The following patch makes sure that CCP when it computes a lattice
value to UNDEFINED ends up replacing uses with default defs (and thus
removes such UNDEFINED producing defs). This optimizes the testcase
below to
<bb 2>:
return _6(D);
in the first CCP. Note this patch isn't mainly for the optimization
it does but for making it easier to discover cases where CCP gets
UNDEFINED wrong (similar to VRP2 re-using the range-info that is
still live on SSA names - that's to catch bogus range-info).
If the experiment works out (read: bootstrap / test completes
successfully) I'm going to enhance VRP and will also look at
how value-numbering could do a similar job.
It might also help removing unreachable (undefined) code earlier
of course.
Bootstrap and regtest running on x86_64-unknown-linux-gnu.
Richard.
2015-11-04 Richard Biener <rguent...@suse.de>
* tree-ssa-ccp.c: Include tree-dfa.h.
(get_constant_value_for_substitution): New function returning
default defs for UNDEFINED.
(ccp_finalize): Use get_constant_value_for_substitution for
substitute_and_fold.
Index: gcc/tree-ssa-ccp.c
===================================================================
*** gcc/tree-ssa-ccp.c (revision 229734)
--- gcc/tree-ssa-ccp.c (working copy)
*************** along with GCC; see the file COPYING3.
*** 139,144 ****
--- 139,145 ----
#include "params.h"
#include "builtins.h"
#include "tree-chkp.h"
+ #include "tree-dfa.h"
/* Possible lattice values. */
*************** get_constant_value (tree var)
*** 365,370 ****
--- 366,411 ----
return NULL_TREE;
}
+ /* Return the constant tree value associated with VAR. */
+
+ static inline tree
+ get_constant_value_for_substitution (tree var)
+ {
+ ccp_prop_value_t *val;
+ if (TREE_CODE (var) != SSA_NAME)
+ {
+ if (is_gimple_min_invariant (var))
+ return var;
+ return NULL_TREE;
+ }
+ val = get_value (var);
+ if (!val)
+ return NULL_TREE;
+ if (val->lattice_val == UNDEFINED)
+ {
+ /* At substitution time replace UNDEFINED values with a default
+ def. We delay creating of the default-def until we actually
+ require substitution. */
+ tree def;
+ if (SSA_NAME_VAR (var)
+ && TREE_CODE (SSA_NAME_VAR (var)) == VAR_DECL)
+ def = get_or_create_ssa_default_def (cfun, SSA_NAME_VAR (var));
+ else
+ def = get_or_create_ssa_default_def (cfun,
+ create_tmp_reg (TREE_TYPE (var)));
+ /* Adjust the lattice. */
+ val->lattice_val = CONSTANT;
+ val->mask = -1;
+ val->value = def;
+ return def;
+ }
+ else if (val->lattice_val == CONSTANT
+ && (TREE_CODE (val->value) != INTEGER_CST
+ || val->mask == 0))
+ return val->value;
+ return NULL_TREE;
+ }
+
/* Sets the value associated with VAR to VARYING. */
static inline void
*************** ccp_finalize (void)
*** 942,948 ****
}
/* Perform substitutions based on the known constant values. */
! something_changed = substitute_and_fold (get_constant_value,
ccp_fold_stmt, true);
free (const_val);
--- 983,989 ----
}
/* Perform substitutions based on the known constant values. */
! something_changed = substitute_and_fold
(get_constant_value_for_substitution,
ccp_fold_stmt, true);
free (const_val);
Index: gcc/testsuite/gcc.dg/tree-ssa/ssa-ccp-40.c
===================================================================
*** gcc/testsuite/gcc.dg/tree-ssa/ssa-ccp-40.c (revision 0)
--- gcc/testsuite/gcc.dg/tree-ssa/ssa-ccp-40.c (working copy)
***************
*** 0 ****
--- 1,14 ----
+ /* { dg-do compile } */
+ /* { dg-options "-O -fdump-tree-ccp1" } */
+
+ int foo (void)
+ {
+ int b;
+ int a = b + 1;
+ return a;
+ }
+
+ /* b + 1 results in an undefined value and CCP should simply replace the
+ return stmt to return a default def so the addition can be DCEed. */
+
+ /* { dg-final { scan-tree-dump "return \[^_\]*_\[0-9\]\+\\(D\\);" "ccp1" } }
*/
