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

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/

On Tuesday 27 November 2007 06:01:34 pm Chris Lattner
wrote:
> > 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?
Some of the hardware that we target has extensive support for these. Granted 
that instead of overloading llvm.sin and others for my vectors it'd be nicer
to just have llvm.sin that accepts arbitrary vectors but it's still useful. 
Of course we could also do regexp matching for multiple sinf's in a row that
operate on the same vector to code-gen something meaningful but with 
intrinsics outputting and visually inspecting the IR before feeding it to 
code-gen gives one a better idea of what's supposed to happen.

z

On Wed, 28 Nov 2007, Zack Rusin wrote:
>> intrinsics.  IOW, if llvm.sin.v4f32 ends up being 4 calls to sinf, why
not
>> encode 4 calls to sinf in the bytecode?
>
> Some of the hardware that we target has extensive support for these.
Granted
> that instead of overloading llvm.sin and others for my vectors it'd be
nicer
> to just have llvm.sin that accepts arbitrary vectors but it's still
useful.
Ok, if there is hardware with support for it, then I agree it makes sense 
to keep them,

-Chris

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