[Bug binutils/19660] [libopcodes] [x86] REP prefixes shown incorrectly

[nholcomb at wisc dot edu]

https://sourceware.org/bugzilla/show_bug.cgi?id=19660

--- Comment #6 from nholcomb at wisc dot edu ---
The fact that the rep prefixes cannot be used with these opcodes is not the
same as machine dependencies. This instruction will be invalid everywhere, so
it should be decoded as invalid.

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[Bug binutils/19660] [libopcodes] [x86] REP prefixes shown incorrectly

[hjl.tools at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=19660

H.J. Lu  changed:

   What|Removed |Added

 Status|WAITING |RESOLVED
 Resolution|--- |WORKSFORME

--- Comment #5 from H.J. Lu  ---
(In reply to njholcomb from comment #4)
> Coming back to this, my concern is that outputting instructions with
> prefixes where the prefixes cause the instruction to be undefined is
> misleading. If the output of the decoder is intended to match the
> instruction that will be executed, it should produce an error, because
> that's what will occur when the instruction is executed on a processor.

UD can happen for various reasons.  Because an AVX instruction is undefined
on Haswell machine, it doesn't mean objdump should decode it based on where
the program runs.

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[Bug binutils/19660] [libopcodes] [x86] REP prefixes shown incorrectly

[njholcomb at wi dot rr.com]

https://sourceware.org/bugzilla/show_bug.cgi?id=19660

--- Comment #4 from njholcomb at wi dot rr.com ---
Coming back to this, my concern is that outputting instructions with prefixes
where the prefixes cause the instruction to be undefined is misleading. If the
output of the decoder is intended to match the instruction that will be
executed, it should produce an error, because that's what will occur when the
instruction is executed on a processor.

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[Bug binutils/19660] [libopcodes] [x86] REP prefixes shown incorrectly

[hjl.tools at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=19660

--- Comment #3 from H.J. Lu  ---
(In reply to njholcomb from comment #2)
> Created attachment 9201 [details]
> Shows rep prefixes on non-string instructions

I got

 :
   0:   55  push   %rbp
   1:   48 89 e5mov%rsp,%rbp
   4:   f3 e0 58repz loopne 5f 
   7:   f2 00 00repnz add %al,(%rax)
   a:   00 b8 00 00 00 00   add%bh,0x0(%rax)
  10:   5d  pop%rbp
  11:   c3  retq   

repz is nop before loopne.  repnz is UD before add:

Program received signal SIGSEGV, Segmentation fault.
0x0040048f in main ()
(gdb) disass
Dump of assembler code for function main:
   0x00400488 <+0>: push   %rbp
   0x00400489 <+1>: mov%rsp,%rbp
   0x0040048c <+4>: repz loopne 0x4004e7 <__libc_csu_init+71>
=> 0x0040048f <+7>: repnz add %al,(%rax)
   0x00400492 <+10>:add%bh,0x0(%rax)
   0x00400498 <+16>:pop%rbp
   0x00400499 <+17>:retq   
End of assembler dump.
(gdb) 

Disassembler output looks OK to me.

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[Bug binutils/19660] [libopcodes] [x86] REP prefixes shown incorrectly

[njholcomb at wi dot rr.com]

https://sourceware.org/bugzilla/show_bug.cgi?id=19660

--- Comment #2 from njholcomb at wi dot rr.com ---
Created attachment 9201
  --> https://sourceware.org/bugzilla/attachment.cgi?id=9201=edit
Shows rep prefixes on non-string instructions

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[Bug binutils/19660] [libopcodes] [x86] REP prefixes shown incorrectly

[hjl.tools at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=19660

H.J. Lu  changed:

   What|Removed |Added

 Status|NEW |WAITING
 CC||hjl.tools at gmail dot com

--- Comment #1 from H.J. Lu  ---
Please provide an ELF object file as the input for "objdump -d".

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