Re: [gfortran] Exponentiation by integral exponents

Re: [gfortran] Exponentiation by integral exponents

[Tobias Schlüter]

[ forwarding to gcc@gcc.gnu.org so more knowledgeable people can comment 
on the point I make in the last paragraph ]

Feng Wang wrote:
> Hi, Tobi, Paul, etc
>   I missed the discussion because of my poor modem. But I have implemented this
> and the patch is under test. IMHO, we have to implement the algorithm described
> by Knuth in the FE. BUILT_IN_POW only deals with Real type. How about complex
> type? There are also many problems to consider when an integer is raised to an
> negative integer. The result is just 0, -1, or 1. So it is not correct to
> convert integer to real. 

This is a good point that I hadn't thought about.

> I think we should also implement the runtime library
> function powi. When a value is raised to an varible integer, we just call the
> library function. The value may be of integer type, real type or complex type.

Yeah, I thought so as well. I was already looking for an implementation.

> I have done these and in a few days I will give the patch. I didn't port Andy's
> implementation. Andy's vertion only deal with exponent under 100 and don't
> consider (integer ** negative integer). The main algorithm in my implementation
> is from builtins.c and can deal with an arbitrary integer exponent. The
> algorithm in powi library function is from libf77.

If your patch is finished I agree that we should go with your patch, as 
mine is far from finished. I chose to port Andy's implementation as this 
was my first step in GCC trees, and I didn't want to do something from 
scratch :-)

I don't necessarily agree that this should be handled in the FE, though. 
There might be other languages that have an "integral exponent" idiom, 
so a language independent solution might be preferable.

- Tobi

Are BUILT_IN_POWII and BUILT_IN_POWCI necessary? (RE: Exponentiation by integral exponents)

[Feng Wang]

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 --- Tobias_Schlüter <tobias.schlueter@physik.uni-muenchen.de> µÄÕýÎÄ£º> [
forwarding to gcc@gcc.gnu.org so more knowledgeable people can comment 
> on the point I make in the last paragraph ]
> 
> Feng Wang wrote:
> > Hi, Tobi, Paul, etc
> >   I missed the discussion because of my poor modem. But I have implemented
> this
> > and the patch is under test. IMHO, we have to implement the algorithm
> described
> > by Knuth in the FE. BUILT_IN_POW only deals with Real type. How about
> complex
> > type? There are also many problems to consider when an integer is raised to
> an
> > negative integer. The result is just 0, -1, or 1. So it is not correct to
> > convert integer to real. 
> 
> This is a good point that I hadn't thought about.
> 
> > I think we should also implement the runtime library
> > function powi. When a value is raised to an varible integer, we just call
> the
> > library function. The value may be of integer type, real type or complex
> type.
> 
> Yeah, I thought so as well. I was already looking for an implementation.
> 
> > I have done these and in a few days I will give the patch. I didn't port
> Andy's
> > implementation. Andy's vertion only deal with exponent under 100 and don't
> > consider (integer ** negative integer). The main algorithm in my
> implementation
> > is from builtins.c and can deal with an arbitrary integer exponent. The
> > algorithm in powi library function is from libf77.
> 
> If your patch is finished I agree that we should go with your patch, as 
> mine is far from finished. I chose to port Andy's implementation as this 
> was my first step in GCC trees, and I didn't want to do something from 
> scratch :-)
> 
> I don't necessarily agree that this should be handled in the FE, though. 
> There might be other languages that have an "integral exponent" idiom, 
> so a language independent solution might be preferable.

To this point it is reasonable. But thus we will have to build and expand
BUILT_IN_POWII (int ** int) and  BUILT_IN_POWCI (complex ** int) in the BE. Is
this necessary?  Maybe the GCC community can give us some suggestion.

Feng Wang

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