Replying to old thread, for full context see
http://thread.gmane.org/gmane.linux.ports.arm.kernel/180226/focus=197914
On Tue, Nov 06, 2012 at 11:02:27AM +1300, Michael Hope wrote:
> On 6 November 2012 02:48, Rob Herring <robherri...@gmail.com> wrote:
> >
> > I tried adding -munaligned-accesses on a v6 build and still get byte
> > accesses rather than unaligned word accesses. So this does seem to be a
> > v7 only issue based on what gcc will currently produce. Copying Michael
> > Hope who can hopefully provide some insight on why v6 unaligned accesses
> > are not enabled.
>
> This looks like a bug. Unaligned access is enabled for armv6 but
> seems to only take effect for cores with Thumb-2. Here's a test case
> both with unaligned field access and unaligned block copy:
>
> struct foo
> {
> char a;
> int b;
> struct
> {
> int x[3];
> } c;
> } __attribute__((packed));
>
> int get_field(struct foo *p)
> {
> return p->b;
> }
>
> int copy_block(struct foo *p, struct foo *q)
> {
> p->c = q->c;
> }
>
> With -march=armv7-a you get the correct:
>
> bar:
> ldr r0, [r0, #1] @ unaligned @ 11 unaligned_loadsi/2
> [length = 4]
> bx lr @ 21 *arm_return [length = 12]
>
> baz:
> str r4, [sp, #-4]! @ 25 *push_multi [length = 4]
> mov r2, r0 @ 2 *arm_movsi_vfp/1 [length = 4]
> ldr r4, [r1, #5]! @ unaligned @ 9 unaligned_loadsi/2
> [length = 4]
> ldr ip, [r1, #4] @ unaligned @ 10 unaligned_loadsi/2
> [length = 4]
> ldr r1, [r1, #8] @ unaligned @ 11 unaligned_loadsi/2
> [length = 4]
> str r4, [r2, #5] @ unaligned @ 12 unaligned_storesi/2
> [length = 4]
> str ip, [r2, #9] @ unaligned @ 13 unaligned_storesi/2
> [length = 4]
> str r1, [r2, #13] @ unaligned @ 14 unaligned_storesi/2
> [length = 4]
> ldmfd sp!, {r4}
> bx lr
>
> With -march=armv6 you get a byte-by-byte field access and a correct
> unaligned block copy:
>
> bar:
> ldrb r1, [r0, #2] @ zero_extendqisi2
> ldrb r3, [r0, #1] @ zero_extendqisi2
> ldrb r2, [r0, #3] @ zero_extendqisi2
> ldrb r0, [r0, #4] @ zero_extendqisi2
> orr r3, r3, r1, asl #8
> orr r3, r3, r2, asl #16
> orr r0, r3, r0, asl #24
> bx lr
>
> baz:
> str r4, [sp, #-4]!
> mov r2, r0
> ldr r4, [r1, #5]! @ unaligned
> ldr ip, [r1, #4] @ unaligned
> ldr r1, [r1, #8] @ unaligned
> str r4, [r2, #5] @ unaligned
> str ip, [r2, #9] @ unaligned
> str r1, [r2, #13] @ unaligned
> ldmfd sp!, {r4}
> bx lr
>
> readelf -A shows that the compiler planned to use unaligned access in
> both. My suspicion is that GCC is using the extv pattern to extract
> the field from memory, and that pattern is only enabled for Thumb-2
> capable cores.
>
> I've logged PR55218. We'll discuss it at our next meeting.
Just tried with gcc-linaro-4.7-2013.01 (gcc-4.7.3 20130102 (prerelease)),
the issue is still unfixed. Do you have any idea how to fix it?
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55218
Thanks,
Johannes
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