On 02/20/12 10:35, Uros Bizjak wrote: > On Mon, Feb 20, 2012 at 7:31 PM, Jakub Jelinek <ja...@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.
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. r~
