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

Author: dtcxzyw

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

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)));

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)));