[Bug breakpoints/12774] New: displaced stepping optimization

[Bug breakpoints/12774] New: displaced stepping optimization

[tromey at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12774

           Summary: displaced stepping optimization
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: breakpoints
        AssignedTo: unassigned@sourceware.org
        ReportedBy: tromey@redhat.com


uprobes has an optimization for 'nop' when doing displaced stepping.
It seems to me that gdb could benefit (perhaps only minorly) from
this optimization as well.  In particular, systemtap probe points will
always be on a nop.

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[Bug breakpoints/12774] displaced stepping optimization

[tromey at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=12774

--- Comment #1 from Tom Tromey <tromey at redhat dot com> ---
My notes say that at one point Josh helpfully
provided a list of standard no-op sequences for x86.

<jistone> tromey, intel's recommendation is:
<jistone> Length
<jistone> Assembly Byte Sequence
<jistone> 2 bytes 66 NOP 66 90H
<jistone> 3 bytes NOP DWORD ptr [EAX] 0F 1F 00H
<jistone> 4 bytes NOP DWORD ptr [EAX + 00H] 0F 1F 40 00H
<jistone> 5 bytes NOP DWORD ptr [EAX + EAX*1 + 00H] 0F 1F 44 00 00H
<jistone> 6 bytes 66 NOP DWORD ptr [EAX + EAX*1 + 00H] 66 0F 1F 44 00 00H
<jistone> 7 bytes NOP DWORD ptr [EAX + 00000000H] 0F 1F 80 00 00 00 00H
<jistone> 8 bytes NOP DWORD ptr [EAX + EAX*1 + 00000000H] 0F 1F 84 00 00 00 00
      00H
<jistone> 9 bytes 66 NOP DWORD ptr [EAX + EAX*1 + 66 0F 1F 84 00 00 00 00
<jistone>         00000000H] 00H

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