[LLVMdev] Math instructions

[Chris Lattner]

On Thu, 6 Jan 2005, Morten Ofstad wrote:

> Chris Lattner wrote:
>> 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.
>
> Is this really safe? If you don't include math.h and don't link with the math 
> library you're free to define sin/cos/etc any way you like, aren't you?

No, not for C programs at least.  In C, all standard C and math library 
function names are reserved.

>> Finally, for fabs, we should just be able to recognize the setcc/select 
>> pair and generate that instruction.
>
> the C math library has abs and absf, so it's easier to recognize the function 
> calls. Of course the previous comment also applies to this...

The problem there is that these functions are inline functions that expand 
to the select/setcc pair.  For this testcase:

#include <math.h>
double test1(double X) { return fabs(X); }
float test2(float X) { return fabsf(X); }

llvm-gcc compiles it to:

double %test1(double %X) {
         %comp = setge double %X, 0.000000e+00
         %abs.pos = sub double 0.000000e+00, %X
         %abs.1 = select bool %comp, double %X, double %abs.pos
         ret double %abs.1
}

float %test2(float %X) {
         %comp = setge float %X, 0.000000e+00
         %abs.pos = sub float 0.000000e+00, %X
         %abs.1 = select bool %comp, float %X, float %abs.pos
         ret float %abs.1
}

... so i won't help the C front-end at least.  If you want to add support 
for fabs and fabsf as calls, I can implement the pattern matching for the 
forms above someday.

> - for now I've just implemented all of them as llvm.xxx intrinsics, but I'm a 
> bit unsure how to lower these if the target does not support them and the 
> program does not link with libmath... Maybe I worry too much ;)

Yeah, I would worry about this one too much for now.

-Chris

-- 
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