llvm dev - Oct 2014 - [LLVMdev] Emit a jump instruction to a place inside basicblock

On Thu, Oct 30, 2014 at 4:48 PM, Eric Christopher <echristo at gmail.com>
wrote:
>
>
> On Thu Oct 30 2014 at 1:16:38 PM Xiaoguang Wang <xgwang09 at
gmail.com>
> wrote:
>
>> On Thu, Oct 30, 2014 at 2:33 PM, Robin Morisset <morisset at
google.com>
>> wrote:
>>
>>> Hi,
>>>
>>> From my understanding of the LLVM IR, it is impossible to jump to
the
>>> middle of a Basic Block, only to its beginning. But there is a
>>> splitBasicBlock function that seems like it might be useful to you,
to make
>>> sure a basic block is starting at the exact place you want to jump.
>>>
>>> Best regards,
>>> Robin
>>>
>>
>> Thanks you all! I'm looking at splitBasicBlock and trying to use
that.
>> BTW, is there a way at lower level of LLVM that can implement this
>> functionality? E.g. MCBasicBlock or MCInst?
>>
>>
> You are unlikely to want to do this at the MC level, maybe the MI level.
> What are you trying to accomplish?
>
> -eric
>
Thanks for the answer. I want to construct a table which can jump to the
address after a call instruction. And replace the call/ret instructions
with that indirection table. So to protect return address on stack.

Sincerely,
Xiaoguang
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20141030/15105541/attachment.html>

On Thu Oct 30 2014 at 2:01:24 PM Xiaoguang Wang <xgwang09 at gmail.com>
wrote:
>
> On Thu, Oct 30, 2014 at 4:48 PM, Eric Christopher <echristo at
gmail.com>
> wrote:
>
>>
>>
>> On Thu Oct 30 2014 at 1:16:38 PM Xiaoguang Wang <xgwang09 at
gmail.com>
>> wrote:
>>
>>> On Thu, Oct 30, 2014 at 2:33 PM, Robin Morisset <morisset at
google.com>
>>> wrote:
>>>
>>>> Hi,
>>>>
>>>> From my understanding of the LLVM IR, it is impossible to jump
to the
>>>> middle of a Basic Block, only to its beginning. But there is a
>>>> splitBasicBlock function that seems like it might be useful to
you, to make
>>>> sure a basic block is starting at the exact place you want to
jump.
>>>>
>>>> Best regards,
>>>> Robin
>>>>
>>>
>>> Thanks you all! I'm looking at splitBasicBlock and trying to
use that.
>>> BTW, is there a way at lower level of LLVM that can implement this
>>> functionality? E.g. MCBasicBlock or MCInst?
>>>
>>>
>> You are unlikely to want to do this at the MC level, maybe the MI
level.
>> What are you trying to accomplish?
>>
>> -eric
>>
> Thanks for the answer. I want to construct a table which can jump to the
> address after a call instruction. And replace the call/ret instructions
> with that indirection table. So to protect return address on stack.
>
>
An MI level pass will likely work, but you might end up having issues
replacing instructions. It'll be some effort though.

-eric
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20141030/2e0e5257/attachment.html>

On Thu, Oct 30, 2014 at 6:28 PM, Eric Christopher <echristo at gmail.com>
wrote:
>
>
> On Thu Oct 30 2014 at 2:01:24 PM Xiaoguang Wang <xgwang09 at
gmail.com>
> wrote:
>
>>
>> On Thu, Oct 30, 2014 at 4:48 PM, Eric Christopher <echristo at
gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Thu Oct 30 2014 at 1:16:38 PM Xiaoguang Wang <xgwang09 at
gmail.com>
>>> wrote:
>>>
>>>> On Thu, Oct 30, 2014 at 2:33 PM, Robin Morisset <morisset at
google.com>
>>>> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> From my understanding of the LLVM IR, it is impossible to
jump to the
>>>>> middle of a Basic Block, only to its beginning. But there
is a
>>>>> splitBasicBlock function that seems like it might be useful
to you, to make
>>>>> sure a basic block is starting at the exact place you want
to jump.
>>>>>
>>>>> Best regards,
>>>>> Robin
>>>>>
>>>>
>>>> Thanks you all! I'm looking at splitBasicBlock and trying
to use that.
>>>> BTW, is there a way at lower level of LLVM that can implement
this
>>>> functionality? E.g. MCBasicBlock or MCInst?
>>>>
>>>>
>>> You are unlikely to want to do this at the MC level, maybe the MI
level.
>>> What are you trying to accomplish?
>>>
>>> -eric
>>>
>> Thanks for the answer. I want to construct a table which can jump to
the
>> address after a call instruction. And replace the call/ret instructions
>> with that indirection table. So to protect return address on stack.
>>
>>
> An MI level pass will likely work, but you might end up having issues
> replacing instructions. It'll be some effort though.
>
> -eric
>
Thanks Eric for your help. I'm reading the function
*Emitter<CodeEmitter>::runOnMachineFunction(MachineFunction
&MF)* in lib/Target/X86/X86CodeEmitter.cpp. And I hope this is the place I
have to modify for my purpose.

Sincerely,
Xiaoguang
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20141031/86326f4d/attachment.html>