microoptimising atomic memory ops

[Michael Hope]

(I've logged this as a potential speed improvement at LP: #681138 so
we don't lose it)

-- Michael

On Thu, Nov 25, 2010 at 8:29 AM, Peter Maydell <peter.maydell at linaro.org> wrote:
> This wiki page came up during the toolchain call:
> https://wiki.linaro.org/Internal/People/KenWerner/AtomicMemoryOperations/
>
> It gives the code generated for __sync_val_compare_and_swap
> as including a push {r4} / pop {r4} pair because it uses too many
> temporaries to fit them all in callee-saves registers. I think you
> can tweak it a bit to get rid of that:
>
> # int __sync_val_compare_and_swap (int *mem, int old, int new);
> # if the current value of *mem is old, then write new into *mem
> # r0: mem, r1 old, r2 new
>        mov     r3, r0       # move r0 into r3
>        dmb     sy           # full memory barrier
>        .LSYT7:
>        ldrex   r0, [r3]     # load (exclusive) from memory pointed to
> by r3 into r0
>        cmp     r0, r1       # compare contents of r0 (mem) with r1
> (old) -> updates the condition flag
>        bne     .LSYB7       # branch to LSYB7 if mem != old
>        # This strex trashes the r0 we just loaded, but since we didn't take
>        # the branch we know that r0 == r1
>        strex   r0, r2, [r3] # store r2 (new) into  memory pointed to
> by r3 (mem)
>                             # r0 contains 0 if the store was
> successful, otherwise 1
>        teq     r0, #0       # compares contents of r0 with zero ->
> updates the condition flag
>        bne     .LSYT7       # branch to LSYT7 if r0 != 0 (if the
> store wasn't successful)
>        # Move the value that was in memory into the right register to return it
>        mov     r0, r1
>        dmb     sy           # full memory barrier
>        .LSYB7:
>        bx      lr           # return
>
> I think you can do a similar trick with __sync_fetch_and_add
> (although you have to use a subtract to regenerate r0 from
> r1 and r2).
>
> On the other hand I just looked at the gcc code that does this
> and it's not simply dumping canned sequences out to the
> assembler, so maybe it's not worth the effort just to drop a
> stack push/pop.
>
> -- PMM
>
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