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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
