On Thu, 8 May 2014, Richard Biener wrote:
>
> Ok, not really predicate aware, but this makes value-numbering
> pessimistically handle non-executable edges. In the following
> patch groundwork is laid and PHI value-numbering is adjusted
> to take advantage of edges known to be not executable.
>
> SCCVN is not well-suited to be control aware, but we still
> can see if value-numbering allows us to mark edges as
> not executable by looking at control statements. Value-numbering
> of PHI nodes is one obvious consumer of such information
> and it also gives a natural order to do that (pessimistic)
> edge executability computation - dominator order.
>
> Thus the following adds a pass over all control statements,
> trying to simplify them after value-numbering their operands
> (and all uses recursively, as SCCVN does).
>
> With followup patches I will try to use this information to
> reduce the amount of work done (also improving optimization,
> of course). One other obvious candidate is the alias walker
> which doesn't have to consider unreachable paths when
> walking into virtual PHIs.
>
> The patch will likely get some more cleanups (due to the hack
> in set_ssa_val_to).
>
> Comments still welcome.
Quiet as usual. Well, the following is what I have committed
after bootstrapping and regtesting on x86_64-unknown-linux-gnu.
It fixes the inliner which is confused by random pass-local
flags on the edges to the exit block, adds one more testcase
and adjusts two.
I figured that followups for more optimizations are not
necessary as virtual operand value-numbering already gets
us most of the benefit. Followups trying to do less work
may still be possible but they are low on priority.
Richard.
2014-05-16 Richard Biener <rguent...@suse.de>
* tree-ssa-sccvn.c: Include tree-cfg.h and domwalk.h.
(set_ssa_val_to): Handle unexpected sets to VN_TOP.
(visit_phi): Ignore edges marked as not executable.
(class cond_dom_walker): New.
(cond_dom_walker::before_dom_children): Value-number
control statements and mark successor edges as not
executable if possible.
(run_scc_vn): First walk all control statements in
dominator order, marking edges as not executable.
* tree-inline.c (copy_edges_for_bb): Be not confused
about random edge flags.
* gcc.dg/tree-ssa/ssa-fre-39.c: New testcase.
* gcc.dg/tree-ssa/ssa-fre-40.c: Likewise.
* gcc.dg/tree-ssa/ssa-pre-8.c: One more elimination.
* gcc.dg/tree-ssa/struct-aliasing-2.c: Scan cddce1 dump.
Index: gcc/tree-ssa-sccvn.c
===================================================================
*** gcc/tree-ssa-sccvn.c.orig 2014-05-15 12:47:20.762286122 +0200
--- gcc/tree-ssa-sccvn.c 2014-05-15 13:04:57.872213342 +0200
*************** along with GCC; see the file COPYING3.
*** 51,56 ****
--- 51,58 ----
#include "params.h"
#include "tree-ssa-propagate.h"
#include "tree-ssa-sccvn.h"
+ #include "tree-cfg.h"
+ #include "domwalk.h"
/* This algorithm is based on the SCC algorithm presented by Keith
Cooper and L. Taylor Simpson in "SCC-Based Value numbering"
*************** set_ssa_val_to (tree from, tree to)
*** 2661,2666 ****
--- 2663,2687 ----
tree currval = SSA_VAL (from);
HOST_WIDE_INT toff, coff;
+ /* The only thing we allow as value numbers are ssa_names
+ and invariants. So assert that here. We don't allow VN_TOP
+ as visiting a stmt should produce a value-number other than
+ that.
+ ??? Still VN_TOP can happen for unreachable code, so force
+ it to varying in that case. Not all code is prepared to
+ get VN_TOP on valueization. */
+ if (to == VN_TOP)
+ {
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ fprintf (dump_file, "Forcing value number to varying on "
+ "receiving VN_TOP\n");
+ to = from;
+ }
+
+ gcc_assert (to != NULL_TREE
+ && (TREE_CODE (to) == SSA_NAME
+ || is_gimple_min_invariant (to)));
+
if (from != to)
{
if (currval == from)
*************** set_ssa_val_to (tree from, tree to)
*** 2680,2692 ****
to = from;
}
- /* The only thing we allow as value numbers are VN_TOP, ssa_names
- and invariants. So assert that here. */
- gcc_assert (to != NULL_TREE
- && (to == VN_TOP
- || TREE_CODE (to) == SSA_NAME
- || is_gimple_min_invariant (to)));
-
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "Setting value number of ");
--- 2701,2706 ----
*************** visit_phi (gimple phi)
*** 3071,3077 ****
tree result;
tree sameval = VN_TOP;
bool allsame = true;
- unsigned i;
/* TODO: We could check for this in init_sccvn, and replace this
with a gcc_assert. */
--- 3085,3090 ----
*************** visit_phi (gimple phi)
*** 3080,3106 ****
/* See if all non-TOP arguments have the same value. TOP is
equivalent to everything, so we can ignore it. */
! for (i = 0; i < gimple_phi_num_args (phi); i++)
! {
! tree def = PHI_ARG_DEF (phi, i);
! if (TREE_CODE (def) == SSA_NAME)
! def = SSA_VAL (def);
! if (def == VN_TOP)
! continue;
! if (sameval == VN_TOP)
! {
! sameval = def;
! }
! else
! {
! if (!expressions_equal_p (def, sameval))
! {
! allsame = false;
! break;
! }
! }
! }
/* If all value numbered to the same value, the phi node has that
value. */
--- 3093,3122 ----
/* See if all non-TOP arguments have the same value. TOP is
equivalent to everything, so we can ignore it. */
! edge_iterator ei;
! edge e;
! FOR_EACH_EDGE (e, ei, gimple_bb (phi)->preds)
! if (e->flags & EDGE_EXECUTABLE)
! {
! tree def = PHI_ARG_DEF_FROM_EDGE (phi, e);
! if (TREE_CODE (def) == SSA_NAME)
! def = SSA_VAL (def);
! if (def == VN_TOP)
! continue;
! if (sameval == VN_TOP)
! {
! sameval = def;
! }
! else
! {
! if (!expressions_equal_p (def, sameval))
! {
! allsame = false;
! break;
! }
! }
! }
/* If all value numbered to the same value, the phi node has that
value. */
*************** set_hashtable_value_ids (void)
*** 4098,4103 ****
--- 4114,4218 ----
set_value_id_for_result (vr->result, &vr->value_id);
}
+ class cond_dom_walker : public dom_walker
+ {
+ public:
+ cond_dom_walker () : dom_walker (CDI_DOMINATORS), fail (false) {}
+
+ virtual void before_dom_children (basic_block);
+
+ bool fail;
+ };
+
+ void
+ cond_dom_walker::before_dom_children (basic_block bb)
+ {
+ edge e;
+ edge_iterator ei;
+
+ if (fail)
+ return;
+
+ /* If any of the predecessor edges are still marked as possibly
+ executable consider this block reachable. */
+ bool reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (cfun);
+ FOR_EACH_EDGE (e, ei, bb->preds)
+ reachable |= (e->flags & EDGE_EXECUTABLE);
+
+ /* If the block is not reachable all outgoing edges are not
+ executable. */
+ if (!reachable)
+ {
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ fprintf (dump_file, "Marking all outgoing edges of unreachable "
+ "BB %d as not executable\n", bb->index);
+
+ FOR_EACH_EDGE (e, ei, bb->succs)
+ e->flags &= ~EDGE_EXECUTABLE;
+ return;
+ }
+
+ gimple stmt = last_stmt (bb);
+ if (!stmt)
+ return;
+
+ /* Value-number the last stmts SSA uses. */
+ ssa_op_iter i;
+ tree op;
+ FOR_EACH_SSA_TREE_OPERAND (op, stmt, i, SSA_OP_USE)
+ if (VN_INFO (op)->visited == false
+ && !DFS (op))
+ {
+ fail = true;
+ return;
+ }
+
+ /* ??? We can even handle stmts with outgoing EH or ABNORMAL edges
+ if value-numbering can prove they are not reachable. Handling
+ computed gotos is also possible. */
+ tree val;
+ switch (gimple_code (stmt))
+ {
+ case GIMPLE_COND:
+ {
+ tree lhs = gimple_cond_lhs (stmt);
+ tree rhs = gimple_cond_rhs (stmt);
+ /* Work hard in computing the condition and take into account
+ the valueization of the defining stmt. */
+ if (TREE_CODE (lhs) == SSA_NAME)
+ lhs = vn_get_expr_for (lhs);
+ if (TREE_CODE (rhs) == SSA_NAME)
+ rhs = vn_get_expr_for (rhs);
+ val = fold_binary (gimple_cond_code (stmt),
+ boolean_type_node, lhs, rhs);
+ break;
+ }
+ case GIMPLE_SWITCH:
+ val = gimple_switch_index (stmt);
+ break;
+ case GIMPLE_GOTO:
+ val = gimple_goto_dest (stmt);
+ break;
+ default:
+ val = NULL_TREE;
+ break;
+ }
+ if (!val)
+ return;
+
+ edge taken = find_taken_edge (bb, vn_valueize (val));
+ if (!taken)
+ return;
+
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ fprintf (dump_file, "Marking all edges out of BB %d but (%d -> %d) as "
+ "not executable\n", bb->index, bb->index, taken->dest->index);
+
+ FOR_EACH_EDGE (e, ei, bb->succs)
+ if (e != taken)
+ e->flags &= ~EDGE_EXECUTABLE;
+ }
+
/* Do SCCVN. Returns true if it finished, false if we bailed out
due to resource constraints. DEFAULT_VN_WALK_KIND_ specifies
how we use the alias oracle walking during the VN process. */
*************** set_hashtable_value_ids (void)
*** 4105,4110 ****
--- 4220,4226 ----
bool
run_scc_vn (vn_lookup_kind default_vn_walk_kind_)
{
+ basic_block bb;
size_t i;
tree param;
*************** run_scc_vn (vn_lookup_kind default_vn_wa
*** 4122,4127 ****
--- 4238,4263 ----
VN_INFO (def)->valnum = def;
}
+ /* Mark all edges as possibly executable. */
+ FOR_ALL_BB_FN (bb, cfun)
+ {
+ edge_iterator ei;
+ edge e;
+ FOR_EACH_EDGE (e, ei, bb->succs)
+ e->flags |= EDGE_EXECUTABLE;
+ }
+
+ /* Walk all blocks in dominator order, value-numbering the last stmts
+ SSA uses and decide whether outgoing edges are not executable. */
+ cond_dom_walker walker;
+ walker.walk (ENTRY_BLOCK_PTR_FOR_FN (cfun));
+ if (walker.fail)
+ {
+ free_scc_vn ();
+ return false;
+ }
+
+ /* Value-number remaining SSA names. */
for (i = 1; i < num_ssa_names; ++i)
{
tree name = ssa_name (i);
Index: gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-39.c
===================================================================
*** /dev/null 1970-01-01 00:00:00.000000000 +0000
--- gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-39.c 2014-05-15 13:04:57.872213342
+0200
***************
*** 0 ****
--- 1,19 ----
+ /* { dg-do compile } */
+ /* { dg-options "-O -fdump-tree-fre1" } */
+
+ int foo (int i)
+ {
+ int k = i + 1;
+ int j = i + 1;
+ if (k != j)
+ k = k + 1;
+ if (k != j)
+ k = k + 1;
+ k = k - i;
+ return k;
+ }
+
+ /* We should be able to value-number the final assignment to k to 1. */
+
+ /* { dg-final { scan-tree-dump "k_. = 1;" "fre1" } } */
+ /* { dg-final { cleanup-tree-dump "fre1" } } */
Index: gcc/testsuite/gcc.dg/tree-ssa/ssa-pre-8.c
===================================================================
*** gcc/testsuite/gcc.dg/tree-ssa/ssa-pre-8.c.orig 2014-05-15
12:47:20.762286122 +0200
--- gcc/testsuite/gcc.dg/tree-ssa/ssa-pre-8.c 2014-05-15 13:04:57.873213341
+0200
*************** foo (__SIZE_TYPE__ i, struct s *array)
*** 17,23 ****
return 0;
}
/* We should eliminate two address calculations, and one load. */
/* We used to eliminate a cast but that was before POINTER_PLUS_EXPR
was added. */
! /* { dg-final { scan-tree-dump-times "Eliminated: 3" 1 "fre1"} } */
/* { dg-final { cleanup-tree-dump "fre1" } } */
--- 17,25 ----
return 0;
}
/* We should eliminate two address calculations, and one load. */
+ /* We also elimiate the PHI node feeding the return because the case
+ returning 1 is unreachable. */
/* We used to eliminate a cast but that was before POINTER_PLUS_EXPR
was added. */
! /* { dg-final { scan-tree-dump-times "Eliminated: 4" 1 "fre1"} } */
/* { dg-final { cleanup-tree-dump "fre1" } } */
Index: gcc/testsuite/gcc.dg/tree-ssa/struct-aliasing-2.c
===================================================================
*** gcc/testsuite/gcc.dg/tree-ssa/struct-aliasing-2.c.orig 2014-05-15
12:47:20.762286122 +0200
--- gcc/testsuite/gcc.dg/tree-ssa/struct-aliasing-2.c 2014-05-15
13:04:57.873213341 +0200
***************
*** 1,5 ****
/* { dg-do compile } */
! /* { dg-options "-O2 -fdump-tree-fre1" } */
struct S { unsigned f; };
--- 1,5 ----
/* { dg-do compile } */
! /* { dg-options "-O2 -fdump-tree-cddce1" } */
struct S { unsigned f; };
*************** foo ( struct S *p)
*** 12,19 ****
}
! /* There should only be one load of p->f because fwprop can change
! *(int *)&p->f into just (int)p->f. */
! /* { dg-final { scan-tree-dump-times "= \[^\n\]*p_.\\\(D\\\)" 1 "fre1" } } */
! /* { dg-final { cleanup-tree-dump "fre1" } } */
--- 12,19 ----
}
! /* There should only be one load of p->f because FRE removes the redundancy
! by realizing it can cast the result of either to the other. */
! /* { dg-final { scan-tree-dump-times "= \[^\n\]*p_.\\\(D\\\)" 1 "cddce1" } }
*/
! /* { dg-final { cleanup-tree-dump "cddce1" } } */
Index: gcc/tree-inline.c
===================================================================
*** gcc/tree-inline.c.orig 2014-05-15 12:47:20.762286122 +0200
--- gcc/tree-inline.c 2014-05-15 13:04:57.888213340 +0200
*************** copy_edges_for_bb (basic_block bb, gcov_
*** 1988,1994 ****
flags = old_edge->flags;
/* Return edges do get a FALLTHRU flag when the get inlined. */
! if (old_edge->dest->index == EXIT_BLOCK && !old_edge->flags
&& old_edge->dest->aux != EXIT_BLOCK_PTR_FOR_FN (cfun))
flags |= EDGE_FALLTHRU;
new_edge = make_edge (new_bb, (basic_block) old_edge->dest->aux, flags);
--- 1988,1995 ----
flags = old_edge->flags;
/* Return edges do get a FALLTHRU flag when the get inlined. */
! if (old_edge->dest->index == EXIT_BLOCK
! && !(old_edge->flags & (EDGE_TRUE_VALUE|EDGE_FALSE_VALUE|EDGE_FAKE))
&& old_edge->dest->aux != EXIT_BLOCK_PTR_FOR_FN (cfun))
flags |= EDGE_FALLTHRU;
new_edge = make_edge (new_bb, (basic_block) old_edge->dest->aux, flags);
Index: gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-40.c
===================================================================
*** /dev/null 1970-01-01 00:00:00.000000000 +0000
--- gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-40.c 2014-05-15 13:38:20.317075476
+0200
***************
*** 0 ****
--- 1,16 ----
+ /* { dg-do compile } */
+ /* { dg-options "-O -fdump-tree-fre1" } */
+
+ int x;
+ int foo (int *p)
+ {
+ x = 0;
+ if (x)
+ *p = 1;
+ return x;
+ }
+
+ /* The final load of x should be replaced as well as the
+ aliasing store via *p is not reachable. */
+
+ /* { dg-final { scan-tree-dump-not "= x;" "fre1" } } */
