[Qemu-devel] [PATCH 6/8] linux-user: Rewrite __get_user/__put_user with __builtin_choose_expr

[Richard Henderson]

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 <address@hidden>
---
 linux-user/qemu.h | 63 +++++++++++++++++++++++++++++--------------------------
 1 file changed, 33 insertions(+), 30 deletions(-)

diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index 8a3538c..31a220a 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -287,36 +287,39 @@ 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.  */
+
+/* Tricky points:
+   - Use __builtin_choose_expr to avoid type promotion from ?:,
+   - Invalid sizes result in a compile time error stemming from
+     the fact that abort has no parameters.
+   - It's easier to use the endian-specific unaligned load/store
+     functions than host-endian unaligned load/store plus tswapN.  */
+
+#define __put_user_e(x, hptr, e)                                        \
+  (__builtin_choose_expr(sizeof(*(hptr)) == 1, stb_p,                   \
+   __builtin_choose_expr(sizeof(*(hptr)) == 2, stw_##e##_p,             \
+   __builtin_choose_expr(sizeof(*(hptr)) == 4, stl_##e##_p,             \
+   __builtin_choose_expr(sizeof(*(hptr)) == 8, stq_##e##_p, abort))))   \
+     ((hptr), (x)), 0)
+
+#define __get_user_e(x, hptr, e)                                        \
+  ((x) =                                                                \
+   __builtin_choose_expr(sizeof(*(hptr)) == 1, ldub_p,                  \
+   __builtin_choose_expr(sizeof(*(hptr)) == 2, lduw_##e##_p,            \
+   __builtin_choose_expr(sizeof(*(hptr)) == 4, ldl_##e##_p,             \
+   __builtin_choose_expr(sizeof(*(hptr)) == 8, ldq_##e##_p, abort))))   \
+     (hptr), 0)
+
+#ifdef TARGET_WORDS_BIGENDIAN
+# define __put_user(x, hptr)  __put_user_e(x, hptr, be)
+# define __get_user(x, hptr)  __get_user_e(x, hptr, be)
+#else
+# define __put_user(x, hptr)  __put_user_e(x, hptr, le)
+# define __get_user(x, hptr)  __get_user_e(x, hptr, le)
+#endif
 
 /* 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,
-- 
1.7.11.7