Mail Archives: djgpp-workers/2004/02/16/14:32:46
Kbwms AT aol DOT com wrote:
>Can anyone help me here?
>
>One set of functions comes to mind immediately because it is on my "to do
>list" and is described in C99 paragraph 7.12.13 -- Floating multiply add with
>subtitle, "The fma functions." My knowledge of assembler would be extended
>beyond its limit trying to figure out how to calculate (x*y)+z as one ternary
>operation.
>
This one is actually pretty difficult to do, if you want to comply with
the letter of the law. The description of the function in C99 talks
about delivering the correctly-rounded result. On the x87 chip is it
possible to come pretty close by using the native 64-bit precision, but
it's not correct rounding in all cases. gcc actually generates this
64-bit code by simply writing
double fma(double x, double y, double z) {
return x*y + z;
}
but it is not guaranteed that the intermediate product will be held in
the 64-bit x87 register, as it currently seems to be. Assembler will
ensure that the intermediate product is held to 64-bit precision, but
that doesn't even guarantee a correctly-rounded result, as the C99
standard requires.
If you want assembler, just compile the above fragment with the -S,
-O2, and -fomit-frame-pointer options and you will get assembler output,
which, for this particular function, is as good as can be done by hand.
-Eric Rudd
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