On 02/03/16 14:25, Renato Golin wrote:
> On 2 March 2016 at 11:35, Edward Nevill <edward.nev...@linaro.org> wrote:
>> cmp x2, 8 <<< (1)
>> (1) If count as a 64 bit unsigned is <= 8 then it is probably still <= 8 as
>> a 32 bit unsigned.
>
> You mean to use "cmp w2, 8" instead? Is there any difference?
>
No, it's code equivalent to
unsigned long x;
if (x <= 8)
{
if ((unsigned) x <= 8)
{
...
}
}
Where the inner test is clearly redundant (for unsigned).
R.
>
>> (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).
>
> My reading of the AAPCS64 is that it's not necessary to have a frame
> at all, only that if you do, it must be quad-word aligned.
>
> Clang/LLVM doesn't seem to bother with the push and pop, but it also
> uses "cmp x".
>
>
>> .L15:
>> adrp x3, .L4
>> add x3, x3, :lo12:.L4
>> ldr x2, [x3, x2, lsl #3]
>> br x2
>
> Hum, this is *exactly* what Clang generates... :)
>
>
>> (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.
>
> Again, Clang seems to do what you want...
>
> Have you tried building OpenJDK with Clang?
>
> cheers,
> --renato
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