Re: [AD] Remove -fno-strength-reduce

[Hein Zelle <hein@xxxxxxxxxx>]

Eric Botcazou wrote:

> [Strength reduction is the operation that turns
> 
>    int i;
>    int array[N];
> 
>    for (i=0; i<N; i++)
>      array[i] = ...;
> 
> into
> 
>   char *p;
>   int array[N];
> 
>   for (p=(char *)array; p < (char*)array+ N*sizeof(int); p+=sizeof(int)) {
>     int *ip = (int *)p;
>     *ip = ...;
>   }]

And the latter is faster because ... ?

I would think that an array index computation would amount to
something like

	  array[i] = *(array + i);

The top loop then has, every iteration, something like

    if (i < N), i++

    *(array + i) = ...

and the bottom loop

    if (p < a + N * sizeof), p += sizeof

    int ip = (int *)p;
    *ip = ...;

I can see how the line "*(array + i) = ..." would be converted to a
multiplication, an addition and a cast, but even then, where is the
speed gain? The bottom case still has the multiplication and the cast
every iteration, and it also has two sizeof's although they could be
optimized out. How does that work? Is the multiplication optimized out
because it is in the condition clause of the for loop?

Hein Zelle

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 Hein Zelle                     hein@xxxxxxxxxx
	                        http://www.icce.rug.nl/~hein
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