Re: Fold pointer range checks with equal spans

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

On Mon, Jul 23, 2018 at 5:05 PM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Marc Glisse <marc.glisse@inria.fr> writes:
> > On Fri, 20 Jul 2018, Richard Sandiford wrote:
> >
> >> --- gcc/match.pd     2018-07-18 18:44:22.565914281 +0100
> >> +++ gcc/match.pd     2018-07-20 11:24:33.692045585 +0100
> >> @@ -4924,3 +4924,37 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> >>    (if (inverse_conditions_p (@0, @2)
> >>         && element_precision (type) == element_precision (op_type))
> >>     (view_convert (cond_op @2 @3 @4 @5 (view_convert:op_type @1)))))))
> >> +
> >> +/* For pointers @0 and @2 and unsigned constant offset @1, look for
> >> +   expressions like:
> >> +
> >> +   A: (@0 + @1 < @2) | (@2 + @1 < @0)
> >> +   B: (@0 + @1 <= @2) | (@2 + @1 <= @0)
> >> +
> >> +   If pointers are known not to wrap, B checks whether @1 bytes starting at
> >> +   @0 and @2 do not overlap, while A tests the same thing for @1 + 1 bytes.
> >> +   A is more efficiently tested as:
> >> +
> >> +   ((sizetype) @0 - (sizetype) @2 + @1) > (@1 * 2)
> >> +
> >> +   as long as @1 * 2 doesn't overflow.  B is the same with @1 replaced
> >> +   with @1 - 1.  */
> >> +(for ior (truth_orif truth_or bit_ior)
> >> + (for cmp (le lt)
> >> +  (simplify
> >> +   (ior (cmp (pointer_plus:s @0 INTEGER_CST@1) @2)
> >> +    (cmp (pointer_plus:s @2 @1) @0))
> >
> > Do you want :c on cmp, in case it appears as @2 > @0 + @1 ? (may need some
> > care with "cmp == LE_EXPR" below)
> > Do you want :s on cmp as well?
> >
> >> +   (if (!flag_trapv && !TYPE_OVERFLOW_WRAPS (TREE_TYPE (@0)))
> >
> > Don't you want TYPE_OVERFLOW_UNDEFINED?
>
> Thanks, fixed below.  Think the cmp == LE_EXPR stuff is still ok with :c,
> since the generated code sets cmp to LE_EXPR when matching GE_EXPR.
>
> Tested as before.  OK to install?
>
> Richard
>
>
> 2018-07-23  Richard Sandiford  <richard.sandiford@arm.com>
>
> gcc/
>         * match.pd: Optimise pointer range checks.
>
> gcc/testsuite/
>         * gcc.dg/pointer-range-check-1.c: New test.
>         * gcc.dg/pointer-range-check-2.c: Likewise.
>
> Index: gcc/match.pd
> ===================================================================
> --- gcc/match.pd        2018-07-23 15:56:47.000000000 +0100
> +++ gcc/match.pd        2018-07-23 15:58:33.480269844 +0100
> @@ -4924,3 +4924,37 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>     (if (inverse_conditions_p (@0, @2)
>          && element_precision (type) == element_precision (op_type))
>      (view_convert (cond_op @2 @3 @4 @5 (view_convert:op_type @1)))))))
> +
> +/* For pointers @0 and @2 and unsigned constant offset @1, look for
> +   expressions like:
> +
> +   A: (@0 + @1 < @2) | (@2 + @1 < @0)
> +   B: (@0 + @1 <= @2) | (@2 + @1 <= @0)
> +
> +   If pointers are known not to wrap, B checks whether @1 bytes starting at
> +   @0 and @2 do not overlap, while A tests the same thing for @1 + 1 bytes.
> +   A is more efficiently tested as:
> +
> +   ((sizetype) @0 - (sizetype) @2 + @1) > (@1 * 2)
> +
> +   as long as @1 * 2 doesn't overflow.  B is the same with @1 replaced
> +   with @1 - 1.  */
> +(for ior (truth_orif truth_or bit_ior)
> + (for cmp (le lt)
> +  (simplify
> +   (ior (cmp:cs (pointer_plus:s @0 INTEGER_CST@1) @2)
> +       (cmp:cs (pointer_plus:s @2 @1) @0))
> +   (if (!flag_trapv && TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (@0)))

no need to check flag_trapv, pointer arithmetic is not covered by -ftrapv.

> +    /* Convert the B form to the A form.  */
> +    (with { offset_int off = wi::to_offset (@1) - (cmp == LE_EXPR ? 1 : 0); }
> +     /* Always fails for negative values.  */
> +     (if (wi::min_precision (off, UNSIGNED) * 2 <= TYPE_PRECISION (sizetype))
> +      /* It doesn't matter whether we use @2 - @0 or @0 - @2, so let
> +        tree_swap_operands_p pick a canonical order.  */

gimple_resimplify takes care of that - well, it doesn't since minus isn't
commutative...  I guess you get better CSE later when doing this thus ok,
but it does look a bit off here ;)

I think you shouldn't use 'sizetype' without guarding this whole thing
with TYPE_PRECISION (sizetype) == TYPE_PRECISION (TREE_TYPE (@0)).

Since the original IL performs an ordered compare of two eventually unrelated
pointers (or pointers adjusted in a way that crosses object
boundaries) (uh... ;))
I wonder if you can use POINTER_DIFF_EXPR here to avoid the sizetype
conversion?  Since POINTER_PLUS_EXPR offsets are supposed to be
interpreted "signed" that should also handle negative offsets correctly then?

Also, when @2 == @0 + (@1+1) then the original condition is true but
((sizetype) @0 - (sizetype) @2 + @1) > (@1 * 2) is not?
   (sizetype) @0 - (sizetype) (@0 + @1 + 1) + @1 > @1 * 2
-> -1 > @1 * 2

which is false.  So I can't really see how you can apply this transform in
general (it would be fine for generating alias checks but a bit more
pessimistic).

But maybe I am missing something?

Richard.

> +      (with { tree ptr1 = @0, ptr2 = @2;
> +             if (tree_swap_operands_p (ptr1, ptr2))
> +               std::swap (ptr1, ptr2); }
> +       (gt (plus (minus (convert:sizetype { ptr1; })
> +                       (convert:sizetype { ptr2; }))
> +                { wide_int_to_tree (sizetype, off); })
> +          { wide_int_to_tree (sizetype, off * 2); }))))))))
> Index: gcc/testsuite/gcc.dg/pointer-range-check-1.c
> ===================================================================
> --- /dev/null   2018-07-09 14:52:09.234750850 +0100
> +++ gcc/testsuite/gcc.dg/pointer-range-check-1.c        2018-07-23 15:58:33.480269844 +0100
> @@ -0,0 +1,37 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fno-ipa-icf -fdump-tree-optimized" } */
> +
> +/* All four functions should be folded to:
> +
> +   ((sizetype) a - (sizetype) b + 15) < 30.  */
> +
> +_Bool
> +f1 (char *a, char *b)
> +{
> +  return (a + 16 <= b) || (b + 16 <= a);
> +}
> +
> +_Bool
> +f2 (char *a, char *b)
> +{
> +  return (a + 15 < b) || (b + 15 < a);
> +}
> +
> +_Bool
> +f3 (char *a, char *b)
> +{
> +  return (a + 16 <= b) || (b + 16 <= a);
> +}
> +
> +_Bool
> +f4 (char *a, char *b)
> +{
> +  return (a + 15 < b) || (b + 15 < a);
> +}
> +
> +/* { dg-final { scan-tree-dump-times { = [^\n]* - [^\n]*;} 4 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times { = [^\n]* \+ [^\n]*;} 4 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times { = [^\n]*\ > [^\n]*;} 4 "optimized" } } */
> +/* { dg-final { scan-tree-dump-not {=[^\n]*\ < [^\n]*;} "optimized" } } */
> +/* { dg-final { scan-tree-dump-times { \+ 15} 4 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times { > 30} 4 "optimized" } } */
> Index: gcc/testsuite/gcc.dg/pointer-range-check-2.c
> ===================================================================
> --- /dev/null   2018-07-09 14:52:09.234750850 +0100
> +++ gcc/testsuite/gcc.dg/pointer-range-check-2.c        2018-07-23 15:58:33.480269844 +0100
> @@ -0,0 +1,31 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fno-ipa-icf -fwrapv-pointer -fdump-tree-optimized" } */
> +
> +_Bool
> +f1 (char *a, char *b)
> +{
> +  return (a + 16 <= b) || (b + 16 <= a);
> +}
> +
> +_Bool
> +f2 (char *a, char *b)
> +{
> +  return (a + 15 < b) || (b + 15 < a);
> +}
> +
> +_Bool
> +f3 (char *a, char *b)
> +{
> +  return (a + 16 <= b) || (b + 16 <= a);
> +}
> +
> +_Bool
> +f4 (char *a, char *b)
> +{
> +  return (a + 15 < b) || (b + 15 < a);
> +}
> +
> +/* { dg-final { scan-tree-dump-not { = [^\n]* - [^\n]*;} "optimized" } } */
> +/* { dg-final { scan-tree-dump-times { = [^\n]* \+ [^\n]*;} 8 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times { \+ 15} 4 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times { \+ 16} 4 "optimized" } } */