[Valgrind-developers] Re: lazy EIP updating sorta working (xedit runs!)

[Julian S. <js...@ac...>]

Hi.  Let me say at the outset that I think your patch (62-) is a much
better solution than mine; it makes it less likely to get things wrong
later on, and extends naturally to skins.  So I like that.

And the performance improvements really are excellent.

However -- I'm getting almost all progs segfault at exit, if not before.

I ran my canonical inner-loop program and looked at the code.  I'm worried
by this:

          47: SUBL      $0x3E7, %eax  (-wOSZACP)                [------]
                159: 81 E8 E7 03 00 00 
                subl    $0x3E7, %eax
          48: INCEIPo   $6              [------]
                165: C6 45 24 37 
                movb    $0x37, 0x24(%ebp)
          49: Jleo      $0x8048508  (-rOSZACP)          [------]
                169: 9C 8F 45 20 
                pushfl ; popl 32(%ebp)
                173: 7F 0D 
                jnle-8  %eip+13
                175: B8 08 85 04 08 
                movl    $0x8048508, %eax
                180: 89 45 24 
                movl    %eax, 0x24(%ebp)
                183: 0F 0B 0F 0B 90 
                ud2; ud2; nop
          50: JMPo      $0x8048539  ($2)                [------]
                188: B8 39 85 04 08 
                movl    $0x8048539, %eax
                193: 89 45 24 
                movl    %eax, 0x24(%ebp)
                196: 0F 0B 0F 0B 90 
                ud2; ud2; nop

The SUBL is the first simd-flag-affecting fn in the block.  So I see what your
scheme does is to note that we are setting the simd flags here, so it lets 
the generated subl set %eflags; the Jleo then copies this to %EFLAGS with
pushfl ; popl 32(%ebp).

Problem is (according to my analysis) is that subl sets O S Z A C and P, but 
it doesn't set D (the string-op direction flag).  Result is that the 
subsequent %EFLAGS := %eflags copy means that the sim'd flags state winds 
up holding the real machine'd D-flag state prior to the subl, which is 
unknown to us.

Am I missing something here?  I'd love to be, considering the speed gains :)

J