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Re: [PATCH, i386] RTM support
On Mon, Feb 20, 2012 at 7:43 PM, Richard Henderson <firstname.lastname@example.org> 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
>>> 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.
> So? ?.+N is generic assembler syntax, not specifying IMM=6.
> With "xbegin .+6" the assembler will of course encode IMM=0,
> because it knows that the xbegin insn is 6 bytes.
Is the "fallback code" the insn just after the xbegin insn?