[LLVMdev] Math instructions

[Chris Lattner]

On Tue, 4 Jan 2005, Morten Ofstad wrote:

> I'm currently adding the floating point math instructions (fabs, fsin, fcos 
> ...) to the x86 instruction set.

Cool!

> I'm a bit unsure how to make the back end actually generate these 
> instructions, though. My current plan is to add llvm intrinsics for 
> these instructions but I've noticed that llvm already handles C math 
> library functions to some extent. It feels a bit strange to add code for 
> example to constant fold these intrinsics when there is already code to 
> constant fold a calls to libmath functions -- but the libmath functions 
> have certain semantics regarding how errno is set which I don't really 
> want to deal with.

The way to deal with this is to add LLVM intrinsics, but only for 
functions that set errno.  For example, you could add llvm.sqrt, which is 
just undefined on a negative value other than -0.0.  For your uses, you 
just emit llvm.sqrt, the C frontend will make use of it and wrap errno 
handling around it as required.

For functions like sin/cos/etc, which do not set errno, you should be able 
to recognize an external function with that name and generate code for it.

> Other compilers have options to enable the direct use of fp instructions for 
> math operations (gcc has -fno-math-errno, -funsafe-math-optimizations and 
> -fno-trapping-math), this could be useful also for LLVM.

Yes, but this is really a separate issue, at least initially.

> Assuming there is such an option, should I modify the visitCallInst 
> function to generate machine instructions directly for calls to math 
> functions or is it better to include a pass which converts the calls 
> into intrinsics? An advantage of the second solution is that it makes it 
> easier for other front ends (like ours) which don't have the semantics 
> of libmath to generate code...

The X86 visitCall should just handle any functions like sin/cos that can 
be directly codegen'd into machine code.  For intrinsics you add, like 
llvm.sqrt, we should have code in the LowerIntrinsics code to lower it to 
a call to sqrt (for code generators that do not support it), but the X86 
backend and many others could support it natively.

Finally, for fabs, we should just be able to recognize the setcc/select 
pair and generate that instruction.

-Chris

-- 
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