https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/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. 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. >From 02065e86c63ab3fbdefd2ce6e963ffeec96e6a24 Mon Sep 17 00:00:00 2001 From: Yingwei Zheng <dtcxzyw2...@gmail.com> Date: Tue, 11 Mar 2025 16:20:08 +0800 Subject: [PATCH] [Clang][CodeGen] Do not set inbounds flag for struct GEP with null base pointers --- clang/lib/CodeGen/CGBuilder.h | 15 ++++++++++----- ...nullptr-and-nonzero-offset-in-offsetof-idiom.c | 2 +- ...llptr-and-nonzero-offset-in-offsetof-idiom.cpp | 2 +- 3 files changed, 12 insertions(+), 7 deletions(-) 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))); _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
