llvm dev - Dec 2009 - [LLVMdev] incorrect x86 instruction size calculation

Hello,

 

I'm trying to write some backends for LLVM that generate code that complies
with SFI validation rules by re-implementing SFI for the LLVM x86 backend
based on the Google NaCl project.

 

However, in trying to implement 32-byte code alignment,
X86InstrInfo::GetInstSizeInBytes() is returning incorrect instruction sizes
for certain instructions (that I have seen so far):  MOV32mi, LEA32r,
MOV32mr, and MOV32rm.  

 

MOV32mi is always calculated incorrectly while the remaining 3 are sometimes
calculated incorrectly.  Just to illustrate:  

 

8d 9c 24 30 0a 00 00                         LEA32r
calculated length: 7         ok

8d 6c 24 28                                          LEA32r
calculated length: 7         incorrect

8b 86 24 0a 39 00                               MOV32rm           calculated
length: 6         ok

8b 44 24 10                                          MOV32rm
calculated length: 7         incorrect

89 84 24 34 14 00 00                         MOV32mr           calculated
length: 7         ok

89 2c 24                                                MOV32mr
calculated length: 7         incorrect

c7 44 24 08 08 0a 00 00                    MOV32mi            calculated
length: 11       incorrect

c7 04 24 20 00 38 00                          MOV32mi            calculated
length: 11       incorrect  

 

Has anyone else encountered this?  If this turns out to be a bug, rather
than some misuse/misinterpretation of the function on my part then I can
resubmit it via that channel.  Also if I need to submit more information let
me know and I will do so.  

 

Regards,

 

--John

 

 

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On Dec 16, 2009, at 11:21 AM, Butterworth, John W. wrote:
> Hello,
>  
> I‘m trying to write some backends for LLVM that generate code that complies
with SFI validation rules by re-implementing SFI for the LLVM x86 backend based
on the Google NaCl project.
>  
> However, in trying to implement 32-byte code alignment,
X86InstrInfo::GetInstSizeInBytes() is returning incorrect instruction sizes for
certain instructions (that I have seen so far):  MOV32mi, LEA32r, MOV32mr, and
MOV32rm.
This piece of code is rather unfortunate.  It is cloned from the JIT and
reimplements some of its logic, apparently incorrectly.  I believe that the
current clients in the tree work ok with over-approximations of the length, they
don't need exact answers.
>  
> Has anyone else encountered this?  If this turns out to be a bug, rather
than some misuse/misinterpretation of the function on my part then I can
resubmit it via that channel.  Also if I need to submit more information let me
know and I will do so.
This definitely sounds like a bug, and improvements are certainly welcome.

Long term, I'm hoping that the MC framework will ultimately provide a more
principled and robust way to do this sort of thing, but it will still be several
months before it will be robust enough to switch this code over to use it.

-Chris
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