Re: When to use std::pow(x,n) instead of times x for n times?

=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?= <>
Sun, 14 Sep 2008 09:37:21 GMT
On 2008-09-13 23:06, Peng Yu wrote:

On Sep 11, 4:56 pm, gpderetta <> wrote:

On Sep 11, 6:39 pm, Juha Nieminen <nos...@thanks.invalid> wrote:

gpderetta wrote:

Why? There is no reason for the compiler not to transformpow(x,
<integral-constant>) to the latter form if it were actually faster
(and in fact some compilers do).

  Some compilers might be able to do that optimizations, others aren't.
And if n is a variable, then it cannot optimize it.

and if it is variable, you can't write an explicit expression either.
You could use a for loop

(At most thepow()
function itself might have optimizations in it, but in my experience it
doesn't: With most compilers it just generates the FPU opcodes necessary
to calculate the result.)

Today hand optimizations are tomorrow pessimizations. Let the compiler
do its job.

The usual rule apply: usepow, and only if the profiler tells it is a
bottleneck, try to optimize it by hand.

  But we don't have to speculate about this as it's trivially easy to
test in practice. Go ahead and try it.

I had already tried. 'pow(x, 16)' is inlined exactly as four
multiplies, at least with a recent gcc.

Would you please let me know the details the procedure on how you
figure this out? Sometimes I want to know what the compiler compile
the code to.

In Visual Studio you can run the program in the debugger and then bring
up the assembly code and it will show you can step through it and switch
back and forth between the code and assembly code. I would imagine you
can do similar things in other IDEs and in gdb.

Erik Wikstr??m

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