Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH 2/2] lib/string.c: Optimize memchr() | Date | Wed, 13 Jul 2022 09:39:43 +0000 |
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From: Yu-Jen Chang > Sent: 12 July 2022 15:59 ... > > I think you're missing the point. Loads at unaligned addresses may not > > be allowed by hardware using conventional load instructions or may be > > inefficient. Given that this memchr implementation is used as a fallback > > when no hardware-specific version is available, you should be > > conservative wrt. hardware capabilities and behavior. You should > > probably have a pre-alignment loop. > > Got it. I add pre-alignment loop. It aligns the address to 8 or 4bytes.
That should be predicated on !HAS_EFFICIENT_UNALIGNED_ACCESS.
... > for (; p <= end - 8; p += 8) { > val = *(u64*)p ^ mask; > if ((val + 0xfefefefefefefeffull) > & (~val & 0x8080808080808080ull)) > break;
I would add a couple of comments, like: // Convert to check for zero byte. // Standard check for a zero byte in a word. (But not the big 4 line explanation you had.
It is also worth looking at how that code compiles on 32bit arch that don't have a carry flag. That is everything based on MIPS, including riscv.
David
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