llvm dev - Aug 2014 - [LLVMdev] Default/initial values for function arguments?

Hi guys,

I’m trying to figure out a way to assign initial values to function arguments.
For a function in IR:

define i32 @main (i32 %0, i32 %1)
{
	%tmp = add i32 %0, %1
	...
}

I would like to make sure %0 has some initial value (e.g. i32 0) under some
circumstances. Is there any easy way to do this? I understand that %0 comes from
a live-in value which is defined from outside of the function. I could,  at the
ISelLowering stage when seeing an ADD, replace the “%0” with a i32 constant 0,
but I don’t feel like this is the right approach.

Should I try to look into SUBREG_TO_REG/INSERT_REG here? Or any other thoughts? 

Thanks,
- Cheng-Chih

Dear Cheng-Chih,

Can you clarify what you mean by ensuring that "%0 has a default
value?"

Are you trying to ensure that the function's arguments have a default 
value if the function is called with too few arguments?

The only way that %0 is going to have an undefined value is if a) a 
function passes in an undefined value (like undef) or b) the function is 
called with few two arguments.  Unless you're trying to address behavior 
that is undefined at the LLVM IR level, your question doesn't make much 
sense.

Regards,

John Criswell


On 8/15/14, 4:54 PM, Cheng-Chih Yang wrote:
> Hi guys,
>
> I’m trying to figure out a way to assign initial values to function
arguments. For a function in IR:
>
> define i32 @main (i32 %0, i32 %1)
> {
> 	%tmp = add i32 %0, %1
> 	...
> }
>
> I would like to make sure %0 has some initial value (e.g. i32 0) under some
circumstances. Is there any easy way to do this? I understand that %0 comes from
a live-in value which is defined from outside of the function. I could,  at the
ISelLowering stage when seeing an ADD, replace the “%0” with a i32 constant 0,
but I don’t feel like this is the right approach.
>
> Should I try to look into SUBREG_TO_REG/INSERT_REG here? Or any other
thoughts?
>
> Thanks,
> - Cheng-Chih
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi John,

I see. Then my question is probably better put as - if there's a good way in
LowerFormalArgument() one can overwrite the input argument's default value?

Now the backend I have does a CopyFromReg(), which reads from a virtual register
mapping to the input argument. I'd like to overwrite its value, so that all
further IR instructions using this register can see the updated value.

Thanks,
- Cheng-Chih

-----Original Message-----
From: John Criswell [mailto:jtcriswel at gmail.com] 
Sent: Friday, August 15, 2014 5:59 PM
To: Yang, Cheng-Chih; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Default/initial values for function arguments?

Dear Cheng-Chih,

Can you clarify what you mean by ensuring that "%0 has a default
value?"

Are you trying to ensure that the function's arguments have a default value
if the function is called with too few arguments?

The only way that %0 is going to have an undefined value is if a) a function
passes in an undefined value (like undef) or b) the function is called with few
two arguments.  Unless you're trying to address behavior that is undefined
at the LLVM IR level, your question doesn't make much sense.

Regards,

John Criswell


On 8/15/14, 4:54 PM, Cheng-Chih Yang wrote:
> Hi guys,
>
> I'm trying to figure out a way to assign initial values to function
arguments. For a function in IR:
>
> define i32 @main (i32 %0, i32 %1)
> {
> 	%tmp = add i32 %0, %1
> 	...
> }
>
> I would like to make sure %0 has some initial value (e.g. i32 0) under some
circumstances. Is there any easy way to do this? I understand that %0 comes from
a live-in value which is defined from outside of the function. I could,  at the
ISelLowering stage when seeing an ADD, replace the "%0" with a i32
constant 0, but I don't feel like this is the right approach.
>
> Should I try to look into SUBREG_TO_REG/INSERT_REG here? Or any other
thoughts?
>
> Thanks,
> - Cheng-Chih
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev