[PATCH v3] ARM: add get_user() support for 8 byte types

On Tue, Jun 17, 2014 at 11:17:23AM +0100, Daniel Thompson wrote:

... at this point there is a narrowing cast followed by an implicit
widening. This results in compiler either ignoring r3 altogether or, if
spilling to the stack, generating code to set r3 to zero before doing
the store.

In actual fact, there's very little difference between the two
implementations in terms of generated code.

The difference between them is what happens on the 64-bit big endian
narrowing case, where we use __get_user_4 with your version.  This
adds one additional instruction.

The little endian case results in identical code except for register
usage - for example, with my test for a 32-bit being widened to 64-bit:

        str     lr, [sp, #-4]!
-       mov     r3, r0
+       mov     ip, r0
        mov     r0, r1
 #APP
 @ 280 "t-getuser.c" 1
        bl      __get_user_4
 @ 0 "" 2
-       str     r2, [r3, #0]
-       mov     r2, #0
-       str     r2, [r3, #4]
+       mov     r3, #0
+       str     r2, [ip, #0]
+       str     r3, [ip, #4]
        ldr     pc, [sp], #4

and 64-bit narrowed to 32-bit:

        str     lr, [sp, #-4]!
-       mov     ip, r0
+       mov     r3, r0
        mov     r0, r1
 #APP
 @ 275 "t-getuser.c" 1
-       bl      __get_user_8
+       bl      __get_user_4
 @ 0 "" 2
-       str     r2, [ip, #0]
+       str     r2, [r3, #0]
        ldr     pc, [sp], #4

In terms of type checking, both seem to get it correct (which is something
I'm concerned about by any implementation since this is just as important
as the generated code).

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