Improve cmpxchg8b inline asm configure test

2 files changed, 15 insertions, 5 deletions

diff --git a/erts/include/internal/ethread_header_config.h.in b/erts/include/internal/ethread_header_config.h.in
index 9cabd0591a..f4b08cfced 100644
--- a/erts/include/internal/ethread_header_config.h.in
+++ b/erts/include/internal/ethread_header_config.h.in
@@ -166,6 +166,10 @@
 /* Define if you use a gcc that supports the double word cmpxchg instruction */
 #undef ETHR_GCC_HAVE_DW_CMPXCHG_ASM_SUPPORT
 
+/* Define if gcc wont let you clobber ebx with cmpxchg8b and position
+   independent code */
+#undef ETHR_CMPXCHG8B_PIC_NO_CLOBBER_EBX
+
 /* Define if you get a register shortage with cmpxchg8b and position independent code */
 #undef ETHR_CMPXCHG8B_REGISTER_SHORTAGE
 
diff --git a/erts/include/internal/i386/ethr_dw_atomic.h b/erts/include/internal/i386/ethr_dw_atomic.h
index e8c4119ef0..79de5d80da 100644
--- a/erts/include/internal/i386/ethr_dw_atomic.h
+++ b/erts/include/internal/i386/ethr_dw_atomic.h
@@ -115,13 +115,19 @@ ethr_native_dw_atomic_addr(ethr_native_dw_atomic_t *var)
     return (ethr_sint_t *) ETHR_DW_NATMC_MEM__(var);
 }
 
-#if ETHR_SIZEOF_PTR == 4 && defined(__PIC__) && __PIC__
+#if defined(ETHR_CMPXCHG8B_PIC_NO_CLOBBER_EBX) && defined(__PIC__) && __PIC__
+#if ETHR_SIZEOF_PTR != 4
+#  error unexpected pic issue
+#endif
 /*
  * When position independent code is used in 32-bit mode, the EBX register
- * is used for storage of global offset table address, and we may not
- * use it as input or output in an asm. We need to save and restore the
- * EBX register explicitly (for some reason gcc doesn't provide this
- * service to us).
+ * is used for storage of global offset table address. When compiling with
+ * an old gcc (< vsn 5) we may not use it as input or output in an inline
+ * asm. We then need to save and restore the EBX register explicitly (for
+ * some reason old gcc compilers didn't provide this service to us).
+ * ETHR_CMPXCHG8B_PIC_NO_CLOBBER_EBX will be defined if we need to
+ * explicitly manage EBX ourselves.
+ *
  */
 #  define ETHR_NO_CLOBBER_EBX__ 1
 #else