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Re: [PATCH, i386] RTM support


On 02/20/2012 01:51 PM, Uros Bizjak wrote:
On Mon, Feb 20, 2012 at 7:43 PM, Richard Henderson<rth@redhat.com> 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.

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?

In the current implementation, yes. It tests the rax value to know if it should execute the fallback or not.
--
Patrick.




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