llvm dev - Nov 2007 - [LLVMdev] Other Intrinsics?

> > Do you have plans to add other intrinsics?  I'm curious as to why
there
 > > is an llvm.sin intrinsic and an llvm.cos intrinsic, but no llvm.atan
 > > intrinsic.  Why is there an llvm.pow intrinsic but no llvm.log
 > > intrinsic?
 >
 > Intrinsics get added on demand.  Generally there has to be a good reason
 > to add them.  llvm.sin was implemented (for example) to allow generation
 > of code that uses vector sin operations.

What is the criteria for adding an intrinsic or a built-in operation?
For example, could the 'frem' instruction be an intrinsic?  Could you 
remove it from LLVM entirely and make it an external function?  What 
distinguishes 'frem' from 'sin', 'pow', 'atan2',
etc.?

Regards,
Jon

On Tue, 27 Nov 2007, Jon Sargeant wrote:
> > Intrinsics get added on demand.  Generally there has to be a good
reason
> > to add them.  llvm.sin was implemented (for example) to allow
generation
> > of code that uses vector sin operations.
>
> What is the criteria for adding an intrinsic or a built-in operation?
> For example, could the 'frem' instruction be an intrinsic?  Could
you
> remove it from LLVM entirely and make it an external function?  What
> distinguishes 'frem' from 'sin', 'pow',
'atan2', etc.?
Historically, intrinsics could not be overloaded, but instructions could. 
However, intrinsics got more powerful, so this is no longer true.  There 
is now very little distinction.

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/

On Tue, Nov 27, 2007 at 10:50:03AM -0700, Jon Sargeant
wrote:
>  > > Do you have plans to add other intrinsics?  I'm curious as
to why there
>  > > is an llvm.sin intrinsic and an llvm.cos intrinsic, but no
llvm.atan
>  > > intrinsic.  Why is there an llvm.pow intrinsic but no llvm.log
>  > > intrinsic?
>  >
>  > Intrinsics get added on demand.  Generally there has to be a good
reason
>  > to add them.  llvm.sin was implemented (for example) to allow
generation
>  > of code that uses vector sin operations.
> 
> What is the criteria for adding an intrinsic or a built-in operation?
> For example, could the 'frem' instruction be an intrinsic?  Could
you
> remove it from LLVM entirely and make it an external function?  What 
> distinguishes 'frem' from 'sin', 'pow',
'atan2', etc.?
The main difference from my perspective between intrinsics and instructions
is that intrinsics don't require bitcode format changes. If someone wants an
atan2 intrinsic for example, it would be fairly easy, and we wouldn't have
to worry very much about collisions with other people adding new intrinsics,
where we would if we were adding new operators.

The main reason for adding intrinsics instead of just using C library
calls is for support for vector types. @llvm.sin.* can be overloaded as
@llvm.sin.v4f32, for example, which is very useful for some users.

The intrinsics for sin, cos, and pow are fairly new; I added them when
I implemented the support for vector types in intrinsics, to exercise this
new infastructure. It would be easy now to add similar intrinsics for other
math functions.

Dan

-- 
Dan Gohman, Cray Inc.

On Tue, 27 Nov 2007, Dan Gohman wrote:
>> > Intrinsics get added on demand.  Generally there has to be a good
reason
>> > to add them.  llvm.sin was implemented (for example) to allow
generation
>> > of code that uses vector sin operations.
>>
>> What is the criteria for adding an intrinsic or a built-in operation?
>> For example, could the 'frem' instruction be an intrinsic? 
Could you
>> remove it from LLVM entirely and make it an external function?  What
>> distinguishes 'frem' from 'sin', 'pow',
'atan2', etc.?
>
> The main difference from my perspective between intrinsics and instructions
> is that intrinsics don't require bitcode format changes. If someone
wants an
> atan2 intrinsic for example, it would be fairly easy, and we wouldn't
have
> to worry very much about collisions with other people adding new
intrinsics,
> where we would if we were adding new operators.
Right, intrinsics trade off easier encoding in the bc format and .ll 
files with having less dense encodings and less natural syntax.

However, this begs the question: what is the advantage of instructions 
over intrinsics?  Why not get rid of fmod?  If we got rid of fmod, why not 
fadd, fdiv, ?
> The main reason for adding intrinsics instead of just using C library
> calls is for support for vector types. @llvm.sin.* can be overloaded as
> @llvm.sin.v4f32, for example, which is very useful for some users.
My question is "how can these be used" by people.  Specifically, these
need to be lowered to some sort of runtime calls (no hardware has support 
for these) and llvm doesn't provide a standard runtime yet.  Unless the 
codegen has a way to lower these, it seems strange to add them as 
intrinsics.  IOW, if llvm.sin.v4f32 ends up being 4 calls to sinf, why not 
encode 4 calls to sinf in the bytecode?

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/