x86/uaccess: Tell the compiler that uaccess is unlikely to fault

GCC doesn't realize that get_user(), put_user(), and their __
variants are unlikely to fail.  Tell it.

I noticed this while playing with the C entry code.

 Before:
       text     data      bss       dec    filename
   21828763  5194760  1277952  28301475    vmlinux.baseline

 After:
      text      data      bss       dec    filename
   21828379  5194760  1277952  28301091    vmlinux.new

The generated code shrunk by 384 bytes.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/dc37bed7024319c3004d950d57151fca6aeacf97.1444091584.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index a8df874f3e8825b0ea6bde91909b8b3d7b380ba7..3e911c68876eebf8ae7c8dfe7e4f80b7bd6a0e78 100644 (file)
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -182,7 +182,7 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
                     : "=a" (__ret_gu), "=r" (__val_gu)                 \
                     : "0" (ptr), "i" (sizeof(*(ptr))));                \
        (x) = (__force __typeof__(*(ptr))) __val_gu;                    \
-       __ret_gu;                                                       \
+       __builtin_expect(__ret_gu, 0);                                  \
 })
 
 #define __put_user_x(size, x, ptr, __ret_pu)                   \
@@ -278,7 +278,7 @@ extern void __put_user_8(void);
                __put_user_x(X, __pu_val, ptr, __ret_pu);       \
                break;                                          \
        }                                                       \
-       __ret_pu;                                               \
+       __builtin_expect(__ret_pu, 0);                          \
 })
 
 #define __put_user_size(x, ptr, size, retval, errret)                  \
@@ -401,7 +401,7 @@ do {                                                                        \
 ({                                                             \
        int __pu_err;                                           \
        __put_user_size((x), (ptr), (size), __pu_err, -EFAULT); \
-       __pu_err;                                               \
+       __builtin_expect(__pu_err, 0);                          \
 })
 
 #define __get_user_nocheck(x, ptr, size)                               \
@@ -410,7 +410,7 @@ do {                                                                        \
        unsigned long __gu_val;                                         \
        __get_user_size(__gu_val, (ptr), (size), __gu_err, -EFAULT);    \
        (x) = (__force __typeof__(*(ptr)))__gu_val;                     \
-       __gu_err;                                                       \
+       __builtin_expect(__gu_err, 0);                                  \
 })
 
 /* FIXME: this hack is definitely wrong -AK */