cselib: add function to check if SET is redundant [PR106187]

A SET operation that writes memory may have the same value as an
earlier store but if the alias sets of the new and earlier store do
not conflict then the set is not truly redundant.  This can happen,
for example, if objects of different types share a stack slot.

To fix this we define a new function in cselib that first checks for
equality and if that is successful then finds the earlier store in the
value history and checks the alias sets.

The routine is used in two places elsewhere in the compiler:
cfgcleanup and postreload.

gcc/ChangeLog:

	PR rtl-optimization/106187
	* alias.h (mems_same_for_tbaa_p): Declare.
	* alias.cc (mems_same_for_tbaa_p): New function.
	* dse.cc (record_store): Use it instead of open-coding
	alias check.
	* cselib.h (cselib_redundant_set_p): Declare.
	* cselib.cc: Include alias.h
	(cselib_redundant_set_p): New function.
	* cfgcleanup.cc: (mark_effect): Use cselib_redundant_set_p instead
	of rtx_equal_for_cselib_p.
	* postreload.cc (reload_cse_simplify): Use cselib_redundant_set_p.
	(reload_cse_noop_set_p): Delete.

diff --git a/gcc/alias.cc b/gcc/alias.cc
index 8c08452e0acfcbf1bfd8fd2e8cd420b5b929d6b4..d54feb1526876386716a7f54225abe9467db30eb 100644 (file)
--- a/gcc/alias.cc
+++ b/gcc/alias.cc
@@ -389,6 +389,20 @@ refs_same_for_tbaa_p (tree earlier, tree later)
          || alias_set_subset_of (later_base_set, earlier_base_set));
 }
 
+/* Similar to refs_same_for_tbaa_p() but for use on MEM rtxs.  */
+bool
+mems_same_for_tbaa_p (rtx earlier, rtx later)
+{
+  gcc_assert (MEM_P (earlier));
+  gcc_assert (MEM_P (later));
+
+  return ((MEM_ALIAS_SET (earlier) == MEM_ALIAS_SET (later)
+          || alias_set_subset_of (MEM_ALIAS_SET (later),
+                                  MEM_ALIAS_SET (earlier)))
+         && (!MEM_EXPR (earlier)
+             || refs_same_for_tbaa_p (MEM_EXPR (earlier), MEM_EXPR (later))));
+}
+
 /* Returns a pointer to the alias set entry for ALIAS_SET, if there is
    such an entry, or NULL otherwise.  */
 

diff --git a/gcc/alias.h b/gcc/alias.h
index b2596518ac95878606ccb363504f810cdcfe4e5b..ee3db46676309d8d130d2cd2abf0f4415ef0fae3 100644 (file)
--- a/gcc/alias.h
+++ b/gcc/alias.h
@@ -40,6 +40,7 @@ tree reference_alias_ptr_type_1 (tree *);
 bool alias_ptr_types_compatible_p (tree, tree);
 int compare_base_decls (tree, tree);
 bool refs_same_for_tbaa_p (tree, tree);
+bool mems_same_for_tbaa_p (rtx, rtx);
 
 /* This alias set can be used to force a memory to conflict with all
    other memories, creating a barrier across which no memory reference

diff --git a/gcc/cfgcleanup.cc b/gcc/cfgcleanup.cc
index 18047da7b2424d440acd12fc3ab0b21b476d79c6..a8b0139bb4dcfcfbc0ac7fb11e2020af8a9de45f 100644 (file)
--- a/gcc/cfgcleanup.cc
+++ b/gcc/cfgcleanup.cc
@@ -208,7 +208,7 @@ mark_effect (rtx exp, regset nonequal)
       return false;
 
     case SET:
-      if (rtx_equal_for_cselib_p (SET_DEST (exp), SET_SRC (exp)))
+      if (cselib_redundant_set_p (exp))
        return false;
       dest = SET_DEST (exp);
       if (dest == pc_rtx)

diff --git a/gcc/cselib.cc b/gcc/cselib.cc
index 6769beeeaf8a606b179a4159caab6300020beb7a..6a5609786fa6cda48297be9402c78bb8df4ae8a9 100644 (file)
--- a/gcc/cselib.cc
+++ b/gcc/cselib.cc
@@ -32,6 +32,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "dumpfile.h"
 #include "cselib.h"
 #include "function-abi.h"
+#include "alias.h"
 
 /* A list of cselib_val structures.  */
 struct elt_list
@@ -1157,6 +1158,75 @@ rtx_equal_for_cselib_1 (rtx x, rtx y, machine_mode memmode, int depth)
   return 1;
 }
 
+/* Wrapper for rtx_equal_for_cselib_p to determine whether a SET is
+   truly redundant, taking into account aliasing information.  */
+bool
+cselib_redundant_set_p (rtx set)
+{
+  gcc_assert (GET_CODE (set) == SET);
+  rtx dest = SET_DEST (set);
+  if (cselib_reg_set_mode (dest) != GET_MODE (dest))
+    return false;
+
+  if (!rtx_equal_for_cselib_p (dest, SET_SRC (set)))
+    return false;
+
+  while (GET_CODE (dest) == SUBREG
+        || GET_CODE (dest) == ZERO_EXTRACT
+        || GET_CODE (dest) == STRICT_LOW_PART)
+    dest = XEXP (dest, 0);
+
+  if (!flag_strict_aliasing || !MEM_P (dest))
+    return true;
+
+  /* For a store we need to check that suppressing it will not change
+     the effective alias set.  */
+  rtx dest_addr = XEXP (dest, 0);
+
+  /* Lookup the equivalents to the original dest (rather than just the
+     MEM).  */
+  cselib_val *src_val = cselib_lookup (SET_DEST (set),
+                                      GET_MODE (SET_DEST (set)),
+                                      0, VOIDmode);
+
+  if (src_val)
+    {
+      /* Walk the list of source equivalents to find the MEM accessing
+        the same location.  */
+      for (elt_loc_list *l = src_val->locs; l; l = l->next)
+       {
+         rtx src_equiv = l->loc;
+         while (GET_CODE (src_equiv) == SUBREG
+                || GET_CODE (src_equiv) == ZERO_EXTRACT
+                || GET_CODE (src_equiv) == STRICT_LOW_PART)
+           src_equiv = XEXP (src_equiv, 0);
+
+         if (MEM_P (src_equiv))
+           {
+             /* Match the MEMs by comparing the addresses.  We can
+                only remove the later store if the earlier aliases at
+                least all the accesses of the later one.  */
+             if (rtx_equal_for_cselib_1 (dest_addr, XEXP (src_equiv, 0),
+                                         GET_MODE (dest), 0))
+               return mems_same_for_tbaa_p (src_equiv, dest);
+           }
+       }
+    }
+
+  /* We failed to find a recorded value in the cselib history, so try
+     the source of this set; this catches cases such as *p = *q when p
+     and q have the same value.  */
+  rtx src = SET_SRC (set);
+  while (GET_CODE (src) == SUBREG)
+    src = XEXP (src, 0);
+
+  if (MEM_P (src)
+      && rtx_equal_for_cselib_1 (dest_addr, XEXP (src, 0), GET_MODE (dest), 0))
+    return mems_same_for_tbaa_p (src, dest);
+
+  return false;
+}
+
 /* Helper function for cselib_hash_rtx.  Arguments like for cselib_hash_rtx,
    except that it hashes (plus:P x c).  */
 

diff --git a/gcc/cselib.h b/gcc/cselib.h
index 9ae65e6459e0cd36eb596e09cd4d5a8f908ab579..b0905053ea581910aff047fb220537235270d116 100644 (file)
--- a/gcc/cselib.h
+++ b/gcc/cselib.h
@@ -83,6 +83,7 @@ extern void cselib_process_insn (rtx_insn *);
 extern bool fp_setter_insn (rtx_insn *);
 extern machine_mode cselib_reg_set_mode (const_rtx);
 extern int rtx_equal_for_cselib_1 (rtx, rtx, machine_mode, int);
+extern bool cselib_redundant_set_p (rtx);
 extern int references_value_p (const_rtx, int);
 extern rtx cselib_expand_value_rtx (rtx, bitmap, int);
 typedef rtx (*cselib_expand_callback)(rtx, bitmap, int, void *);

diff --git a/gcc/dse.cc b/gcc/dse.cc
index 90a4c1f22dba6a672a8b4ea8ad9fe9755084c3a3..0f7b0fb9796769e7ff09c57d4a3c947cae7a337f 100644 (file)
--- a/gcc/dse.cc
+++ b/gcc/dse.cc
@@ -1570,12 +1570,7 @@ record_store (rtx body, bb_info_t bb_info)
                                         width)
              /* We can only remove the later store if the earlier aliases
                 at least all accesses the later one.  */
-             && ((MEM_ALIAS_SET (mem) == MEM_ALIAS_SET (s_info->mem)
-                  || alias_set_subset_of (MEM_ALIAS_SET (mem),
-                                          MEM_ALIAS_SET (s_info->mem)))
-                 && (!MEM_EXPR (s_info->mem)
-                     || refs_same_for_tbaa_p (MEM_EXPR (s_info->mem),
-                                              MEM_EXPR (mem)))))
+             && mems_same_for_tbaa_p (s_info->mem, mem))
            {
              if (GET_MODE (mem) == BLKmode)
                {

diff --git a/gcc/postreload.cc b/gcc/postreload.cc
index d1c99fe6dc9b21b191cd3d5fb0ac5ee059b3642e..41f61d3264825b1f2b53e64c7eb022b1ad3de950 100644 (file)
--- a/gcc/postreload.cc
+++ b/gcc/postreload.cc
@@ -43,7 +43,6 @@ along with GCC; see the file COPYING3.  If not see
 #include "function-abi.h"
 #include "rtl-iter.h"
 
-static int reload_cse_noop_set_p (rtx);
 static bool reload_cse_simplify (rtx_insn *, rtx);
 static void reload_cse_regs_1 (void);
 static int reload_cse_simplify_set (rtx, rtx_insn *);
@@ -74,16 +73,6 @@ reload_cse_regs (rtx_insn *first ATTRIBUTE_UNUSED)
     }
 }
 
-/* See whether a single set SET is a noop.  */
-static int
-reload_cse_noop_set_p (rtx set)
-{
-  if (cselib_reg_set_mode (SET_DEST (set)) != GET_MODE (SET_DEST (set)))
-    return 0;
-
-  return rtx_equal_for_cselib_p (SET_DEST (set), SET_SRC (set));
-}
-
 /* Try to simplify INSN.  Return true if the CFG may have changed.  */
 static bool
 reload_cse_simplify (rtx_insn *insn, rtx testreg)
@@ -118,7 +107,7 @@ reload_cse_simplify (rtx_insn *insn, rtx testreg)
          this out, so it's safer to simplify before we delete.  */
       count += reload_cse_simplify_set (body, insn);
 
-      if (!count && reload_cse_noop_set_p (body))
+      if (!count && cselib_redundant_set_p (body))
        {
          if (check_for_inc_dec (insn))
            delete_insn_and_edges (insn);
@@ -157,7 +146,7 @@ reload_cse_simplify (rtx_insn *insn, rtx testreg)
          rtx part = XVECEXP (body, 0, i);
          if (GET_CODE (part) == SET)
            {
-             if (! reload_cse_noop_set_p (part))
+             if (! cselib_redundant_set_p (part))
                break;
              if (REG_P (SET_DEST (part))
                  && REG_FUNCTION_VALUE_P (SET_DEST (part)))