On 17/06/14 12:09, Russell King - ARM Linux wrote:
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.
Good point.
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
The later case avoids allocating r3 for the __get_user_x and should reduce register pressure and, potentially, saves a few instructions elsewhere (one of my rather large test functions does demonstrate this effect).
I don't know if we care about that. If we do I'm certainly happy to put a patch together than exploits this (whilst avoiding the add in the big endian case).
Daniel.