Re: Implementation dependent behaviour (WAS: Re: Programming language and IDE design)

glen herrmannsfeldt <gah@ugcs.caltech.edu> writes:
>It has been hardware tradition for a long time to offer double width
>product on multiply and dividend on divide, but rare for high-level
>languages to allow for its use.
>
>I believe that gcc recognizes casts to (long long) on the operands of
>multiply to generate a single multiply instruction with a double
>length product. (That is, it doesn't do the three multiplies by zero
>that the casts imply.) I don't know about divide.


Yes, gcc does that for multiplication (suitably written in C, posted
elsewhere in this thread). It cannot do this for divide, because
there is no way to express the operation in C (boundary conditions
like overflow don't match).


>I have at times needed to multiply two 32 bit values, for a 64 bit
>product, then divide by another 32 bit value. I sometimes hope that
>the compiler figures it out.


No, it won't. Better use a language that can express this. Forth has
*/, which does exactly this sequence of operations; it also has
double-by-single division words, as well as
single-by-single-giving-double multiplication words.


- anton
--
M. Anton Ertl
anton@mips.complang.tuwien.ac.at
http://www.complang.tuwien.ac.at/anton/

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