[llvm-dev] How to insert instructions before each function calls?

[SHUCAI YAO via llvm-dev]

On Sun, Sep 4, 2016 at 8:37 PM, Dean Michael Berris <dean.berris at gmail.com>
wrote:

> We do something very similar here for XRay, and I would think the approach
> would be similar. What XRay does are the following:
>
> - Find the machine instructions in a MachineFunctionPass that look
> interesting from the XRay perspective. Theses turn out to be: the beginning
> of the function (not really an instruction but a location), tail calls, and
> returns. I suspect you can very simply find the call instructions for the
> platform you're interested in and insert/wrap it in a pseudo instruction.
> - When lowering, emit the actual assembly sequence that you want.
>
> For your use-case though I think you may need to hook into function call
> lowering so you can insert your instruction sequence before stack
> adjustments are performed (if you want to insert your intercepts before any
> stack operations as opposed to just before actually calling the function).
>
> Hi Dean,
Thank you very much!

For the function call lowering, do you mean lowercall function? I did
insert the instruction before the stack adjustments, but the inserted code
appears in the prologue of the function, other than before the function
call. Maybe I did something wrong with the iterator.

So you mean I should insert a pseudo instruction in the machinefunction
pass, then replace it when function call lowering?  (Like segemented stack
implementation?)

Thanks!
Shucai


> Hope this helps.
>
> -- Dean
>
> > On 5 Sep 2016, at 00:23, Ryan Taylor via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Personally I would add a new pass that iterates, looks for the call you
> want then inserts the new instruction.
> >
> >
> > On Sep 4, 2016 10:20, "SHUCAI YAO" <yaos4 at mcmaster.ca> wrote:
> >
> >
> > On Sun, Sep 4, 2016 at 7:44 AM, Ryan Taylor <ryta1203 at gmail.com> wrote:
> > Mehdi,
> >
> >  Sorry, I misread his original post.
> >
> >  So something like:
> >
> >  XXXInsrtInfo   *XII;    // target instruction info
> >   MachineBasicBlock::iterator MI = MachineBasicBlock(YourCallInst);
> >  MachineBasicBlock *MBB = YourCallInst->getParent(); // basic block
> location of your call inst
> >  BuildMI(*MBB, MI, DebugLoc(), XII->get(XXX:::INSTRUCTION)......);
> >
> > The BuildMI params are going to depend on what you want to do with the
> instruction being inserted.
> >  http://llvm.org/docs/doxygen/html/MachineInstrBuilder_8h.html
> >
> > -Ryan
> >
> > Hi Ryan,
> >     I need to add two instructions for each function call. Do you mean I
> should add this snippet in the Lowercall function? Or I should add a new
> pass?
> >
> > Thanks!
> >
> >
> >
> >
> > On Sun, Sep 4, 2016 at 1:45 AM, Mehdi Amini <mehdi.amini at apple.com>
> wrote:
> >
> >> On Sep 3, 2016, at 6:18 PM, Ryan Taylor via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> So one way might look like this:
> >>
> >> IRBuilder<> Builder(&*BB); // BB = Function::iterator OR IRBuilder<>
> Builder(CallInst->getParent());
> >> Builder.SetInsertPoint(CallInst);
> >> InstructionClass *YourNewInstruction = builder.CreateInstructionClass(.....);
> // InstructionClass = type of instruction you are inserting
> >>
> >
> > I’m not sure how the IRBuilder would work at the MI level, as Shucai was
> asking.
> >
> >>
> >>
> >>
> >> On Sat, Sep 3, 2016 at 6:04 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:
> >> Take a look at IRBuilder and SetInsertPoint().
> >>
> >>
> >> On Sep 3, 2016 18:02, "SHUCAI YAO via llvm-dev" <
> llvm-dev at lists.llvm.org> wrote:
> >> I'm trying to insert some instructions before each function calls
> (before arguments push):
> >>    lea %EAX, label                      ----- new instructions
> >>    mov [ESP+stacksize], %EAX  ----- new instructions
> >>    push arg1
> >>    push arg2
> >>    ...
> >>    push argn
> >>    call callee_name
> >>
> >> I am a newbie to LLVM. I tried to use buildMI() to insert the
> instructions in the lowercall() function. But I couldn't put these
> instructions in the right positions. Is there a way to locate the position
> by using MachineBasicBlock iterator?
> >
> > Can you describe more precisely what are you trying to achieve?
> > I.e. what are these instructions? Why do you want to do that? It may
> lead to a different answer.
> >
> > I'm trying to implement something similar to segmented stack mechanism
> by using LLVM. Instead of inserting comparison code in the prologue of the
> function, I would like do the probe before arguments pushed. The segmentd
> stacks append a guarded page. This guard page will call the addmorestack
> function if the probe instructions touch this guarded page. Otherwise, it
> only stroe the return address in the bottom of the callee stack frame.
> >
> > In order to achieve this, for each function call, two instructions are
> needed to be inserted:
> >      LEA  %EAX, callee_return_label
> >      MOV [ESP- callee_stack_frame_size - arguments_size], %EAX
> >      PUSH argn
> >      ...
> >      PUSH arg1
> >      JMP callee_name
> > callee_return_label:
> >      ...
> >
> > So I need to insert two instructions (LEA and MOV) before each function
> call. I don't know when how to insert these two instructions.
> >
> > Thanks!
> > Shucai
> >
> >
> > —
> > Mehdi
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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