Messages in this thread | | | Date | Tue, 17 Jun 2014 14:36:20 +0100 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH v3] ARM: add get_user() support for 8 byte types |
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On Tue, Jun 17, 2014 at 02:28:44PM +0100, Daniel Thompson wrote: > 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).
No need - the + case is your version, the - case is my version. So your version wins on this point. :)
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