On 12/11/2017 10:17 PM, Alexandre Oliva wrote:
> On Dec 11, 2017, Jeff Law <l...@redhat.com> wrote:
>
> I've updated it according to richi's and your feedbacks. Regstrapped on
> {x86_64,i686}-linux-gnu. Ok to install?
>
>
> We limit the amount of copying for jump threading based on counting
> stmts. This counting is overly pessimistic, because we will very
> often delete stmts as a consequence of jump threading: when the final
> conditional jump of a block is removed, earlier SSA names computed
> exclusively for use in that conditional are killed. Furthermore, PHI
> nodes in blocks with only two predecessors are trivially replaced with
> their now-single values after threading.
>
> This patch scans blocks to be copied in the path constructed so far
> and estimates the number of stmts that will be removed in the copies,
> bumping up the stmt count limit.
>
> for gcc/ChangeLog
>
> PR tree-optimization/81165
> * tree-ssa-threadedge.c (uses_in_bb): New.
> (estimate_threading_killed_stmts): New.
> (estimate_threading_killed_stmts): New overload.
> (record_temporary_equivalences_from_stmts_at_dest): Add path
> parameter; adjust caller. Expand limit when it's hit.
>
> for gcc/testsuite/ChangeLog
>
> PR tree-optimization/81165
> * gcc.dg/pr81165.c: New.
> diff --git a/gcc/tree-ssa-threadedge.c b/gcc/tree-ssa-threadedge.c
> index 91793bfa59d3..0f5b943aa9a0 100644
> --- a/gcc/tree-ssa-threadedge.c
> +++ b/gcc/tree-ssa-threadedge.c
> @@ -170,6 +170,160 @@ threadedge_valueize (tree t)
> return t;
> }
>
> +/* Return how many uses of T there are within BB, as long as there
> + aren't any uses outside BB. If there are any uses outside BB,
> + return -1 if there's at most one use within BB, or -2 if there is
> + more than one use within BB. */
> +
> +static int
> +uses_in_bb (tree t, basic_block bb)
> +{
> + int uses = 0;
> + bool outside_bb = false;
> +
> + imm_use_iterator iter;
> + use_operand_p use_p;
> + FOR_EACH_IMM_USE_FAST (use_p, iter, t)
> + {
> + if (is_gimple_debug (USE_STMT (use_p)))
> + continue;
> +
> + if (gimple_bb (USE_STMT (use_p)) != bb)
> + outside_bb = true;
> + else
> + uses++;
> +
> + if (outside_bb && uses > 1)
> + return -2;
> + }
> +
> + if (outside_bb)
> + return -1;
> +
> + return uses;
> +}
> +
> +/* Starting from the final control flow stmt in BB, assuming it will
> + be removed, follow uses in to-be-removed stmts back to their defs
> + and count how many defs are to become dead and be removed as
> + well. */
> +
> +static int
> +estimate_threading_killed_stmts (basic_block bb)
> +{
> + int killed_stmts = 0;
> + hash_map<tree, int> ssa_remaining_uses;
> + auto_vec<gimple *, 4> dead_worklist;
> +
> + /* If the block has only two predecessors, threading will turn phi
> + dsts into either src, so count them as dead stmts. */
> + bool drop_all_phis = EDGE_COUNT (bb->preds) == 2;
> +
> + if (drop_all_phis)
> + for (gphi_iterator gsi = gsi_start_phis (bb);
> + !gsi_end_p (gsi); gsi_next (&gsi))
> + {
> + gphi *phi = gsi.phi ();
> + tree dst = gimple_phi_result (phi);
> +
> + /* We don't count virtual PHIs as stmts in
> + record_temporary_equivalences_from_phis. */
> + if (virtual_operand_p (dst))
> + continue;
> +
> + killed_stmts++;
> + }
> +
> + if (gsi_end_p (gsi_last_bb (bb)))
> + return killed_stmts;
> +
> + gimple *stmt = gsi_stmt (gsi_last_bb (bb));
> + if (gimple_code (stmt) != GIMPLE_COND
> + && gimple_code (stmt) != GIMPLE_GOTO
> + && gimple_code (stmt) != GIMPLE_SWITCH)
> + return killed_stmts;
> +
> + dead_worklist.quick_push (stmt);
> + while (!dead_worklist.is_empty ())
> + {
> + stmt = dead_worklist.pop ();
> +
> + ssa_op_iter iter;
> + use_operand_p use_p;
> + FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_USE)
> + {
> + tree t = USE_FROM_PTR (use_p);
> + gimple *def = SSA_NAME_DEF_STMT (t);
> +
> + if (gimple_bb (def) == bb
> + && (gimple_code (def) != GIMPLE_PHI
> + || !drop_all_phis)
> + && !gimple_has_side_effects (def))
> + {
> + int *usesp = ssa_remaining_uses.get (t);
> + int uses;
> +
> + if (usesp)
> + uses = *usesp;
> + else
> + uses = uses_in_bb (t, bb);
> +
> + gcc_assert (uses);
> +
> + /* Don't bother recording the expected use count if we
> + won't find any further uses within BB. */
> + if (!usesp && (uses < -1 || uses > 1))
> + {
> + usesp = &ssa_remaining_uses.get_or_insert (t);
> + *usesp = uses;
> + }
> +
> + if (uses < 0)
> + continue;
> +
> + --uses;
> + if (usesp)
> + *usesp = uses;
> +
> + if (!uses)
> + {
> + killed_stmts++;
> + if (usesp)
> + ssa_remaining_uses.remove (t);
> + if (gimple_code (def) != GIMPLE_PHI)
> + dead_worklist.safe_push (def);
> + }
> + }
> + }
> + }
> +
> + if (dump_file)
> + fprintf (dump_file, "threading bb %i kills %i stmts\n",
> + bb->index, killed_stmts);
> +
> + return killed_stmts;
So it appears the loop counts a statement as killed when its uses are no
longer needed. AFAICT we fail to count the conditional as killed.
That's trivially fixed.
I also think we want to use this elsewhere. I think putting it in
tree-ssa-threadupdate.c and exporting from tree-ssa-threadupdate.h
should work.
> +}
> +
> +/* Estimate, over all src blocks to be copied in PATH and E's dest,
> + the number of stmts that become dead as a consequence of removing
> + their final control flow stmts. If PATH is not empty, E must be
> + its last entry. */
> +
> +static int
> +estimate_threading_killed_stmts (edge e, vec<jump_thread_edge *> *path)
> +{
> + gcc_assert (path->length () == 0 || path->last ()->e == e);
> + int total = estimate_threading_killed_stmts (e->dest);
> +
> + int i;
> + jump_thread_edge *je;
> + FOR_EACH_VEC_ELT_REVERSE (*path, i, je)
> + if (je->type == EDGE_COPY_SRC_BLOCK)
> + total += estimate_threading_killed_stmts (je->e->src);
> +
> + return total;
> +}
I don't think we need this function at all.
At the point where you're calling it, we're at the one and only block
within the jump threading path where we're copying a block where
statements are going to be killed.
So I think you should just remove this function and call the other one
(that works on just a basic block). THat also means we don't have to
pass the path around.
I've got those minor tweaks in a tree here along with the changes to use
it to fix 36550 as well. Let me run them through some deeper testing.
jeff
