Messages in this thread | | | Date | Mon, 8 Aug 2016 21:17:01 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/hweight: Don't clobber %rdi |
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On Mon, Aug 08, 2016 at 08:53:16PM +0200, Jiri Kosina wrote: > Ville's patch is the proper thing to do; I still think the comments > probably should be removed as they might provide more confusion than help, > but that's a separate discussion.
Right, so while trying to debug this with a pen and paper, I found that adding a subsript to the w variable makes it more understandable. I.e., something like this:
w_1 -= (w >> 1) & 0x55555555; w_2 = (w_1 & 0x33333333) + ((w_1 >> 2) & 0x33333333);
and then you'd have
movl %eax, %edx # w shrl %edx # w >>= 1 andl $0x55555555, %edx # (w >> 1) & 0x55555555 subl %edx, %eax # w_1 = w - ...
movl %eax, %edx # w_1 shrl $2, %eax # w_1 >>= 2 andl $0x33333333, %edx # (w_1 >> 2) & 0x33333333 andl $0x33333333, %eax # w_1 &= 0x33333333 addl %edx, %eax # w_2 = ... + ...
...
Anyway, something like that. I find that more readable TBH.
Thanks and thanks for confirming Ville's patch fixes it on your box.
-- Regards/Gruss, Boris.
ECO tip #101: Trim your mails when you reply. --
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