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