On Fri, Nov 30, 2018 at 9:57 AM Jose Abreu <jose.abreu@xxxxxxxxxxxx> wrote:
> On 29-11-2018 21:20, Arnd Bergmann wrote:
> > On Thu, Nov 29, 2018 at 5:14 PM Jose Abreu <jose.abreu@xxxxxxxxxxxx> wrote:
> >> See how the if condition added in this version is checked in
> >> <test_readsl+0xe92> and then it takes two different loops.
> > This looks good to me. I wonder what the result is for CPUs
> > that /do/ support unaligned accesses. Normally put_unaligned()
> > should fall back to a simple store in that case, but I'm not
> > sure it can fold the two stores back into one and skip the
> > alignment check. Probably not worth overoptimizing for that
> > case (the MMIO access latency should be much higher than
> > anything you could gain here), but I'm still curious about
> > how well our get/put_unaligned macros work.
>
> Here is disassembly for an ARC CPU that supports unaligned accesses:
>
> -->8---
> 00000d48 <test_readsl>:
> d48: breq_s r1,0,28 /* if (count) */
> d4a: tst r0,0x3
> d4e: bne_s 32 /* if (bptr % ((t) / 8)) */
>
> d50: ld r2,[0xdeadbeef] /* first loop */
> d58: sub_s r1,r1,0x1
> d5a: tst_s r1,r1
> d5c: bne.d -12
> d60: st.ab r2,[r0,4]
>
> d64: dmb 0x1 /* common exit point */
> d68: j_s [blink]
> d6a: nop_s
>
> d6c: ld r2,[0xdeadbeef] /* second loop */
> d74: sub_s r1,r1,0x1
> d76: tst_s r1,r1
> d78: bne.d -12
> d7c: st.ab r2,[r0,4]
>
> d80: b_s -28 /* jmp to 0xd64 */
> d82: nop_s
> --->8---
>
> Notice how first and second loop are exactly equal ...
Ok, so it's halfway there: it managed to optimize the the unaligned
case correctly, but it failed to notice that both sides are
identical now.
Arnd
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