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Re: [PATCH] Improve jump threading using VRP information
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Jeff Law <law at redhat dot com>
- Cc: gcc-patches <gcc-patches at gcc dot gnu dot org>
- Date: Tue, 27 Aug 2013 09:50:43 +0200
- Subject: Re: [PATCH] Improve jump threading using VRP information
- Authentication-results: sourceware.org; auth=none
- References: <51CBBFA7 dot 9010808 at redhat dot com>
On Thu, Jun 27, 2013 at 6:29 AM, Jeff Law <law@redhat.com> wrote:
>
> Just something else I saw while analyzing dumps from an unrelated set of
> changes.
>
> It's relatively common to see sequences like this:
>
> # parent_1 = PHI <parent_7(3), parent_6(D)(4), parent_6(D)(5)>
> _11 = single_tree_10(D) != 0;
> _12 = parent_1 == 0B;
> _13 = _11 & _12;
> if (_13 != 0)
> goto <bb 8>;
> else
> goto <bb 7>;
>
> Where VRP can deduce that the value of parent_6 has a nonzero value on one
> (or more) of the paths reaching this block (because those paths dereference
> parent_6).
>
> Obviously when VRP knows parent_6 is nonzero, then we know the current block
> will always transfer control to BB 7.
>
> Or something like this:
>
> <bb 6>:
> # prephitmp_49 = PHI <1(4), pretmp_48(5)>
> prev_line.31_6 = prev_line;
> _8 = prev_line.31_6 != line_7(D);
> line_differs_9 = (unsigned char) _8;
> _10 = prephitmp_49 | line_differs_9;
> if (_10 != 0)
> goto <bb 8>;
> else
> goto <bb 7>;
>
>
> We may not know the exact value of _10, but VRP can determine that when BB6
> is reached from BB4 that BB6 will always transfer control to BB8.
>
>
> I never coded up the bits to utilize VRP information to simplify statements
> for threading except for COND_EXPRs. It wasn't clear how often it would be
> useful, thus I took the easy way out. This picks up a hundred or so
> additional threading opportunities in my testfiles. Not huge, but worth it
> IMHO.
>
> Bootstrapped and regression tested on x86_64-unknown-linux-gnu.
>
> OK for trunk?
Ok if it still applies.
Thanks,
Richard.
>
>
>
> * tree-vrp.c (simplify_stmt_for_jump_threading): Try to
> simplify assignments too. If the RHS collapses to a singleton
> range, then return the value for the range.
>
> * gcc.dg/tree-ssa/ssa-vrp-thread-1.c: New test.
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-vrp-thread-1.c
> b/gcc/testsuite/gcc.dg/tree-ssa/ssa-vrp-thread-1.c
> new file mode 100644
> index 0000000..9d9473e
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-vrp-thread-1.c
> @@ -0,0 +1,31 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-vrp1-details" } */
> +
> +
> +struct basic_block_def;
> +typedef struct basic_block_def *basic_block;
> +enum gimple_code
> +{
> + LAST_AND_UNUSED_GIMPLE_CODE
> +};
> +struct omp_region
> +{
> + struct omp_region *outer;
> + basic_block cont;
> +};
> +void
> +build_omp_regions_1 (basic_block bb, struct omp_region *parent,
> + unsigned char single_tree, enum gimple_code code)
> +{
> + if (code == 25)
> + parent = parent->outer;
> + else if (code == 42)
> + parent->cont = bb;
> + if (single_tree && !parent)
> + return;
> + oof ();
> +}
> +
> +/* { dg-final { scan-tree-dump-times "Threaded" 1 "vrp1" } } */
> +/* { dg-final { cleanup-tree-dump "vrp1" } } */
> +
> diff --git a/gcc/tree-vrp.c b/gcc/tree-vrp.c
> index ec7ef8f..683a3db 100644
> --- a/gcc/tree-vrp.c
> +++ b/gcc/tree-vrp.c
> @@ -9109,15 +9109,27 @@ static vec<tree> equiv_stack;
> static tree
> simplify_stmt_for_jump_threading (gimple stmt, gimple within_stmt)
> {
> - /* We only use VRP information to simplify conditionals. This is
> - overly conservative, but it's unclear if doing more would be
> - worth the compile time cost. */
> - if (gimple_code (stmt) != GIMPLE_COND)
> - return NULL;
> + if (gimple_code (stmt) == GIMPLE_COND)
> + return vrp_evaluate_conditional (gimple_cond_code (stmt),
> + gimple_cond_lhs (stmt),
> + gimple_cond_rhs (stmt), within_stmt);
> +
> + if (gimple_code (stmt) == GIMPLE_ASSIGN)
> + {
> + value_range_t new_vr = VR_INITIALIZER;
> + tree lhs = gimple_assign_lhs (stmt);
> +
> + if (TREE_CODE (lhs) == SSA_NAME
> + && (INTEGRAL_TYPE_P (TREE_TYPE (lhs))
> + || POINTER_TYPE_P (TREE_TYPE (lhs))))
> + {
> + extract_range_from_assignment (&new_vr, stmt);
> + if (range_int_cst_singleton_p (&new_vr))
> + return new_vr.min;
> + }
> + }
>
> - return vrp_evaluate_conditional (gimple_cond_code (stmt),
> - gimple_cond_lhs (stmt),
> - gimple_cond_rhs (stmt), within_stmt);
> + return NULL_TREE;
> }
>
> /* Blocks which have more than one predecessor and more than
>