[Qemu-devel] [PATCH 4/6] linux-user: Rewrite __get_user/__put_user with __builtin_choose_expr

[Richard Henderson <rth@twiddle.net>]

The previous formuation with multiple assignments to __typeof(*hptr) falls
down when hptr is qualified const.  E.g. with const struct S *p, p->f is
also qualified const.

With this formulation, there's no assignment to any local variable.

Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 linux-user/qemu.h | 50 ++++++++++++++++++++------------------------------
 1 file changed, 20 insertions(+), 30 deletions(-)

diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index fc4cc00..39b2c9c 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -284,36 +284,26 @@ static inline int access_ok(int type, abi_ulong addr, abi_ulong size)
                             (type == VERIFY_READ) ? PAGE_READ : (PAGE_READ | PAGE_WRITE)) == 0;
 }
 
-/* NOTE __get_user and __put_user use host pointers and don't check access. */
-/* These are usually used to access struct data members once the
- * struct has been locked - usually with lock_user_struct().
- */
-#define __put_user(x, hptr)\
-({ __typeof(*hptr) pu_ = (x);\
-    switch(sizeof(*hptr)) {\
-    case 1: break;\
-    case 2: pu_ = tswap16(pu_); break; \
-    case 4: pu_ = tswap32(pu_); break; \
-    case 8: pu_ = tswap64(pu_); break; \
-    default: abort();\
-    }\
-    memcpy(hptr, &pu_, sizeof(pu_)); \
-    0;\
-})
-
-#define __get_user(x, hptr) \
-({ __typeof(*hptr) gu_; \
-    memcpy(&gu_, hptr, sizeof(gu_)); \
-    switch(sizeof(*hptr)) {\
-    case 1: break; \
-    case 2: gu_ = tswap16(gu_); break; \
-    case 4: gu_ = tswap32(gu_); break; \
-    case 8: gu_ = tswap64(gu_); break; \
-    default: abort();\
-    }\
-    (x) = gu_; \
-    0;\
-})
+/* NOTE __get_user and __put_user use host pointers and don't check access.
+   These are usually used to access struct data members once the struct has
+   been locked - usually with lock_user_struct.  */
+#define __put_user(x, hptr)                                          \
+(*(hptr) =                                                           \
+ __builtin_choose_expr(sizeof(*(hptr)) == 1, (uint8_t)(x),           \
+ __builtin_choose_expr(sizeof(*(hptr)) == 2, tswap16((uint16_t)(x)), \
+ __builtin_choose_expr(sizeof(*(hptr)) == 4, tswap32((uint32_t)(x)), \
+ __builtin_choose_expr(sizeof(*(hptr)) == 8, tswap64((uint64_t)(x)), \
+                       abort())))),                                  \
+ 0)
+
+#define __get_user(x, hptr)                                     \
+((x) =                                                          \
+ __builtin_choose_expr(sizeof(*(hptr)) == 1, *(hptr),           \
+ __builtin_choose_expr(sizeof(*(hptr)) == 2, tswap16(*(hptr)),  \
+ __builtin_choose_expr(sizeof(*(hptr)) == 4, tswap32(*(hptr)),  \
+ __builtin_choose_expr(sizeof(*(hptr)) == 8, tswap64(*(hptr)),  \
+                       abort())))),                             \
+ 0)
 
 /* put_user()/get_user() take a guest address and check access */
 /* These are usually used to access an atomic data type, such as an int,
-- 
1.7.11.4