MN10300: gcc 4.6 vs am33 inline assembly
Richard Henderson [Wed, 23 Mar 2011 17:42:49 +0000 (17:42 +0000)]
GCC 4.6 explicitly represents the MDR register.  It may be accessed
via the "z" constraint.  Perhaps more importantly, it tracks when
the MDR register is clobbered and uses the RETF instruction if the
incoming value is still valid.

Thus it is important to (at least) clobber the MDR register in
relevant inline assembly fragments, lest RETF be used incorrectly.

The only instances I could find are here.  There are reads of the
MDR register in kernel/gdb-stub.c, but that's harmless.  Although,
frankly, __builtin_return_address(0) might be a better thing in
those cases.  Certainly MDR isn't going to contain anything else
that might be useful...

Signed-off-by: Richard Henderson <rth@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>

arch/mn10300/include/asm/div64.h

index 34dcb8e..503efab 100644 (file)
 extern void ____unhandled_size_in_do_div___(void);
 
 /*
+ * Beginning with gcc 4.6, the MDR register is represented explicitly.  We
+ * must, therefore, at least explicitly clobber the register when we make
+ * changes to it.  The following assembly fragments *could* be rearranged in
+ * order to leave the moves to/from the MDR register to the compiler, but the
+ * gains would be minimal at best.
+ */
+#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
+# define CLOBBER_MDR_CC                "mdr", "cc"
+#else
+# define CLOBBER_MDR_CC                "cc"
+#endif
+
+/*
  * divide n by base, leaving the result in n and returning the remainder
  * - we can do this quite efficiently on the MN10300 by cascading the divides
  *   through the MDR register
@@ -29,7 +42,7 @@ extern void ____unhandled_size_in_do_div___(void);
                    "mov        mdr,%1  \n"                             \
                    : "+r"(n), "=d"(__rem)                              \
                    : "r"(base), "1"(__rem)                             \
-                   : "cc"                                              \
+                   : CLOBBER_MDR_CC                                    \
                    );                                                  \
        } else if (sizeof(n) <= 8) {                                    \
                union {                                                 \
@@ -48,7 +61,7 @@ extern void ____unhandled_size_in_do_div___(void);
                    : "=d"(__rem), "=r"(__quot.w[1]), "=r"(__quot.w[0]) \
                    : "r"(base), "0"(__rem), "1"(__quot.w[1]),          \
                      "2"(__quot.w[0])                                  \
-                   : "cc"                                              \
+                   : CLOBBER_MDR_CC                                    \
                    );                                                  \
                n = __quot.l;                                           \
        } else {                                                        \
@@ -72,7 +85,7 @@ unsigned __muldiv64u(unsigned val, unsigned mult, unsigned div)
                                         * MDR = MDR:val%div */
            : "=r"(result)
            : "0"(val), "ir"(mult), "r"(div)
-           : "cc"
+           : CLOBBER_MDR_CC
            );
 
        return result;
@@ -93,7 +106,7 @@ signed __muldiv64s(signed val, signed mult, signed div)
                                         * MDR = MDR:val%div */
            : "=r"(result)
            : "0"(val), "ir"(mult), "r"(div)
-           : "cc"
+           : CLOBBER_MDR_CC
            );
 
        return result;