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Re: [PATCH, i386] RTM support


On Mon, Feb 20, 2012 at 7:31 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Mon, Feb 20, 2012 at 07:27:48PM +0100, Uros Bizjak wrote:
>> IIUC the documentation, the fallback label is a parameter to xbegin
>> insn, but the insn itself doesn't jump anywhere - it just records the
>> parameter as a fallback address. However, there is no guarantee that
>> the fallback code is exactly at (pc)+6, so we have to use asm labels
>> here.
>
> 6 bytes is the length of the xbegin instruction, so xbegin .+6
> says that it has the fallback address at the immediately next insn
> after xbegin. ?Which is the _xbegin () semantics.

No! From 319433-012a.pdf, page 523, it says that

tempRIP = RIP + SignExtend (IMM),

where RIP is instruction following XBEGIN instruction.

cmp and jcc that follows xbegin are 5 bytes long...

Uros.


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