From 62bc7b7c4247a62c23ea93cd960c3c0434925c49 Mon Sep 17 00:00:00 2001
From: Kim Barrett <kbarrett@openjdk.org>
Date: Thu, 4 Sep 2025 05:42:18 +0000
Subject: [PATCH] 8300080: offset_of for GCC/Clang exhibits undefined behavior
 and is not always a compile-time constant

Reviewed-by: stefank, jsjolen
---
 make/hotspot/lib/CompileJvm.gmk                |  4 +++-
 .../share/utilities/globalDefinitions.hpp      |  3 +++
 .../share/utilities/globalDefinitions_gcc.hpp  | 18 ------------------
 .../utilities/globalDefinitions_visCPP.hpp     |  2 --
 4 files changed, 6 insertions(+), 21 deletions(-)

diff --git a/make/hotspot/lib/CompileJvm.gmk b/make/hotspot/lib/CompileJvm.gmk
index 6b5edc85b23..bf92c50576a 100644
--- a/make/hotspot/lib/CompileJvm.gmk
+++ b/make/hotspot/lib/CompileJvm.gmk
@@ -97,11 +97,13 @@ CFLAGS_VM_VERSION := \
 
 DISABLED_WARNINGS_gcc := array-bounds comment delete-non-virtual-dtor \
     empty-body format-zero-length implicit-fallthrough int-in-bool-context \
+    invalid-offsetof \
     maybe-uninitialized missing-field-initializers \
     shift-negative-value unknown-pragmas unused-but-set-variable \
     unused-local-typedefs unused-variable
 
-DISABLED_WARNINGS_clang := delete-non-abstract-non-virtual-dtor missing-braces \
+DISABLED_WARNINGS_clang := delete-non-abstract-non-virtual-dtor \
+    invalid-offsetof missing-braces \
     sometimes-uninitialized unknown-pragmas unused-but-set-variable \
     unused-function unused-local-typedef unused-private-field unused-variable
 
diff --git a/src/hotspot/share/utilities/globalDefinitions.hpp b/src/hotspot/share/utilities/globalDefinitions.hpp
index 1ef548d6510..64ccae539a3 100644
--- a/src/hotspot/share/utilities/globalDefinitions.hpp
+++ b/src/hotspot/share/utilities/globalDefinitions.hpp
@@ -85,6 +85,9 @@ class oopDesc;
 // they cannot be defined and potential callers will fail to compile.
 #define NONCOPYABLE(C) C(C const&) = delete; C& operator=(C const&) = delete /* next token must be ; */
 
+// offset_of was a workaround for UB with offsetof uses that are no longer an
+// issue.  This can be removed once all uses have been converted.
+#define offset_of(klass, field) offsetof(klass, field)
 
 //----------------------------------------------------------------------------------------------------
 // Printf-style formatters for fixed- and variable-width types as pointers and
diff --git a/src/hotspot/share/utilities/globalDefinitions_gcc.hpp b/src/hotspot/share/utilities/globalDefinitions_gcc.hpp
index 41b7868f8c7..f82f9b1386a 100644
--- a/src/hotspot/share/utilities/globalDefinitions_gcc.hpp
+++ b/src/hotspot/share/utilities/globalDefinitions_gcc.hpp
@@ -84,24 +84,6 @@ inline int g_isnan(double f) { return isnan(f); }
 inline int g_isfinite(jfloat  f)                 { return isfinite(f); }
 inline int g_isfinite(jdouble f)                 { return isfinite(f); }
 
-
-// gcc warns about applying offsetof() to non-POD object or calculating
-// offset directly when base address is null. The -Wno-invalid-offsetof
-// option could be used to suppress this warning, but we instead just
-// avoid the use of offsetof().
-//
-// FIXME: This macro is complex and rather arcane. Perhaps we should
-// use offsetof() instead, with the invalid-offsetof warning
-// temporarily disabled.
-#define offset_of(klass,field)                          \
-([]() {                                                 \
-  alignas(16) char space[sizeof (klass)];               \
-  klass* dummyObj = (klass*)space;                      \
-  char* c = (char*)(void*)&dummyObj->field;             \
-  return (size_t)(c - space);                           \
-}())
-
-
 #if defined(_LP64) && defined(__APPLE__)
 #define JLONG_FORMAT          "%ld"
 #define JLONG_FORMAT_W(width) "%" #width "ld"
diff --git a/src/hotspot/share/utilities/globalDefinitions_visCPP.hpp b/src/hotspot/share/utilities/globalDefinitions_visCPP.hpp
index 536b79165ae..b9d25096cd5 100644
--- a/src/hotspot/share/utilities/globalDefinitions_visCPP.hpp
+++ b/src/hotspot/share/utilities/globalDefinitions_visCPP.hpp
@@ -85,8 +85,6 @@ inline int g_isnan(jdouble f)                    { return _isnan(f); }
 inline int g_isfinite(jfloat  f)                 { return _finite(f); }
 inline int g_isfinite(jdouble f)                 { return _finite(f); }
 
-#define offset_of(klass,field) offsetof(klass,field)
-
 #define THREAD_LOCAL __declspec(thread)
 
 // Inlining support