linux-kernel - Re: [PATCH v3] ARM: add get_user() support for 8 byte types

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Message-ID: <20140617110908.GH23430@n2100.arm.linux.org.uk>
Date:	Tue, 17 Jun 2014 12:09:08 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Rob Clark <robdclark@...il.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Arnd Bergmann <arnd.bergmann@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org
Subject: Re: [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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