what set of casts will in fact be consistently folded. Even if all the constructs you care about get folded right now, it's not clear that will remain true in the future. And with CIR coming along, they also need a consistent rule to follow.

Expr::isNullPointerConstant is unavailable in clang CodeGen unless we store the result into Address.

Again, the check should not be in CreateStructGEP; this check needs to happen at a higher level. And at that level, you should have an Expr*, not just an Address.

Do you care at all about patterns like ((uintptr_t)(&(((S *)nullptr)->y.z)));?

dtcxzyw · 2025-03-13T08:53:46Z

this check needs to happen at a higher level. And at that level, you should have an Expr*, not just an Address.

I tried to use Expr::isNullPointerConstant in CodeGenFunction::EmitMemberExpr:

llvm-project/clang/lib/CodeGen/CGExpr.cpp

Lines 4774 to 4781 in 59fd287

    
           LValue CodeGenFunction::EmitMemberExpr(const MemberExpr *E) { 
        
             if (DeclRefExpr *DRE = tryToConvertMemberExprToDeclRefExpr(*this, E)) { 
        
               EmitIgnoredExpr(E->getBase()); 
        
               return EmitDeclRefLValue(DRE); 
        
             } 
        
             Expr *BaseExpr = E->getBase(); 
        
             // If this is s.x, emit s as an lvalue.  If it is s->x, emit s as a scalar.

struct S {
  int x, y;
};

using uintptr_t = unsigned long long;
uintptr_t get_offset_of_y_naively() {
  return ((uintptr_t)(&(((S *)nullptr)->y)));
}

The base expr is:

ParenExpr 0x58d207ad2048 'S *'
`-CStyleCastExpr 0x58d207ad2020 'S *' <NoOp>
  `-ImplicitCastExpr 0x58d207ad2008 'S *' <NullToPointer> part_of_explicit_cast
    `-CXXNullPtrLiteralExpr 0x58d207ad1f80 'nullptr_t'

isNullPointerConstant returns false regardless of the value of NullPointerConstantValueDependence.

Do you care at all about patterns like ((uintptr_t)(&(((S *)nullptr)->y.z)));?

Yes. This pattern is used by openssl.

efriedma-quic · 2025-03-13T22:18:14Z

Just ran into some related code: see 3d0a540 . Maybe using IgnoreParenCasts() like that code does is appropriate. (I don't really like using IgnoreParenCasts() because there's such a big variety of casts, but it's probably okay in this context.)

Do you care at all about patterns like ((uintptr_t)(&(((S *)nullptr)->y.z)));?

Yes. This pattern is used by openssl.

Note this has two MemberExprs, so if you want to handle this, you'll have to look through the MemberExpr when you're generating the second GEP, I think.

dtcxzyw · 2025-03-19T13:03:27Z

@efriedma-quic Any more comments?

efriedma-quic

Needs release note.

The big thing I'm not sure about is whether we want a corresponding warning. We're detecting a pattern we know has undefined behavior, so we should maybe warn? CC @AaronBallman @erichkeane

clang/lib/CodeGen/CGExpr.cpp

clang/lib/CodeGen/CGBuilder.h

AaronBallman · 2025-03-20T13:53:39Z

Needs release note.

The big thing I'm not sure about is whether we want a corresponding warning. We're detecting a pattern we know has undefined behavior, so we should maybe warn? CC @AaronBallman @erichkeane

This pattern is undefined behavior, but we're wanting to define the behavior in this one specific case. My preference is to be honest about that: add documentation to clang/docs/LanguageExtensions.rst saying that we define the behavior of dereferencing a null pointer constant in these specific circumstances (and be clear that it's UB in all other circumstances), issue a pedantic warning about the extension, and make sure we have enough test coverage to help us avoid regressing the extension in the future. Bonus points if the pedantic warning comes with a fixit that suggest offsetof (not __builtin_offsetof because that's still an extension), but I don't think it's strictly required (fixits that require including a header file might be tricky?).

clang/docs/ReleaseNotes.rst

AaronBallman · 2025-03-20T13:59:52Z

clang/test/CodeGenCXX/catch-nullptr-and-nonzero-offset-in-offsetof-idiom.cpp

+// CHECK-NEXT:    ret i64 ptrtoint (ptr getelementptr ([[STRUCT_S:%.*]], ptr getelementptr ([[STRUCT_T:%.*]], ptr null, i32 0, i32 1), i32 0, i32 1) to i64)
+//
+uintptr_t get_offset_of_y_naively_nested() {
+  return ((uintptr_t)(&(((T *)nullptr)->s.y)));


It would be good to have tests for other null pointer constants like 0, but there's a question of how deep down the rabbit hole we want the extension to go. I think it makes sense to allow it only for null pointer constants, not just any constant expression which converts to a null pointer. But maybe that's too restrictive?

Suggested tests:

nullptr_t{} // Should probably be accepted because this is a valid null pointer constant constexpr void *null_ptr = nullptr; // Should probably be rejected, null_ptr is not a null pointer constant 0 // Should be accepted because this is a valid null pointer constant (void *)0 // Should only be accepted in C because it's a valid null pointer constant there, but not in C++

and you should also ensure the C tests cover these cases as well.

clang/lib/CodeGen/CGBuilder.h

clang/docs/ReleaseNotes.rst

… type (#134221) If `CreateConstInBoundsGEP2_32` returns a constant null/gep, the cast to GetElementPtrInst will fail. This patch uses two static helpers `GEPOperator::accumulateConstantOffset/GetElementPtrInst::getIndexedType` to infer offset and result type instead of depending on the GEP result. This patch is extracted from #130734.

… and result type (#134221) If `CreateConstInBoundsGEP2_32` returns a constant null/gep, the cast to GetElementPtrInst will fail. This patch uses two static helpers `GEPOperator::accumulateConstantOffset/GetElementPtrInst::getIndexedType` to infer offset and result type instead of depending on the GEP result. This patch is extracted from llvm/llvm-project#130734.

…134269) This patch turns off inbounds/nuw flags for member accesses when `-fwrapv-pointer` is set. Closes #132449. It is required by #130734.

…uct GEPs (#134269) This patch turns off inbounds/nuw flags for member accesses when `-fwrapv-pointer` is set. Closes llvm/llvm-project#132449. It is required by llvm/llvm-project#130734.

…se pointers

dtcxzyw · 2025-04-10T01:57:28Z

Rebased on the top of #134269 and #134221

…lvm#134269) This patch turns off inbounds/nuw flags for member accesses when `-fwrapv-pointer` is set. Closes llvm#132449. It is required by llvm#130734.

efriedma-quic

LGTM

…dress` when the base pointer is null (#130952) See also #130734 for the original motivation. This pattern (`container_of`) is also widely used by real-world programs. Examples: https://github.com/llvm/llvm-project/blob/1d89d7d5d76e391b035f50343e2a4890506c6f2b/llvm/include/llvm/IR/SymbolTableListTraits.h#L77-L87 https://github.com/nodejs/node/blob/a2a53cb728ec5f776ac16879ce0f480a8e838320/src/util-inl.h#L134-L137 https://github.com/search?q=*%29nullptr-%3E*&type=code

…taPointerAddress` when the base pointer is null (#130952) See also llvm/llvm-project#130734 for the original motivation. This pattern (`container_of`) is also widely used by real-world programs. Examples: https://github.com/llvm/llvm-project/blob/1d89d7d5d76e391b035f50343e2a4890506c6f2b/llvm/include/llvm/IR/SymbolTableListTraits.h#L77-L87 https://github.com/nodejs/node/blob/a2a53cb728ec5f776ac16879ce0f480a8e838320/src/util-inl.h#L134-L137 https://github.com/search?q=*%29nullptr-%3E*&type=code

…lvm#134269) This patch turns off inbounds/nuw flags for member accesses when `-fwrapv-pointer` is set. Closes llvm#132449. It is required by llvm#130734.

…se pointers (llvm#130734) In the LLVM middle-end we want to fold `gep inbounds null, idx -> null`: https://alive2.llvm.org/ce/z/5ZkPx- This pattern is common in real-world programs (dtcxzyw/llvm-opt-benchmark#55 (comment)). Generally, it exists in some (actually) unreachable blocks, which is introduced by JumpThreading. However, some old-style offsetof macros are still widely used in real-world C/C++ code (e.g., hwloc/slurm/luajit). To avoid breaking existing code and inconvenience to downstream users, this patch removes the inbounds flag from the struct gep if the base pointer is null.

…dress` when the base pointer is null (llvm#130952) See also llvm#130734 for the original motivation. This pattern (`container_of`) is also widely used by real-world programs. Examples: https://github.com/llvm/llvm-project/blob/1d89d7d5d76e391b035f50343e2a4890506c6f2b/llvm/include/llvm/IR/SymbolTableListTraits.h#L77-L87 https://github.com/nodejs/node/blob/a2a53cb728ec5f776ac16879ce0f480a8e838320/src/util-inl.h#L134-L137 https://github.com/search?q=*%29nullptr-%3E*&type=code

Proof: https://alive2.llvm.org/ce/z/5ZkPx- See also #130734 for the motivation.

…ull` (#130742) Proof: https://alive2.llvm.org/ce/z/5ZkPx- See also llvm/llvm-project#130734 for the motivation.

…130742) Proof: https://alive2.llvm.org/ce/z/5ZkPx- See also llvm#130734 for the motivation.

dtcxzyw requested review from nikic and efriedma-quic March 11, 2025 08:37

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[Clang][CodeGen] Do not set inbounds flag in EmitMemberDataPointerAddress when the base pointer is null #130952

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[Clang][CodeGen] Do not use the GEP result to infer offset and result type #134221

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[Clang][CodeGen] Respect -fwrapv-pointer when emitting struct GEPs #134269

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[Clang][CodeGen] Do not set inbounds flag for struct GEP with null ba…

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…se pointers

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[Clang][CodeGen] Fix test failures.

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dtcxzyw added a commit that referenced this pull request Apr 17, 2025

[InstSimplify] Fold getelementptr inbounds null, idx -> null (#130742)

5a99355

Proof: https://alive2.llvm.org/ce/z/5ZkPx- See also #130734 for the motivation.

llvm-sync bot pushed a commit to arm/arm-toolchain that referenced this pull request Apr 17, 2025

Automerge: [InstSimplify] Fold `getelementptr inbounds null, idx -> n…

9943894

…ull` (#130742) Proof: https://alive2.llvm.org/ce/z/5ZkPx- See also llvm/llvm-project#130734 for the motivation.

lukel97 mentioned this pull request Apr 21, 2025

[InstSimplify] Fold getelementptr inbounds null, idx -> null #130742

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[InstSimplify] Fold getelementptr inbounds null, idx -> null (llvm#…

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…130742) Proof: https://alive2.llvm.org/ce/z/5ZkPx- See also llvm#130734 for the motivation.

IanWood1 pushed a commit to IanWood1/llvm-project that referenced this pull request May 6, 2025

[InstSimplify] Fold getelementptr inbounds null, idx -> null (llvm#…

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…130742) Proof: https://alive2.llvm.org/ce/z/5ZkPx- See also llvm#130734 for the motivation.

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dtcxzyw commented Mar 11, 2025 •

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