On 2 March 2016 at 12:35, Edward Nevill <edward.nev...@linaro.org> wrote:
> Hi,
>
> I have just switched to gcc 5.2 from 4.9.2 and the code quality does seem to
> have improved significantly. For example, it now seems much better at using
> ldp/stp and it seems to has stopped gratuitous use of the SIMD registers.
Hi Ed,
Thanks for the feedback.
Can you be more specific on the GCC versions you are using?
Do you mean Linaro TCWG releases?
>
> However, I still have a few whinges:-)
>
> See attached copy.c / copy.s (This is a performance critical function from
> OpenJDK)
>
> pd_disjoint_words:
> cmp x2, 8 <<< (1)
> sub sp, sp, #64 <<< (2)
> bhi .L2
> cmp w2, 8 <<< (1)
> bls .L15
> .L2:
> add sp, sp, 64 <<< (2)
>
> (1) If count as a 64 bit unsigned is <= 8 then it is probably still <= 8 as a
> 32 bit unsigned.
>
> (2) Nowhere in the function does it store anything on the stack, so why
> drop and restore the stack every time. Also, minor quibble in the
> disass, why does sub use #64 whereas add uses just '64' (appreciate this
> is probably binutils, not gcc).
>
> .L15:
> adrp x3, .L4
> add x3, x3, :lo12:.L4
> ldrb w2, [x3,w2,uxtw] <<< (3)
> adr x3, .Lrtx4
> add x2, x3, w2, sxtb #2
> br x2
>
> (3) Why use a byte table, this is not some sort of embedded system. Use
> a word table and this becomes.
>
> .L15:
> adrp x3, .L4
> add x3, x3, :lo12:.L4
> ldr x2, [x3, x2, lsl #3]
> br x2
>
> An aligned word load takes exactly the same time as a byte load and we
> save the faffing about calculating the address.
>
> .L10:
> ldp x6, x7, [x0]
> ldp x4, x5, [x0, 16]
> ldp x2, x3, [x0, 32] <<< (4)
> stp x2, x3, [x1, 32] <<< (4)
> stp x6, x7, [x1]
> stp x4, x5, [x1, 16]
>
> (4) Seems to be something wrong with the load scheduler here? Why not
> move the stp x2, x3 to the end. It does this repeatedly.
>
> Unfortunately as this function is performance critical it means I will
> probably end up doing it in inline assembler which is time consuming,
> error prone and non portable.
>
> * Whinge mode off
>
I've just tried with our 5.3-2015.12 snapshot, and all your comments
except (4) are still valid.
The same is true with today's trunk. FWIW, (4) now looks like this:
.L10:
ldp x6, x7, [x0]
ldp x4, x5, [x0, 16]
ldp x2, x3, [x0, 32]
stp x6, x7, [x1]
stp x4, x5, [x1, 16]
stp x2, x3, [x1, 32]
Christophe
> Ed
>
>
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