Re: [PATCH v7 2/5] x86/asm: add volatile, clobbers and zero-length check in inline memcmp

[David Laight <david.laight.linux@xxxxxxxxx>]

On Thu, 23 Jul 2026 20:49:13 -0400
Brian Gerst <brgerst@xxxxxxxxx> wrote:

...
> I had originally thought that the test for RCX==0 was also at the end
> of the loop, after the decrement.  But a closer look at the
> pseudo-code in the SDM does show that the test is at the start of the
> loop.  Starting with RCX==0 does indeed skip the comparison and does
> not touch ZF.  Your suggestion is good, but should have a comment
> explaining why it's there to someone reading the code in the future.

I have a feeling that was a change between the 8086 and 80286.
I don't have an 8086 book (any more) but I'm sure I remember fixing
bugs where %cx of zero caused 65536 iterations.
(Fun to find because the vector table is 'first in the line to be
overwritten.)

        David