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? Uros.
