diff --git a/llvm/lib/Analysis/MemoryBuiltins.cpp b/llvm/lib/Analysis/MemoryBuiltins.cpp
--- a/llvm/lib/Analysis/MemoryBuiltins.cpp
+++ b/llvm/lib/Analysis/MemoryBuiltins.cpp
@@ -149,7 +149,9 @@
     {LibFunc_vec_realloc,                       {ReallocLike,      2,  1, -1, -1, MallocFamily::VecMalloc}},
     {LibFunc_reallocf,                          {ReallocLike,      2,  1, -1, -1, MallocFamily::Malloc}},
     {LibFunc_strdup,                            {StrDupLike,       1, -1, -1, -1, MallocFamily::Malloc}},
+    {LibFunc_dunder_strdup,                     {StrDupLike,       1, -1, -1, -1, MallocFamily::Malloc}},
     {LibFunc_strndup,                           {StrDupLike,       2,  1, -1, -1, MallocFamily::Malloc}},
+    {LibFunc_dunder_strndup,                    {StrDupLike,       2,  1, -1, -1, MallocFamily::Malloc}},
     {LibFunc___kmpc_alloc_shared,               {MallocLike,       1,  0, -1, -1, MallocFamily::KmpcAllocShared}},
     // TODO: Handle "int posix_memalign(void **, size_t, size_t)"
 };
diff --git a/llvm/test/Transforms/InstCombine/builtin-object-size-strdup-family.ll b/llvm/test/Transforms/InstCombine/builtin-object-size-strdup-family.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/builtin-object-size-strdup-family.ll
@@ -0,0 +1,51 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare dso_local noalias noundef i8* @malloc(i64 noundef) local_unnamed_addr
+declare i64 @llvm.objectsize.i64.p0i8(i8*, i1 immarg, i1 immarg, i1 immarg)
+declare noalias i8* @strdup(i8*);
+declare noalias i8* @__strdup(i8*);
+declare noalias i8* @strndup(i8*, i64);
+declare noalias i8* @__strndup(i8*, i64);
+
+@str = dso_local constant [11 x i8] c"toulbroc'h\00"
+
+define dso_local i64 @check_strdup(i32 noundef %n) local_unnamed_addr {
+; CHECK-LABEL: @check_strdup(
+; CHECK-NEXT:    ret i64 11
+;
+  %ptr = call noalias i8* @strdup(i8* noundef getelementptr inbounds ([11 x i8], [11 x i8]* @str, i64 0, i64 0))
+  %size = call i64 @llvm.objectsize.i64.p0i8(i8* %ptr, i1 false, i1 true, i1 false)
+  ret i64 %size
+}
+
+define dso_local i64 @check_dunder_strdup(i32 noundef %n) local_unnamed_addr {
+; CHECK-LABEL: @check_dunder_strdup(
+; CHECK-NEXT:    ret i64 11
+;
+  %ptr = call noalias i8* @__strdup(i8* noundef getelementptr inbounds ([11 x i8], [11 x i8]* @str, i64 0, i64 0))
+  %size = call i64 @llvm.objectsize.i64.p0i8(i8* %ptr, i1 false, i1 true, i1 false)
+  ret i64 %size
+}
+
+define dso_local i64 @check_strndup(i32 noundef %n) local_unnamed_addr {
+; CHECK-LABEL: @check_strndup(
+; CHECK-NEXT:    ret i64 5
+;
+  %ptr = call noalias i8* @strndup(i8* noundef getelementptr inbounds ([11 x i8], [11 x i8]* @str, i64 0, i64 0), i64 4)
+  %size = call i64 @llvm.objectsize.i64.p0i8(i8* %ptr, i1 false, i1 true, i1 false)
+  ret i64 %size
+}
+
+define dso_local i64 @check_dunder_strndup(i32 noundef %n) local_unnamed_addr {
+; CHECK-LABEL: @check_dunder_strndup(
+; CHECK-NEXT:    ret i64 5
+;
+  %ptr = call noalias i8* @__strndup(i8* noundef getelementptr inbounds ([11 x i8], [11 x i8]* @str, i64 0, i64 0), i64 4)
+  %size = call i64 @llvm.objectsize.i64.p0i8(i8* %ptr, i1 false, i1 true, i1 false)
+  ret i64 %size
+}