llvm dev - Apr 2016 - Questions on writing a machine pass in LLVM

Dear all,


Hi, my name is Wenqing(Wendell) Zhuang who is a Ph.D student in the college of
EECS of Syracuse University in US, I am currently writing a machine pass of LLVM
which checks if a pointer points to an address that is out of a certain range.
So in that pass, I need to determine if a pointer points to a stack slot or to a
non-stack area(e.g heap), I call the function:
X86InstrInfo::isLoadFromStackSlot(const MachineInstr *MI, int &FrameIndex)
in my pass which is added in X86PassConfig::addPreEmitPass(), however, that
function always return 0 for every Machine Instruction, and returned FrameIndex
is always 0, my pass is executed before code emit and after register allocation,
but isLoadFromStackSlot and isStoreToStackSlot do not work as I expected in my
pass,I can hardly figure out why.


I really appreciate if anyone can help me with this. Thanks. The best!




Wenqing(Wendell) Zhuang

wzhuang at syr.edu

Department of Electrical Engineering and Computer Science

Syracuse University, US
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On 4/19/2016 9:25 PM, Wenqing Zhuang via llvm-dev wrote:
>
> Hi, my name is Wenqing(Wendell) Zhuang who is a Ph.D student in the
> college of EECS of Syracuse University in US, I am currently writing a
> machine pass of LLVM which checks if a pointer points to an address that
> is out of a certain range. So in that pass, I need to determine if a
> pointer points to a stack slot or to a non-stack area(e.g heap), I call
> the function: X86InstrInfo::isLoadFromStackSlot(const MachineInstr *MI,
> int &FrameIndex) in my pass which is added in
> X86PassConfig::addPreEmitPass(), however, that function always return 0
> for every Machine Instruction, and returned FrameIndex is always 0, my
> pass is executed before code emit and after register allocation, but
> isLoadFromStackSlot and isStoreToStackSlot do not work as I expected in
> my pass,I can hardly figure out why.
>
>
> I really appreciate if anyone can help me with this. Thanks. The best!
Hi Wenqing,
The functions isLoadFromStackSlot and isStoreToStackSlot only work while 
the frame indices are still present as instruction operands. After 
prolog/epilog insertion, they will be replaced with actual 
register/offset values and determining whether any load or store 
accesses a stack slot is likely beyond the intended scope of these 
functions.

If your pass runs before prolog/epilog insertion, it should work fine, 
but I suspect it runs too late.

-Krzysztof


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