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Re: [PATCH] Disable an unsafe VRP transformation when -fno-strict-overflow is set
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Patrick Palka <patrick at parcs dot ath dot cx>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 20 Nov 2014 14:17:19 +0100
- Subject: Re: [PATCH] Disable an unsafe VRP transformation when -fno-strict-overflow is set
- Authentication-results: sourceware.org; auth=none
- References: <1416453660-19905-1-git-send-email-patrick at parcs dot ath dot cx>
On Thu, Nov 20, 2014 at 4:21 AM, Patrick Palka <patrick@parcs.ath.cx> wrote:
> VRP may simplify a conditional like i <= 5 to i == 5 if it is known that
> the lower bound of i's range is 5, e.g. [5, +INF]. But if the upper
> bound of i's range is also overflow infinity, i.e. [5, +INF(OVF)] then
> this transformation is only valid if -fstrict-overflow is in effect.
> Likewise for transforming i > 10 to i != 10 given i's range is
> [10, +INF(OVF)] and for transforming i <= 20 to i == 20 given i's range
> is [-INF(OVF), 20].
>
> This patch makes this transformation only get performed if strict
> overflow rules are in effect and potentially emits a -Wstrict-overflow=3
> warning when the transformation takes place.
>
> Bootstrap and regtesting in progress on x86_64-unknown-linux-gnu. Does
> the patch look OK if there are no new regressions?
Ok.
Thanks,
Richard.
> gcc/
> * tree-vrp.c (test_for_singularity): New parameter
> strict_overflow_p. Set *strict_overflow_p to true if signed
> overflow must be undefined for the return value to satisfy the
> conditional.
> (simplify_cond_using_ranges): Don't perform the simplification
> if it violates overflow rules.
>
> gcc/testsuite/
> * gcc.dg/no-strict-overflow-8.c: New test.
> ---
> gcc/testsuite/gcc.dg/no-strict-overflow-8.c | 25 +++++++++++++
> gcc/tree-vrp.c | 57 +++++++++++++++++++++++++----
> 2 files changed, 74 insertions(+), 8 deletions(-)
> create mode 100644 gcc/testsuite/gcc.dg/no-strict-overflow-8.c
>
> diff --git a/gcc/testsuite/gcc.dg/no-strict-overflow-8.c b/gcc/testsuite/gcc.dg/no-strict-overflow-8.c
> new file mode 100644
> index 0000000..11ef935
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/no-strict-overflow-8.c
> @@ -0,0 +1,25 @@
> +/* { dg-do compile } */
> +/* { dg-options "-fno-strict-overflow -O2 -fdump-tree-optimized" } */
> +
> +/* We cannot fold i > 0 because p->a - p->b can be larger than INT_MAX
> + and thus i can wrap. Dual of Wstrict-overflow-18.c */
> +
> +struct c { unsigned int a; unsigned int b; };
> +extern void bar (struct c *);
> +int
> +foo (struct c *p)
> +{
> + int i;
> + int sum = 0;
> +
> + for (i = 0; i < p->a - p->b; ++i)
> + {
> + if (i > 0)
> + sum += 2;
> + bar (p);
> + }
> + return sum;
> +}
> +
> +/* { dg-final { scan-tree-dump "i_.* > 0" "optimized" } } */
> +/* { dg-final { cleanup-tree-dump "optimized" } } */
> diff --git a/gcc/tree-vrp.c b/gcc/tree-vrp.c
> index bcf4c2b..444af71 100644
> --- a/gcc/tree-vrp.c
> +++ b/gcc/tree-vrp.c
> @@ -9117,11 +9117,15 @@ simplify_bit_ops_using_ranges (gimple_stmt_iterator *gsi, gimple stmt)
> a known value range VR.
>
> If there is one and only one value which will satisfy the
> - conditional, then return that value. Else return NULL. */
> + conditional, then return that value. Else return NULL.
> +
> + If signed overflow must be undefined for the value to satisfy
> + the conditional, then set *STRICT_OVERFLOW_P to true. */
>
> static tree
> test_for_singularity (enum tree_code cond_code, tree op0,
> - tree op1, value_range_t *vr)
> + tree op1, value_range_t *vr,
> + bool *strict_overflow_p)
> {
> tree min = NULL;
> tree max = NULL;
> @@ -9172,7 +9176,16 @@ test_for_singularity (enum tree_code cond_code, tree op0,
> then there is only one value which can satisfy the condition,
> return that value. */
> if (operand_equal_p (min, max, 0) && is_gimple_min_invariant (min))
> - return min;
> + {
> + if ((cond_code == LE_EXPR || cond_code == LT_EXPR)
> + && is_overflow_infinity (vr->max))
> + *strict_overflow_p = true;
> + if ((cond_code == GE_EXPR || cond_code == GT_EXPR)
> + && is_overflow_infinity (vr->min))
> + *strict_overflow_p = true;
> +
> + return min;
> + }
> }
> return NULL;
> }
> @@ -9252,9 +9265,12 @@ simplify_cond_using_ranges (gcond *stmt)
> able to simplify this conditional. */
> if (vr->type == VR_RANGE)
> {
> - tree new_tree = test_for_singularity (cond_code, op0, op1, vr);
> + enum warn_strict_overflow_code wc = WARN_STRICT_OVERFLOW_CONDITIONAL;
> + bool sop = false;
> + tree new_tree = test_for_singularity (cond_code, op0, op1, vr, &sop);
>
> - if (new_tree)
> + if (new_tree
> + && (!sop || TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (op0))))
> {
> if (dump_file)
> {
> @@ -9275,16 +9291,30 @@ simplify_cond_using_ranges (gcond *stmt)
> fprintf (dump_file, "\n");
> }
>
> + if (sop && issue_strict_overflow_warning (wc))
> + {
> + location_t location = input_location;
> + if (gimple_has_location (stmt))
> + location = gimple_location (stmt);
> +
> + warning_at (location, OPT_Wstrict_overflow,
> + "assuming signed overflow does not occur when "
> + "simplifying conditional");
> + }
> +
> return true;
> }
>
> /* Try again after inverting the condition. We only deal
> with integral types here, so no need to worry about
> issues with inverting FP comparisons. */
> - cond_code = invert_tree_comparison (cond_code, false);
> - new_tree = test_for_singularity (cond_code, op0, op1, vr);
> + sop = false;
> + new_tree = test_for_singularity
> + (invert_tree_comparison (cond_code, false),
> + op0, op1, vr, &sop);
>
> - if (new_tree)
> + if (new_tree
> + && (!sop || TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (op0))))
> {
> if (dump_file)
> {
> @@ -9305,6 +9335,17 @@ simplify_cond_using_ranges (gcond *stmt)
> fprintf (dump_file, "\n");
> }
>
> + if (sop && issue_strict_overflow_warning (wc))
> + {
> + location_t location = input_location;
> + if (gimple_has_location (stmt))
> + location = gimple_location (stmt);
> +
> + warning_at (location, OPT_Wstrict_overflow,
> + "assuming signed overflow does not occur when "
> + "simplifying conditional");
> + }
> +
> return true;
> }
> }
> --
> 2.2.0.rc1.23.gf570943
>