Re: combine: Undo canonicalizations when splitting (minus x (mult))

[Richard Earnshaw <rearnsha at arm dot com>]

On Sat, 2010-08-21 at 01:29 +0200, Bernd Schmidt wrote:
> A week or two ago I posted a patch which added some new
> canonicalizations in the combiner, to change
>   (plus (mult X -Y) Z)
> to
>   (minus Z (mult X Y))
> 
> This helped on ARM, and to a lesser degree on x86.  I mentioned at the
> time that I was aware of one counterexample, in the 20040709-2 testcase.
>  Here, we have two successive pairs of multiply and add insns, which can
> be combined to a single multiply and add - however, that becomes
>  (minus (const_int) (mult X (const_int))
> which is not recognizable on i686 even when split.
> 
> Since then I've also found a couple additional examples on x86_64 with
> vectorization enabled.
> 
> The patch below corrects the problem by tweaking find_split_point.  If
> we have
>   (minus Z (mult X Y))
> at the top level, and Y is not a constant power of 2, assume it is
> easier for the target to split the (plus (mult X -Y) Z) form.
> 
> -       imull   $1103515245, %ecx, %r8d
> -       addl    $12345, %r8d
> -       imull   $1103515245, %r8d, %r8d
> -       addl    $12345, %r8d
> +       imull   $-1029531031, %ecx, %r8d
> +       subl    $740551042, %r8d
> 
> Bootstrapped and tested on x86_64-linux.  Looking at code generation on
> ARMv7 and x86_64, a few sequences were improved, and I did not discover
> additional regressions.
> 
> Ok?
> 
> 
This is OK.

R.