[gfortran] Exponentiation by integral exponents

[Tobias Schlüter]

Paul Brook wrote:
> On Tuesday 27 April 2004 23:07, Tobias Schlüter wrote:
> 
>>Andrew Pinski wrote:
>>
>>>On Apr 27, 2004, at 17:14, Tobias Schlüter wrote:
>>>
>>>>FYI I have ported Andy's code for exponentiation by an integral
>>>>exponent to our tree. This is an implementation of Knuth's algorithm
>>
>>...
>>
>>
>>>You should not need it if you use __builtin_pow and you turn on
>>>flag_unsafe_math_optimizations on.  Maybe the best way is to have
>>>another flag
>>>to say expand __builtin_pow always.
>>
>>Does the builtin recognize the case pow(x,(float)i), where i is an
>>integer? Then this sounds like a very good idea.
> 
> 
> Maybe the way to indicate we always want to expand __builtin_pow would be to 
> pass the second parameter with an integer type. It may be easier to add 
> __builtin_powi to avoid needing polymorphic builtins.

Definitely.

Anyway, I tried to implement Andrew's suggestion as a quick'n'dirty 
hack. Right now I get an ICE. What is the correct way to generate the 
equivalent of a typecast in C?

What I tried was this: gfc_conv_power_op has this code:

   type = TREE_TYPE (lse.expr);

   kind = expr->op1->ts.kind;
   switch (expr->op2->ts.type)
     {
     case BT_INTEGER:
       /* Integer powers are expanded inline as multiplications.  */
       gfc_conv_integer_power (se, lse.expr, rse.expr);
       return;

Instead of calling gfc_integer_power and returning, I want to convert 
rse.expr to the type of lse.expr. So I tried both of the following 
(gfc_conv_expr_type has no explanatory comment, so I'm just guessing 
that this might be the function I'm looking for):
       gfc_conv_expr_type (&rse, expr->op2, type); // first try
       rse.expr = convert (type, rse.expr);        // second try

After that I select the function based on the type of lse.expr as in the 
real case and continue as if rse.expr had not been int all the time.

What am I missing? I'm still within my first three hours of messing with 
gcc trees, so I might well be missing the obvious.

- Tobi