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.

1 files changed, 70 insertions, 0 deletions

diff --git a/gcc/cselib.cc b/gcc/cselib.cc
index 6769beeeaf8..6a5609786fa 100644
--- 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).  */