i386: Explain why __HAVE_64B_ATOMICS has to be 0

diff --git a/sysdeps/x86/atomic-machine.h b/sysdeps/x86/atomic-machine.h
index 695222e4faa057c262c26314ae28f4a7310a7516..2692d94a929f9bdb600a8c7799a27dcfc1428fc8 100644 (file)
--- a/sysdeps/x86/atomic-machine.h
+++ b/sysdeps/x86/atomic-machine.h
@@ -60,6 +60,10 @@ typedef uintmax_t uatomic_max_t;
 # define BR_CONSTRAINT                 "q"
 # define IBR_CONSTRAINT                        "iq"
 #else
+/* Since the Pentium, i386 CPUs have supported 64-bit atomics, but the
+   i386 psABI supplement provides only 4-byte alignment for uint64_t
+   inside structs, so it is currently not possible to use 64-bit
+   atomics on this platform.  */
 # define __HAVE_64B_ATOMICS            0
 # define SP_REG                                "esp"
 # define SEG_REG                       "gs"