Passing array as const slows down code?

[Steven Schveighoffer]

On Fri, 27 Apr 2012 11:12:07 -0400, Joseph Rushton Wakeling  
<joseph.wakeling at webdrake.net> wrote:

> Hello all,
>
> A surprise today when tweaking some code.
>
> I have a function which takes as input an array of values (the values  
> are a custom struct).  So, in pseudocode, it's something like this:
>
>      struct MyStruct
>      {
>          size_t x;
>          size_t y;
>          double z;
>      }
>
>      void foo(MyStruct[] input)
>      {
>          ...
>      }
>
> Since the input isn't modified, I thought I'd tag it as const[1]:
>
>      void foo(const MyStruct[] input) { ... }
>
> ... but this turned out to noticeably slow down the program.   
> (Noticeably as in, a 25-second as opposed to 23-second running time,  
> consistently observed on repeated trials of different code versions.)
>
> I'm surprised by this as I would have thought if anything tagging a  
> variable as const would if anything have increased the speed, or at  
> worst left it the same as before.
>
> Can anyone explain why?  It's no big deal, but I'm curious.

const should not affect code generation *at all*, except for name mangling  
(const MyStruct is a different type from MyStruct), and generating an  
extra TypeInfo for const MyStruct and const MyStruct[].  Const is purely a  
compile-time concept.

This cannot account for an extra 2 seconds.  Something else is happening.

Without more of your code, nobody can give a definitive answer except it  
is not supposed to affect it.

-Steve