On Sat, Feb 4, 2017 at 3:52 PM, Jeff Law <l...@redhat.com> wrote:
>
> This is the second in the 4 part series to address 79095. This patch
> introduces a new function into tree-vrp.c to allow for the detection of
> overflow checks of the form A OP A + CST (for unsigned/wrapping A).
>
> This is implemented by first checking for A OP B, we then conditionally walk
> the ASSERT_EXPR chain for B to produce B'. We then look at the defining
> statement for B or B' to see if it has the form B = X + CST or B' = X + CST
> respectively.
>
> Then we conditionally walk the ASSERT_EXPR chain for A to see if it resolves
> to X at any point. There have been cases where no walking was necessary to
> show that X resolves to A. Other cases have required walking part or the
> entire ASSERT_EXPR chain.
>
> We do not walk during propagation, but do walk during
> folding/simplification.
>
> At this point we have an overflow check of the appropriate form. We compute
> an updated constant so that we can check for overflow with expressions like
>
> A > 0xfffffffe
>
> or
>
> A <= 0
>
> Those are particularly interesting forms as they collapse into equality
> tests (next patch). The code supports other forms, but they're not as
> useful because they don't end up generating equality tests or allow for
> constant propagation.
>
> Bootstrapped and regression tested with the other patches in this series.
> OK for the trunk?
>
>
>
>
> * tree-vrp.c (overflow_comparison_p): New function.
>
> diff --git a/gcc/tree-vrp.c b/gcc/tree-vrp.c
> index 3338d8b..6459c71 100644
> --- a/gcc/tree-vrp.c
> +++ b/gcc/tree-vrp.c
> @@ -5189,6 +5189,94 @@ masked_increment (const wide_int &val_in, const
> wide_int &mask,
> return val ^ sgnbit;
> }
>
> +/* OP0 CODE OP1 is a comparison. Examine the comparison and potentially
> + OP1's defining statement to see if it ultimately has the form
> + OP0 CODE (OP0 PLUS INTEGER_CST)
> +
> + If so, return TRUE indicating this is an overflow test and store into
> + *NEW_CST an updated constant that can be used in a narrowed range test.
> +
> + REVERSED indicates if the comparison was originally:
> +
> + OP1 CODE' OP0.
> +
> + This affects how we build the updated constant. */
> +
> +static bool
> +overflow_comparison_p (enum tree_code code, tree op0, tree op1,
> + bool follow_assert_exprs, bool reversed, tree
> *new_cst)
> +{
> + /* See if this is a relational operation between two SSA_NAMES with
> + unsigned, overflow wrapping values. If so, check it more deeply. */
> + if ((code == LT_EXPR || code == LE_EXPR
> + || code == GE_EXPR || code == GT_EXPR)
> + && TREE_CODE (op0) == SSA_NAME
> + && TREE_CODE (op1) == SSA_NAME
> + && INTEGRAL_TYPE_P (TREE_TYPE (op0))
> + && TYPE_UNSIGNED (TREE_TYPE (op0))
> + && TYPE_OVERFLOW_WRAPS (TREE_TYPE (op0)))
> + {
> + gimple *op1_def = SSA_NAME_DEF_STMT (op1);
> +
> + /* If requested, follow any ASSERT_EXPRs backwards for OP1. */
> + if (follow_assert_exprs)
> + {
> + while (gimple_assign_single_p (op1_def)
> + && TREE_CODE (gimple_assign_rhs1 (op1_def)) == ASSERT_EXPR)
> + {
> + op1 = TREE_OPERAND (gimple_assign_rhs1 (op1_def), 0);
> + if (TREE_CODE (op1) != SSA_NAME)
> + break;
> + op1_def = SSA_NAME_DEF_STMT (op1);
> + }
> + }
> +
> + /* Now look at the defining statement of OP1 to see if it adds
> + or subtracts a nonzero constant from another operand. */
> + if (op1_def
> + && is_gimple_assign (op1_def)
> + && gimple_assign_rhs_code (op1_def) == PLUS_EXPR
> + && TREE_CODE (gimple_assign_rhs2 (op1_def)) == INTEGER_CST
> + && wi::ne_p (gimple_assign_rhs2 (op1_def), 0))
! integer_zerop ()
> + {
> + tree target = gimple_assign_rhs1 (op1_def);
> +
> + /* If requested, follow ASSERT_EXPRs backwards for op0 looking
> + for one where TARGET appears on the RHS. */
> + if (follow_assert_exprs)
> + {
> + /* Now see if that "other operand" is op0, following the chain
> + of ASSERT_EXPRs if necessary. */
> + gimple *op0_def = SSA_NAME_DEF_STMT (op0);
> + while (op0 != target
> + && gimple_assign_single_p (op0_def)
> + && TREE_CODE (gimple_assign_rhs1 (op0_def)) ==
> ASSERT_EXPR)
> + {
> + op0 = TREE_OPERAND (gimple_assign_rhs1 (op0_def), 0);
> + if (TREE_CODE (op0) != SSA_NAME)
> + break;
> + op0_def = SSA_NAME_DEF_STMT (op0);
> + }
> + }
> +
> + /* If we did not find our target SSA_NAME, then this is not
> + an overflow test. */
> + if (op0 != target)
> + return false;
> +
> + tree type = TREE_TYPE (op0);
> + wide_int max = wi::max_value (TYPE_PRECISION (type), UNSIGNED);
> + HOST_WIDE_INT inc = TREE_INT_CST_LOW (gimple_assign_rhs2
> (op1_def));
You nowhere tested that rhs2 fits a HOST_WIDE_INT. You can simply make inc
a tree and use that in the max + inc expression below.
I'll have to see where the function is used to make sense of it.
> + if (reversed)
> + *new_cst = wide_int_to_tree (type, max + inc);
> + else
> + *new_cst = wide_int_to_tree (type, max - inc);
> + return true;
> + }
> + }
> + return false;
> +}
> +
> /* Try to register an edge assertion for SSA name NAME on edge E for
> the condition COND contributing to the conditional jump pointed to by
> BSI.
> Invert the condition COND if INVERT is true. */
>
