From: Arnd Bergmann > Sent: 30 November 2018 13:44 > > 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. There're even identical opcodes... The barrier() (etc) in the asm output probably stopped the optimisation. It also seems to have used a different type of loop to the other example, probably less efficient. (Not that I'm an expert on ARC opcodes.) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales) _______________________________________________ linux-snps-arc mailing list linux-snps-arc@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/linux-snps-arc
