Re: [PATCH 4/4] match.pd: Add x + ((-x) & m) -> (x + m) & ~m pattern

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Jan 21, 2015 at 11:49 AM, Rasmus Villemoes
<rv@rasmusvillemoes.dk> wrote:
> Generalizing the x+(x&1) pattern, one can round up x to a multiple of
> a 2^k by adding the negative of x modulo 2^k. But it is fewer
> instructions, and presumably requires fewer registers, to do the more
> common (x+m)&~m where m=2^k-1.
>
> Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
> ---
>  gcc/match.pd                      |  9 ++++++
>  gcc/testsuite/gcc.dg/20150120-4.c | 59 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 68 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.dg/20150120-4.c
>
> diff --git gcc/match.pd gcc/match.pd
> index 47865f1..93c2298 100644
> --- gcc/match.pd
> +++ gcc/match.pd
> @@ -273,6 +273,15 @@ along with GCC; see the file COPYING3.  If not see
>   (if (TREE_CODE (@2) != SSA_NAME || has_single_use (@2))
>    (bit_ior @0 (bit_not @1))))
>
> +/* x + ((-x) & m) -> (x + m) & ~m when m == 2^k-1.  */
> +(simplify
> + (plus:c @0 (bit_and@2 (negate @0) CONSTANT_CLASS_P@1))

I think you want to restrict this to INTEGER_CST@1

> + (with { tree cst = fold_binary (PLUS_EXPR, TREE_TYPE (@1),
> +                                @1, build_one_cst (TREE_TYPE (@1))); }

We shouldn't dispatch to fold_binary in patterns.  int_const_binop would
be the appropriate function to use - but what happens for @1 == INT_MAX
where @1 + 1 overflows?  Similar, is this also valid for negative @1
and thus signed mask types?  IMHO we should check whether @1
is equal to wi::mask (TYPE_PRECISION (TREE_TYPE (@1)) - wi::clz (@1),
false, TYPE_PRECISION (TREE_TYPE (@1)).

As with the other patch a ChangeLog entry is missing as well as stating
how you tested the patch.

Thanks,
Richard.

> +  (if ((TREE_CODE (@2) != SSA_NAME || has_single_use (@2))
> +       && cst && integer_pow2p (cst))
> +   (bit_and (plus @0 @1) (bit_not @1)))))
> +
>  (simplify
>   (abs (negate @0))
>   (abs @0))
> diff --git gcc/testsuite/gcc.dg/20150120-4.c gcc/testsuite/gcc.dg/20150120-4.c
> new file mode 100644
> index 0000000..c3552bf
> --- /dev/null
> +++ gcc/testsuite/gcc.dg/20150120-4.c
> @@ -0,0 +1,59 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-original" } */
> +
> +/* x + ((-x) & m) -> (x + m) & ~m for m one less than a pow2.  */
> +int
> +fn1 (int x)
> +{
> +       return x + ((-x) & 7);
> +}
> +int
> +fn2 (int x)
> +{
> +       return ((-x) & 7) + x;
> +}
> +unsigned int
> +fn3 (unsigned int x)
> +{
> +       return x + ((-x) & 7);
> +}
> +unsigned int
> +fn4 (unsigned int x)
> +{
> +       return ((-x) & 7) + x;
> +}
> +unsigned int
> +fn5 (unsigned int x)
> +{
> +       return x + ((-x) % 8);
> +}
> +unsigned int
> +fn6 (unsigned int x)
> +{
> +       return ((-x) % 8) + x;
> +}
> +int
> +fn7 (int x)
> +{
> +       return x + ((-x) & 9);
> +}
> +int
> +fn8 (int x)
> +{
> +       return ((-x) & 9) + x;
> +}
> +unsigned int
> +fn9 (unsigned int x)
> +{
> +       return x + ((-x) & ~0U);
> +}
> +unsigned int
> +fn10 (unsigned int x)
> +{
> +       return ((-x) & ~0U) + x;
> +}
> +
> +
> +/* { dg-final { scan-tree-dump-times "x \\+ 7" 6 "original" } } */
> +/* { dg-final { scan-tree-dump-times "-x & 9" 2 "original" } } */
> +/* { dg-final { scan-tree-dump-times "return 0" 2 "original" } } */
> --
> 2.1.3
>