On Thu, 30 Jun 2016, Jan Hubicka wrote:
> Hi,
> i sent the email before, but it seems it never left my machine. Sorry for
> possible duplicates. This is updated version which does not overflow (by
> capping nit), but still using widest_int for the calculatoins. It seems more
> readable to do so than using wi:add/wi:mult/wi:lt and overflow checks, but
> i can definitly update the patch to do it, too.
>
> Bootstrapped/regtested x86_64-linux, OK?
>
> Honza
>
> * tree-scalar-evolution.h (iv_can_overflow_p): Declare.
> * tree-scalar-evolution.c (iv_can_overflow_p): New funcition.
> (simple_iv): Use it.
> * tree-ssa-loop-niter.c (nowrap_type_p): Use ANY_INTEGRAL_TYPE_P
>
> * gcc.dg/tree-ssa/scev-14.c: New testcase.
>
> Index: testsuite/gcc.dg/tree-ssa/scev-14.c
> ===================================================================
> --- testsuite/gcc.dg/tree-ssa/scev-14.c (revision 0)
> +++ testsuite/gcc.dg/tree-ssa/scev-14.c (working copy)
> @@ -0,0 +1,11 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-ivopts-details" } */
> +int a[100];
> +void t(unsigned int n)
> +{
> + unsigned int i;
> + for (i=0; i<n; i++)
> + a[i]++;
> +}
> +/* { dg-final { scan-tree-dump "Overflowness wrto loop niter:
> No-overflow" "ivopts" } } */
> +/* { dg-final { scan-tree-dump-not "Overflowness wrto loop niter:
> Overflow" "ivopts" } } */
> Index: tree-scalar-evolution.c
> ===================================================================
> --- tree-scalar-evolution.c (revision 237856)
> +++ tree-scalar-evolution.c (working copy)
> @@ -280,6 +280,7 @@ along with GCC; see the file COPYING3.
> #include "params.h"
> #include "tree-ssa-propagate.h"
> #include "gimple-fold.h"
> +#include "print-tree.h"
Don't see you need this.
> static tree analyze_scalar_evolution_1 (struct loop *, tree, tree);
> static tree analyze_scalar_evolution_for_address_of (struct loop *loop,
> @@ -3309,6 +3310,60 @@ scev_reset (void)
> }
> }
>
> +/* Return true if the IV calculation in TYPE can overflow based on the
> knowledge
> + of the upper bound on the number of iterations of LOOP, the BASE and STEP
> + of IV.
> +
> + We do not use information whether TYPE can overflow so it is safe to
> + use this test even for derived IVs not computed every iteration or
> + hypotetical IVs to be inserted into code. */
> +
> +bool
> +iv_can_overflow_p (struct loop *loop, tree type, tree base, tree step)
Exporting this is also not necessary?
> +{
> + widest_int nit;
> + wide_int base_min, base_max, step_min, step_max, type_min, type_max;
> + signop sgn = TYPE_SIGN (type);
> + signop base_sgn = TYPE_SIGN (TREE_TYPE (base));
> + signop step_sgn = TYPE_SIGN (TREE_TYPE (step));
> +
> + if (step == 0)
> + return false;
Err - you probably mean
if (integer_zerop (step))
here? your check is a NULL pointer check ...
> +
> + if (TREE_CODE (base) == INTEGER_CST)
> + base_min = base_max = base;
> + else if (TREE_CODE (base) == SSA_NAME && !POINTER_TYPE_P (TREE_TYPE (base))
> + && get_range_info (base, &base_min, &base_max) == VR_RANGE)
split &&s into separate lines. Also use && INTEGRAL_TYPE_P rather
than a negative check. This means that pointer IVs are always
considered to overflow.
> + ;
> + else
> + return true;
> +
> + if (TREE_CODE (step) == INTEGER_CST)
> + step_min = step_max = step;
> + else if (TREE_CODE (step) == SSA_NAME && !POINTER_TYPE_P (TREE_TYPE (step))
> + && get_range_info (step, &step_min, &step_max) == VR_RANGE)
Likewise.
> + ;
> + else
> + return true;
> +
> + if (!get_max_loop_iterations (loop, &nit))
> + return true;
> +
> + type_min = wi::min_value (type);
> + type_max = wi::max_value (type);
> + /* Watch overflow. */
> + if ((widest_int)1 << TYPE_PRECISION (type) < nit)
> + return true;
TYPE_PRECISION (type) - 1? The comment can be improved I think.
> + if ((widest_int::from (base_max, base_sgn)
> + + widest_int::from (step_max, step_sgn) * (nit + 1))
> + > widest_int::from (type_max, sgn)
> + || (widest_int::from (type_min, sgn)
> + > (widest_int::from (base_min, base_sgn)
> + + widest_int::from (step_min, step_sgn) * (nit + 1))))
> + return true;
and this lacks any comment... so it decodes to
(base_max + step_max * (nit + 1) > type_max)
|| (type_min > base_min + step_min * (nit + 1))
and it basically assumes infinite precision arithmetic for
the computation. As mentioned previously for __int128 widest_int
does _not_ guarantee this so you need to use FIXED_WIDE_INT
with a precision of WIDE_INT_MAX_PRECISION * 2 (+1?) like VRP does.
Note as both type_min/max and base_min/max are wide_ints the above
might be simplified
step_max * (nit + 1) > type_max - base_max
where the subtraction can be carried out in wide_int(?) and you
should also CSE nit + 1 or transform it further to
step_max * nit > type_max - base_max - step_max
where if I understand correctly, if type_max - base_max - step_max
underflows we already wrap.
Thanks,
Richard.
> + return false;
> +}
> +
> /* Checks whether use of OP in USE_LOOP behaves as a simple affine iv with
> respect to WRTO_LOOP and returns its base and step in IV if possible
> (see analyze_scalar_evolution_in_loop for more details on USE_LOOP
> @@ -3375,8 +3430,12 @@ simple_iv (struct loop *wrto_loop, struc
> if (tree_contains_chrecs (iv->base, NULL))
> return false;
>
> - iv->no_overflow = (!folded_casts && ANY_INTEGRAL_TYPE_P (type)
> - && TYPE_OVERFLOW_UNDEFINED (type));
> + iv->no_overflow = !folded_casts && nowrap_type_p (type);
> +
> + if (!iv->no_overflow
> + && !iv_can_overflow_p (wrto_loop, type, iv->base, iv->step))
> + iv->no_overflow = true;
> +
>
> /* Try to simplify iv base:
>
> Index: tree-scalar-evolution.h
> ===================================================================
> --- tree-scalar-evolution.h (revision 237856)
> +++ tree-scalar-evolution.h (working copy)
> @@ -38,6 +38,7 @@ extern unsigned int scev_const_prop (voi
> extern bool expression_expensive_p (tree);
> extern bool simple_iv (struct loop *, struct loop *, tree, struct affine_iv
> *,
> bool);
> +extern bool iv_can_overflow_p (struct loop *, tree, tree, tree);
> extern tree compute_overall_effect_of_inner_loop (struct loop *, tree);
>
> /* Returns the basic block preceding LOOP, or the CFG entry block when
> Index: tree-ssa-loop-niter.c
> ===================================================================
> --- tree-ssa-loop-niter.c (revision 237856)
> +++ tree-ssa-loop-niter.c (working copy)
> @@ -4105,7 +4105,7 @@ n_of_executions_at_most (gimple *stmt,
> bool
> nowrap_type_p (tree type)
> {
> - if (INTEGRAL_TYPE_P (type)
> + if (ANY_INTEGRAL_TYPE_P (type)
> && TYPE_OVERFLOW_UNDEFINED (type))
> return true;
>
>
>
--
Richard Biener <rguent...@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
