[LLVMdev] Question. about Machinefunction pass, funtion Prolog/Epilog code, stack frame

John Criswell criswell at uiuc.edu
Wed Apr 14 07:49:55 PDT 2010

Dear All,

Gang is a student at University of Illinois, and I've been asked to help 
advise him on using LLVM for a project.  However, I lack experience with 
LLVM's machine-level IR (used for code generation), so I asked Gang to 
post to LLVMdev for help.

To clarify Gang's questions, we need to insert machine code into the 
beginning of a function that does some prefetching of the stack frame 
and the function's code.  We'd like to avoid modifying the code 
generators, so we're hoping we can implement this as a MachineFunctionPass.

So, to rephrase Gang's questions:

1) Can we implement this functionality as a MachineFunctionPass?  If so, 
is there another MachineFunctionPass that does something similar that we 
can use as an example?  Also, can this MachineFunctionPass be target 
independent, or will it have to know how to insert processor-specific 

2) We believe we can get the size of the stack frame by using the 
MachineFrameInfo::getStackSize() method.  What sort of objects does this 
method include (e.g., does it contain function parameters)?  Also, how 
do we find where it begins?  Is this guaranteed to be stored in a 
specific register after the Prologue code (e.g, the %esp or %ebp 
registers on x86)?

3) Is there a way to determine the location and size (in bytes) of a 

On a related note, does implementing a transform on the machine IR level 
make sense for this work, or is there an LLVM IR way to do it?  I think 
that, on x86, we could find the stack frame by using a combination of 
the stacksave and frameaddress intrinsics, but I'm not sure if this 
method is reliable.

Thanks in advance for any answers.

-- John T.

Gang.Yao wrote:
> I am new to the LLVM, and need some help with this points.
> 1. how can we add special code for the Prolog/Epilog for some
> certain functions, this should be done with machinefunction pass, rt?
> 2. Basically, I want to get the function stack frame, that is the size and
> the initial position. 
>   I found
> int64_t llvm::MachineFrameInfo::getObjectSize 	( 	int  
> /ObjectIdx/ 	 )  	const| [inline]|
>   This method is done before or after the mapping of virtual registers?
> 3. the program code will be at .text section, suppose we are going to
> move the code from .text to somewhere else, how can we get the
> start and the length of the effective code,
> Thanks.
> -- 

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